02. Acids, Bases and Salts

Indicator :- Those substances which indicate the presence of acid and base in a solution by change in their color and smell are called Indicator
Natural indicator :- An indicator prepared from natural substances is known as natural indicator.
Example- Litmus Paper, Turmuric, Red cabbage.
Litmus solution is a purple dye extracted from lichen plants of the Thallophyta group. When litmus solution is neither acidic nor basic, it is purple in colour
Synthetic Indicator :- An indicator prepared from atrificical substances in laboratory is known as synthetic indicator.
Example- Phenolphthalein, Methyl orange
Olfactory Indicators : Those substances whose odour changes in acidic or basic media are called olfactory indicators. For example: onion, vanilla and clove oil.
Universal Indicator: A universal indicator is a mixture of several indicators, which display different colors at different concentrations of hydrogen ions in a solution.
Example - pH scale
Acid
        1. Acids are sour in taste
        2. Acids turn the colour of blue litmus to red
        3. Acids give H⁺ ions in aqueous solution.
Base
        1. Bases are bitter to taste and shoapy in touch.
        2. Bases turn the colour of the red litmus to blue.
        3. Bases give OH⁻ ions in aqueous solution.
The Chemical Propeties Of Acids And Bases
1. Reaction of Acids & Bases with Metals
Metal displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt.
Acid reacts with metal to form corresponding salt and hydrogen gas
Acid + Metal → Salt + Hydrogen gas
H₂SO₄ + Zn → ZnSO₄ + H₂↑

Sodium hydroxide reacts with zinc to form sodium zincate and hydrogen gas.. But such reactions are not possible with all metals.
         2NaOH + Zn → Na₂ZnO₂ + H₂
2. Reaction of Metal Carbonates & Bicarbonates with Acids
Acids react with metal carbonates or bicarbonates to form salt and water with the release of carbon dioxide gas.
Metal carbonate + acid ⟶ salt + carbon dioxide + water
Hydrochloric acid reacts with sodium carbonate to form sodium chloride and water with the release of carbon dioxide gas.
Na₂CO₃ + HCl ⟶ 2NaCl + H₂O + CO₂
On passing the carbon dioxide gas evolved through lime water,
Ca(OH)₂    +      CO₂    ⟶      CaCO₃ (s)  +   H₂O   
(Lime water)                   (White precipitate)
On passing excess carbon dioxide the following reaction takes place:
CaCO₃   +  H₂O    +CO₂    ⟶   Ca(HCO₃) (aq)
                                        (Soluble in water)

Similarly, sodium bicarbonate also reacts with hydrochloric acid to form sodium chloride and water with the release of carbon dioxide gas.
NaHCO₃ + HCl ⟶ NaCl + H₂O + CO₂
Limestone, chalk and marble are different forms of calcium carbonate.
3. Neutralisation :
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Base + Acid ⟶ Salt + Water
For example:
Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. HCl + NaOH → NaCl + H₂O
4. Reaction of Metallic Oxides with Acids
Similar to the reaction of acids and bases, acids react with metallic oxides to form salts and water, hence metallic oxides are also called basic oxides.
Acid + Metal Oxide ⟶ Salt + Water
Example-
Copper oxide reacts with hydrochloric acid to give copper chloride and water.;
CuO + 2HCl → CuCl₂ + H₂O
In this reaction the colour of the solution becomes blue-green due to the formation of copper(II) chloride.
5. Reaction of a Non-metallic Oxide with Base
Similar to the reaction of acids and bases, Bases react with non- metallic oxides to form salts and water, hence non- metallic oxides are also called acidic oxides.
Base +     Nonmetal Oxide     ⟶     Salt     +     Water
Example-
When carbon dioxide gas is passed through calcium hydroxide (lime water), which is a base, reacts with carbon dioxide to form a salt and water.
Ca(OH)₂ + CO₂ → CaCO₃  + H₂O
Acids and Bases in Water
Acids produce hydrogen ions [H⁺] in presence of water which are responsible for their acidic properties. Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as
H⁺(aq) or hydronium ion (H₃O⁺).
HCl + H₂O ⟶ H⁺+ Cl⁻
H⁺ + H₂O ⟶ H₃O⁺
HCl + H₂O ⟶ H₃O⁺ + Cl⁻
Hydrogen ions are produced in HCl in the presence of water. The separation of H⁺ ion from HCl molecules cannot occur in the absence of water
Therefore aqueous solution of HCl is acidic whereas H⁺ ions cannot dissociate from HCl molecules in the absence of water. Hence dry HCl does not show acidic property


Bases produce hydroxide ions [OH⁻] in in water.
NaOH → Na⁺ + OH⁻
All bases do not dissolve in water. Bases which are soluble in water are called alkalis. The process of dissolving an acid or a base in water is a highly exothermic process. Therefore extreme care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.
Dilution
Mixing an acid or base with water results in decrease in concentration of H₃O⁺ and OH⁻ ions per unit volume. Such a process is called dilution
Strength of Acids and Bases
pH Scale
A scale for measuring hydrogen ion concentration in a solution, called pH scale
In the pH scale, p is the German word 'potenz', which is indicator of power and H is the indicator of hydrogen ion. On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline).
pH = 7 → neutral solution
pH less than 7 → on the pH acidic solution.
pH more than 7 → on the pH basic solution.




pH value of various substances
Gastric juice ⟹ 1.2
Lemon juice ⟹ 2.2
Pure water ⟹ 7
Blood ⟹ 7.4
Milk of Magnesia ⟹ 10
NaOH solution ⟹ 14
Strong acid :
Acids which produce large number of H⁺ ions or completely ionize in aqueous solution are called strong acids. Example : Hydrochloric acid (HCl) Sulfuric acid (H₂SO₄) Nitric acid (HNO₃)
Weak Acids :
Acids which produce less H⁺ ions or do not completely ionize in aqueous solution are called weak acids. Ex : Acetic acid, Organic acid
Strong base :
Those bases which ionize completely in aqueous solution are called strong bases.
Example Sodium hydroxide (NaOH), Potassium hydroxide (KOH)
Weak base :
Those bases which do not completely ionize in aqueous solution are called weak bases.
Example Magnesium hydroxide, Calcium hydroxide
Importance of pH in Everyday Life
1. pH sensitivity of plants and animals : Plants and animals are sensitive to pH. Our body works within the pH range of 7.0 to 7.8 When the pH value of rain water becomes less than 5.6, it is called acid rain. When acid rain water flows into a river, the pH value of the river water decreases. The survival of aquatic life in such rivers becomes difficult.
2. pH in our digestive system
Our stomach produces hydrochloric acid. It helps in digestion of food without harming the stomach. In case of indigestion, the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids. These antacids neutralize the excess of acid. A weak base such as magnesium hydroxide (Milk of Magnesia) is used for this purpose.
3. pH change as the cause of tooth decay :
Tooth decay starts when the pH value of the mouth is less than 5.5. Tooth enamel is made of calcium hydroxyapatite, which is the hardest substance in the body. It does not dissolve in water but it corrodes when the pH value of the mouth is less than 5.5. Bacteria present in the mouth produce acid by degradation of sugar and food particles remaining in the mouth after eating.
The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralize the excess acid and prevent tooth decay.
4. Self defence by animals and plants through chemical warfare
Bee-sting releases an acid which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief. The stinging hair of nettle leaves inject methanoic acid which causes burning pain.
Nature provides neutralisation options
Nettle is a herbaceous plant which is grows in the wild Its leaves have stinging hairs. which cause painful stings when touched accidentally. The secretion of methanoic acid from these hairs causes pain. Traditionally it is treated by rubbing the leaves of the dock plant on the stung area. These plants are mostly grows near Natal.
Acids found in various natural substances
Natural Source ⟹ Acid
Vinegar ⟹ Acetic acid
Sour milk (yogurt) ⟹ Lactic acid
Orange ⟹ Citric acid
Lemon ⟹ Citric acid
Tamarind ⟹ Tartaric acid
Ant sting ⟹ Methanoic acid
Tomato ⟹ Oxalic acid
Nettle sting ⟹ Methanoic acid
Salts : 
Salts are ionic products formed from the neutralization reaction of an acid and a base.
Salts of strong acid and a strong base are neutral, with a pH value of 7. For Example: NaCl, Na₂SO₄
Salts of strong acid and weak base are acidic, with a pH value less than 7. For Example: NH₄Cl
Salts of weak acid and strong base are basic, with a pH value more than 7. For Example : Sodium carbonate 
Rock salt.:
Deposits of solid salt are found in several parts of the world. These large crystals
are often brown due to impurities. This is called rock salt.
Chemicals from common salt
Sodium chloride is a common salt.  Common salt is a neutral salt and can be prepared in the laboratory by the reaction of sodium hydroxide and hydrochloric acid.
The common salt  an important raw material for many substances of our daily use such as sodium hydroxide, baking soda, washing soda, bleaching powder, etc.
Sodium hydroxide.
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
2NaCl + 2H₂O   →   2NaOH + Cl₂ + H₂
Chlorine gas is released at the anode and hydrogen gas is released at the cathode, Sodium hydroxide remains in the solution. Sodium hydroxide remains in the solution.

Bleaching powder
Chemical name =  Calcium Oxy Chloride.
Chemical formula = CaOCl₂,
Common name =  Bleaching powder 
Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂].
Ca(OH)₂ + Cl₂ →    CaOCl₂ + H₂O
Use of bleaching powder:
1. As a bleaching agent in the textile and paper industries
2. To to make drinking water free from germs.
3. As an oxidizing agent in chemical industries
Baking soda
Chemical name = Sodium  hydrogen carbonate 
Chemical formula = NaHCO₃
Common name =  Baking soda
It is a mild non-corrosive basic salt.
Sodium hydrogen carbonate is an antacid. 
Baking soda is made by reacting an aqueous solution of Sodium chloride with CO₂ and Ammonia.
NaCl + H₂O + CO₂  + NH₃ →  NaHCO₃     +  NH₄Cl 
Use of baking soda
1.For making baking powder [baking soda + tartaric]
When baking powder is heated or mixed in water, carbon dioxide is produced wich cause bread or cake to rise making them soft and spongy.
2NaHCO₃    →       Na₂CO₃+ H₂O + CO₂
2. Used as an antacid to treat acidity of the stomach
3. It is also used in soda-acid fire extinguishers.
Washing soda
Chemical name = Sodium  carbonate 
Chemical formula = Na₂CO₃.10H₂O
Common name =  Washing soda
Recrystallization of sodium carbonate gives washing soda.
Na₂CO₃ + 10H₂O   →      Na₂CO₃.10H₂O
Uses of washing soda
1. Sodium carbonate is used in glass, soap and paper industries.
2. It is used in the manufacture of sodium compounds such as borax.
3. Sodium carbonate is used for cleaning in homes.
4. It is used to remove hardness of water.
Plaster of Paris
Chemical name = calcium sulphate hemihydrate 
Chemical formula = CaSO₄ . 1/2H₂O
Common name =  Plaster of Paris (POP)
On heating gypsum at 373 K, it loses water molecules and becomes plaster of paris (POP)
CaSO₄.2H₂O    373  →   CaSO₄.½H₂O +1½H₂O 
Plaster of Paris is a white powder. on mixing with water, it becomes solid and hard immediately and changes to gypsum 
CaSO₄.½H₂O +1½H₂O   →   CuSO₄.2H₂O
Use of plaster of paris
1. Doctors use plaster of paris  for supporting fractured bones
2. Plaster of Paris is used for making toys, materials for decoration
3. Plaster of Paris is used for  make the surface smooth
Water of crystallisation
The fix number of water molecules present in the unit formula of a salt is called water of crystallization.
Example-
1. Copper sulphate [CuSO₄.5H₂O] has five molecules of water of crystallisation. Copper sulfate is colored due to the presence of water of crystallisation. When we heat the crystals, this water is removed and the salt turns white. 

2. Washing soda [Na₂CO₃.10H₂O]  has ten molecules of water of crystallisation.
3. Gypsum [CaSO₄.2H₂] has two molecules of water of crystallisation.

1. Which gas is evolved when dilute hydrochloric acid reacts with zinc metal? Write the molecular formula of this gas. 
   Hydrogen gas, H₂
2. Which among distilled water, tap water and sea water is the best conductor of electricity? 
   Sea water is a better conductor due to the presence of ions.
3. A compound which is prepared from gypsum has the property of hardening when mixed with a proper quantity of water. Identify the compound.
   Plaster of Paris
4. Name the sodium compound which is used for softening hard water.
   sodium carbonate
5. Name a salt which does not contain water of crystallisation. 
   NaHCO₃ is a salt that does not contain water of crystallisation. 
6. Name one indicator which specifies the various levels of H⁺ ion concentration.
   Universal indicator specifies the various levels of H⁺ ion concentration. 
7. Write chemical equation for the reaction of zinc metal on sodium hydroxide. 
   
   Zn(s) + 2NaOH(aq)   →  Heat → Na₂ZnO₂ (aq) + H₂ (g) 
8. Write a balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid indicating the physical state of reactants and the products.
   Na₂CO₃(s) + HCl (aq) ⟶ 2NaCl(aq) + H₂O(l) + CO₂(g)
9. Which gas is usually liberated when an acid reacts with a metal? 
   Hydrogen gas 
10. Name the acid and base that have constituted the salt ammonium nitrate. 
    Acid: HNO₃/Nitric acid , Base: NH₄OH/Ammonium hydroxide.
    
11. Write a balanced chemical equation for the neutralisation reaction, mentioning the physical state of reactants and products. 
    NaOH(aq) + HCl(aq)  → NaCl(aq) + H₂O(l) 
12. Define alkalies and give an example.
    Bases which are soluble in water are called alkalis. e.g., NaOH. 
13. Fresh milk has pH = 6. When it changes to curd will its pH value increase or decrease? 
    pH value will decrease when milk changes to curd.
    
14. The pH of a sample of vegetable soup was found to be 6.5. How is this soup likely to taste? 
    It will be sour in taste. 
    
15. How is the pH of solution of an acid influenced when it is diluted? 
    pH of the solution increases when it is diluted.
    
16. How does the pH change when solution of a base is diluted? 
    When solution of a base is diluted, its pH decreases.
17. Mention the range of pH for identification of a base
    7.1 to 14 is the pH range for bases. 
    
18. Which is a stronger acid, with pH = 5 or with pH=2? 
    The acid with pH = 2 is a stronger acid. 
    
19. What effect does an increase in concentration of H⁺(aq) ions in a solution have on pH of solution? 
    Increase in H⁺ concentration will lead to decrease in pH
    
20. Arrange the following in an increasing order of their pH values:      
    NaOH solution, Blood, Lemon juice.
    Lemon juice<Blood<NaOH solution.
21. You have two solutions A and B. The pH of solution ‘A’ is 6 and the pH of solution ‘B’ is 8. Which solution has more hydrogen ion concentration? Which one of this is acidic and which one is basic?
    Solutions ‘A’ has more H⁺ ion concentration. ‘A’ is acidic while ‘B’ is basic. 
22. What is the nature of salt if pH of its aqueous solution is greater than 7
    The salt is basic if pH>7. 
23. Name the hardest substance in the body. 
    Tooth enamel (Calcium phosphate).
    
24. At what pH rain water is said to be acidic?
    When the pH value of rain water becomes less than 5.6, it is called acid rain.
25. Write the names of two salts belonging to sodium family. 
    NaCl, Na₂CO₃ are two salts belonging to sodium family.
26. Write the chemical equation representing the action of atmospheric CO₂ gas on bleaching powder when left exposed in open. (Kept open in air.)
    CaOCl₂  + CO₂   →  CaCO₃ +  Cl₂  
27. What are olfactory indicators? Name two substances which can be used as olfactoiy 
    Those substances whose odour changes in acidic or basic media are called olfactory indicators. For example : onion, vanilla and clove oil.
28. A few drops of sulphuric acid are added to water before electrolysis, why? 
    Pure water is a bad conductor of electricity. By adding few drops of sulphuric acid  it becomes better conductor of electricity.
    
29. What would be the colour of litmus in a solution of sodium carbonate?
    Sodium carbonate is a basic salt. So when red litmus paper is dipped in sodium carbonate solution it will turn blue.
30. What is the action of litmus on (i) dry ammonia gas (ii) solution of ammonia gas in water? 
    (i) There is no effect of dry litmus on dry ammonia gas. 
    (ii) Solution of ammonia will turn red litmus blue. 
31. What is the difference between slaked lime and lime water? 
    Slaked lime is the solid form of Calcium Hydroxide/Ca(OH)₂(s) whereas lime water is the aqueous solution of Calcium hydroxide/Ca(OH)₂(aq)
32. Write a balanced chemical equation for the reaction taking place when sodium oxide reacts with water. How will this solution behave towards phenolphthalein and red litmus paper? 
    Na₂O + H₂O   →  2NaOH
    The Solution of NaOH will turn phenolphthalein pink and red litmus paper blue. 
33. While constructing a house, a builder selects marble flooring and marble table top for the kitchen where vinegar and lemon juice, tamarind etc., and more often used for cooking are to be kept. Will you agree to this selection and why?
    No, the substance like vinegar, tamarind etc. contain acids which. Marble will react with these acids and get corroded.  
34. Dry HCl gas does not change the colour of dry blue litmus. Give reason.
    Litmus paper change colour only in the presence of H⁺ ions. Dry HCl gas does not form H⁺ ion in the absence of water, so there is no effect on litmus.
35. Write the name given to the bases that are highly soluble in water. Give an example. 
    The bases which are highly soluble in water are known as alkalis. For example NaOH, KOH.
36. What is meant by p and H in pH? 
    In the pH scale, p is the German word 'potenz', which is indicator of power and H is the indicator of hydrogen ion.
    
37. Why is HCl a stronger acid than acetic acid? 
    HCl is completely ionised in aqueous solution whereas acetic acid is only partially ionised in aqueous solution.Therefore HCl is a stronger acid than acetic acid.
38. Why does 1 M HCl solution have a high concentration of H⁺ ions than 1 M CH₃COOH solution? 
    It is because 1M  HCl is a strong acid and it is completely ionised in aqueous solution whereas CH₃ COOH is a weak acid, so it is only partially ionised.
39. Equal lengths of magnesium ribbon are taken in test tube ‘A’ and ‘B’. Hydrochloric acid (HCl) is added to test tube A’ while acetic acid (CH₃COOH) is added to test tube ‘B’. In which test tube, will fizzing occur more vigorously and why? 
    The fizzing will occur more vigorously in test tube ‘A’ because HCl is a strong acid and reacts faster than acetic acid which is a weak acid.
40. How does flow of acid rain water into river makes the survival of aquatic life in the river difficult? 
    When acid rain water flows into a river, the pH value of the river water decreases. So the survival of aquatic life in such rivers becomes difficult.
    
41. At what pH in the mouth is tooth decay faster and why? 
    At pH less than 5.5, tooth decay becomes faster because calcium phosphate (enamel) reacts with acid and gets corroded. 
    
42. Explain how antacid works. 
    Antacids are weakly basic in nature. They neutralise excess of HCl present in our stomach and gives us relief from hyper-acidity. 
43. Name the acids present in  nettle sting and curd. 
    Nettle sting - HCOOH, Formic acid/Methanoic acid
    Curd - Lactic acid, CH₃-CH(OH)-COOH 
44. Curd is not kept in copper and brass utensils, why?
    Curd contains lactic acid which can make poisonous compounds with brass and copper vessels. 
45. Identify the acid and the base whose combination forms the common salt that you use in your food. Write its chemical formula and chemical name of the salt. 
    NaOH (Sodium hydroxide) and HCl (Hydrochloric acid) form common salt. 
    its chemical formula is NaCl and chemical name sodium chloride. 
46. What effect does the concentration of H+(aq) ions have on the nature of the solution?
    If the concentration of hydrogen ions in a solution increases, the solution becomes acidic; conversely, if the concentration of hydrogen ions in a solution falls, the solution becomes basic.
47. Why do the acids not show acidic behaviour in the absence of water
    The acidic behaviour of acid is due to the presence of hydrogen ions. The acids do not show its acidic behaviour in the absence of water,  because acids do not dissociate to produce  H+(aq) ions in the absence of water.
48. Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
    Yes, H+ ions can be found in all basic solutions. They are basic since hydrogen ions (H⁺) have a much lower concentration than hydroxide ions.
49. How is the concentration of hydroxide ions (OH⁻) affected when the excess base is dissolved in a solution of sodium hydroxide?
    When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions (OH⁻) increases
50. What is rock salt? Mention its colour and the reason due to which it has this colour. 
    Rock salt is sodium chloride found in the form of rocks. 
    It is brown in colour due to the presence of impurities.
51. How chloride of lime differs from calcium chloride? 
    Chloride of lime is calcium hypochlorite CaOCl₂ which is alkaline. Calcium chloride is CaCl₂ which is neutral.
52. Name one antacid. How does it help to relieve indigestion in stomach?
    NaHCO₃ (baking soda) is an antacid. It neutralises excess of HCl in stomach and gives relief.
53. A farmer treats the soil with quicklime or calcium carbonate. What is the nature of the soil? Why does the farmer treat the soil with quicklime? 
    The nature of soil is acidic. The farmer treats the soil with quicklime (basic in nature) to neutralise the acidity of soil and make it fit for crops. 
54. A substance ‘X’ is used as antacid reacts with hydrochloric acid to produce a gas Y which is used in fire extinguishers: Name the substance X and ‘Y’. 
    Substance ‘X’ is NaHCO₃ (Sodium hydrogen carbonate).
    Substance ‘Y’ is CO₂ gas, which is used in fire extinguishers.
55. Define indicator. Name two indicators obtained from plants.
    Those substances which indicate the presence of acid and base in a solution by change in their color and smell are called Indicator
    Litmus and turmeric are indicators obtained from plants.
56. Define an acid and a base. Name one weak acid and one strong acid. 
    Acid is a substance which gives H⁺ ions in an aqueous solution. Base is substance which gives OH- ions in the aqueous solution. CH₃COOH is a weak acid, H₂SO₄ is a strong acid. 
57. List two differences between acids and bases on the basis of chemical properties.  
    1.Acids turn blue litmus red. Bases turn red litmus blue. 
    2.Acids liberate CO₂ with metal carbonates and hydrogen carbonates. Bases do not react with metal carbonates and hydrogen carbonates. 
58. What happens when chlorine is passed over slaked lime at 313 K? Write chemical equation of the reaction involved .
    when chlorine is passed over slaked lime at 313 K bleaching powder (CaOCl₂) is formed
    Ca(OH)₂ + Cl₂ →    CaOCl₂ + H₂O
59. Name the chemicals used in acid fire extinguisher and the gas evolved from it when it is used?
    NaHCO₃ (Sodium hydrogen carbonate) and H₂SO₄ (Sulphuric acid) are used in acid fire extinguisher.
    When they are used the gas evolved is Carbon dioxide
60. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal? 
    Hydrogen gas is liberated when hydrochloric acid reacts with an active metal. We can test  the presence of hydrogen gas by bringing a burning matchstick near the test tube. If it burns with a pop sound, the gas liberated is hydrogen.
61. What is a neutralisation reaction? Give one example. 
    The reaction in which acid reacts with a base to form salt and water is called neutralisation reaction 
    e.g., KOH(aq) + HNO₃(aq)  →   KNO₃(aq) + H₂O(l)
62. What is meant by term pH of solution? The pH of rain water collected from two cities A’ and ‘B’ were found to be 6.0 and 5.0 respectively. The water of which city will be more acidic? 
    The measurement of hydrogen ion concentration present in the solution is known as pH. It determines the strength of acid and base. 
    Rainwater with pH = 5 is more acidic. 
    
63. Explain why is hydrochloric acid a strong acid and acetic acid, a weak acid. How can it be verified?
    Hydrochloric acid is a strong acid because it is completely ionised in its aqueous solution. Acetic acid is  a weak acid because it is  partially ionised. 
    HCl reacts with Mg vigorously whereas acetic acid reacts with Mg less vigorously.
    
64. Explain why aqueous solution of an acid conducts electricity. 
    An aqueous solution of an acid conducts electricity because the acid molecules dissociate into ions when dissolved in water. These ions are responsible for carrying electric current in the solution. 
65. Mention the pH of aqueous solution of the following salts as 7, more than 7, less than 7. KCl, Na₂CO₃ , NH₄C1, NaNO₃ (Sodium nitrate) 
    (i) pH less than 7 - NH₄Cl
    (ii) pH equal to 7 -KCl and NaNO₃
    (iii) pH more than 7- Na₂CO₃ 
66. Classify the following salts into acidic, basic and neutral salts: Potassium sulphate, ammonium chloride, sodium carbonate, sodium chloride.
    Acidic: Ammonium chloride
    Basic: Sodium carbonate
    Neutral: Potassium sulphate, sodium chloride. 
67. Why does bee sting causes pain and irritation? Rubbing of baking soda on the sting area gives relief. How? 
    Bee sting contains HCOOH (formic acid) which causes irritation. 
    Rubbing baking soda of the sting are gives relief as baking soda is a base and ant sting is an acid. When they react the baking soda neutralizes this acid.
    
68. The pH of the mouth of a person is lower I than 5.5. What changes will occur in his mouth? How these changes can be controlled? Write any two measures. 
    When the pH in the mouth of a person is lower I than 5.5, tooth decay starts.
    (i) Wash your mouth with water after every meal.
    (ii) Clean your teeth with toothpaste after meals.
    
69. What is baking powder? How does it make the cake soft and spongy? 
    Baking powder is made up of NaHCO₃ and tartaric acid. 
    When baking powder is heated or mixed in water, carbon dioxide is produced wich cause bread or cake to rise making them soft and spongy.
    
70. State the chemical properties on which the following uses of baking soda are based: (i) as an antacid, (ii) as a soda acid fire extinguisher, (iii) to make bread and cake soft and spongy. 
    (i) It is weakly basic in nature and naturalize hyperacidity.
    (ii) It liberates CO₂ with acid which extingui shes fire. 
    (iii) It releases CO₂ gas on heating which makes bread and cake soft and spongy.
71. What is meant by water of crystallisation in a substance? 
    The fix number of water molecules present in the unit formula of a salt is called water of crystallization.
    Ex. Copper sulphate [CuSO₄. 5H₂O] has five molecules of water of crystallisation.
72.  A metal compound ‘X’ reacts with dilute H₂SO₄ to produce effervescence. The gas evolved extinguishes a burning candle. If one of the compound formed is calciumsulphate, then what is ‘X’ and the gas evolved? Also write a balanced chemical equation for the reaction which has occurred. 
    The 'X' is is calcium carbonate
    The gas evolved is carbon dioxide.
    CaCO₃ + H₂SO₄   ⟶   CaSO₄ + H₂O + CO₂
73. Three acidic solutions A, B and C have pH = 0, 3 and 5 respectively. 
    (i) Which solution has the highest concentration of H⁺ ions? 
    The solution with pH = 0 has highest concentration of H⁺ ions. 
    (ii) Which solution has the lowest concentration of H⁺ ions? 
    The solution with pH = 5 has lowest concentration of H⁺ ions.
74. Write the common/chemical name and chemical formula of the product formed by action of chlorine on slaked lime. 
    Chemical name =  Calcium Oxy Chloride.
    Common name =  Bleaching powder 
    Chemical formula = CaOCl₂.
75. Give two uses of baking soda. 
    Use of baking soda: 
    (i) It is used in making of bread, biscuits, cakes. 
    (ii) It is used as an antacid  to treat acidity of the stomach.
76. A student dropped few pieces of marble in dilute HCl contained in a test tube. The evolved gas was passed through lime water. 
    (i) What change would be observed in lime water? 
    Lime water will turn milky. 
    (ii) Write a balanced chemical equation for the above change. 
    Ca(OH)₂  + CO₂  → CaCO₃  + H₂O
77. Two solutions ‘A’ and ‘B’ have pH value 3.0 and 10.5 respectively. Which of these will turn (i) Blue litmus solution to red, (ii) Phenolphthalein from colourless to pink? Justify your answer in each case.  
     (i) Solution A will turn blue litmus  red as it has pH 3 which is acidic in nature. 
    (ii) Solution B will turn phenolphthalein from colorless to pink as it is basic in nature having pH 10.5.
78. (i) Write the chemical name and chemical formula of washing soda.  
    Na₂CO₃ .10H₂O, sodium carbonate deca-hydrate is washing soda.
    (ii) Give two uses of  washing soda each. 
     It is used as a cleaning agent. 
     It is used to remove hardness of water. 
79. Name a gas evolved when dilute HCl reacts with sodium hydrogen carbonate. How is it recognised? 
    Carbon dioxide, it turns lime water milky. In this way, CO₂ gas is recognised. 
    Carbon dioxide gas is liberated when dilute HCl reacts with sodium hydrogen carbonate. When the gas liberated in the reaction is passed through lime water, lime water turns milky. This confirms that the gas is carbon dioxide.
80. What is universal indicator? State the purpose for which this indicator is used. 
    A universal indicator is a mixture of several indicators, which display different coluors at different concentrations of hydrogen ions in a solution.
    Example - pH scale
    It is used to detect the acidic or basic nature of a substance or a solution. 
81. On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue green. Predict the new compound formed which imparts a blue green colour to the solution. 
    Copper oxide reacts with hydrochloric acid to give copper chloride and water.;
    CuO + 2HCl → CuCl₂ + H₂O
    In this reaction the colour of the solution becomes blue-green due to the formation of copper(II) chloride
82. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? 
    The process of dissolving an acid or a base in water is a highly exothermic process.  If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive  heating.
83. Why should curd and sour substances not be kept in brass and copper vessels?

    Curd and sour substances are both acids. When sour things, such as curd, are stored in brass or copper vessels, the lactic acid in the curd reacts with the metals produce poisonous substances. Hence, curd and sour substances are not kept in brass and copper vessels.
84. Explain why sodium hydroxide solution cannot be kept in aluminiumcontainers? Write the equation for the reaction that may take place for the same. 
    Sodium hydroxide (NaOH) solution cannot be kept in aluminum containers because NaOH reacts with ‘Al’ to form sodium meta-aluminate and highly flammable hydrogen gas and it corrodes the Al metal.
    2Al + 2NaOH + 2H₂O  → NaAlO₂ +3H₂
85. HCl and HNO₃ show acidic characteristics in aqueous solution while alcohol and glucose solutions do not. Give reasons. 
    HCl and HNO₃ form H⁺ or H₃O⁺ ions in aqueous solution whereas alcohol and glucose do not dissociate into ions. 
    HCl + H₂O →  H₃O⁺ + Cl 
    HNO₃ + H₂O →H₃O⁺ + NO₃
86. What happens when electricity is passed through brine? Write chemical equation for it. 
    When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide and the evolution of chlorine gas at anode and hydrogen gas at cathode. 
    2NaCl + 2H₂O   →   2NaOH + Cl₂ + H₂
87. Plaster of Paris should be stored in a moisture-proof container. Explain why?

    it absorbs moisture (water) from the atmosphere easily and turns into a hard substance (gypsum).

    CaSO₄.½H₂O +1½H₂O   →   CuSO₄.2H₂O

    Therefore the plaster of paris should be stored in a moisture-proof container.
88. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

    The pH of fresh milk is 6, which is acidic in nature.  When milk changes into curd, its pH decreases from 6 to less than 6.  This is because during curd formation, lactic acid is produced which makes it acidic
89. Write name and chemical formulae of two such compounds whose one unit is associated with 10 and 2 water molecules respectively. 
    Sodium carbonate decahydrate [Na₂CO₃.10H₂O] has 10 molecules of water of crystallisation. 
    calcium sulphate dihydrate [CaSO₄.2H₂O] has 2 molecules of water of crystallisation. 
90. State what does pH of solution signify? Three solutions A, B and C have pH values of 6, 2 and 10 respectively. Which one of these solutions is highly acidic? Which solution will turn red litmus blue? 
    pH of solution signifies the nature (acidic/basic/neutral)of the solution i.e., it is weakly acidic, strongly  acidic, neutral, weakly basic, strongly basic. 
    ‘B’ with pH = 2 is strongly acidic. 
    ‘C’ with pH = 10 will turn red litmus blue. 
91. Explain the following chemical properties of acids with the help of balanced chemical equations only: 
    (i) when an acid reacts with a metal carbonate.
    Na₂CO₃ + HCl ⟶ 2NaCl + H₂O + CO₂ 
    (ii) when an acid reacts with a metal bicarbonate, 
    NaHCO₃ + HCl ⟶ NaCl + H₂O + CO₂
    (iii) when an acid reacts with a metal oxide. 
    CuO + 2HCl → CuCl₂ + H₂O
92. Explain the action of dilute hydrochloric acid on the following with suitable chemical equations: 
    (i) Magnesium ribbon
    Mg + 2HCl →  MgCl₂  + H₂
    

    (ii) Sodium hydroxide
     NaOH + HCl  →   NaCl + H₂O 
    (iii) Crushed egg shells.
    CaCO₃ + HCl ⟶ CaCl₂ + H₂O + CO₂
93. Oxides of metals are basic while those of non-metals are acidic. Explain. 
    Similar to the reaction of acids and bases, acids react with metallic oxides to form salts and water, hence metallic oxides are basic oxides. 
    Acid + Metal Oxide ⟶ Salt + Water
    Similar to the reaction of acids and bases, Bases react with non- metallic oxides to form salts and water, hence non- metallic oxides areacidic oxides.
    Base +     Nonemetal Oxide     ⟶     Salt     +     Water
94. A student detected the pH of four unknown solutions A, B, C and D as follows: 11, 5, 7 and 2. Predict the nature of these solutions.
    Solution with pH = 11 is basic 
    Solution with pH = 5 is acidic 
    Solution with pH = 7 is neutral 
    Solution with pH = 2 is strongly acidic
95. Name the natural source of each of the following acid: A. Citric acid, B. Oxalic acid, C. Lactic acid, D. Tartaric acid. 
    A. Citric acid   = Lemon, Orange. 
    B. Oxalic acid  = Tomato, Guava 
    C. Lactic acid   = Curd, Sour milk 
    D. Tartaric acid = Tamarind
96. Write the chemical equation to describe how baking soda is produced on a large scale. Also write the chemical name of  the products formed in the reaction.
    Baking soda is made by reacting an aqueous solution of Sodium chloride with CO₂ and Ammonia.
    NaCl + H₂O + CO₂  + NH₃ →  NaHCO₃     +  NH₄Cl 
    The chemical name of  the products : 
    Sodiumhydrogen carbonate and   Ammonium chloride.
97. (i) Why is Plaster of paris written as CaSO₄.½H₂O ?
    It has one molecule of water associated with 2 molecules of CaSO₄. It means that there is half a molecule of water per formula unit of CaSO₄. 
    (ii) How is it possible to have a half water molecule attached with CaSO₄ ?
    it is not possible to have half molecule of water attached to calcium sulphate (CaSO₄). The formula CaSO₄.½H₂O means that two formula units of CaSO₄ shares one molecule of water.
98. “Sodium hydrogen carbonate is a basic salt.” Justify the statement. How is it converted into washing soda? Explain.
    NaHCO₃ is a salt of NaOH which is a strong base and H₂CO₃ (Carbonic acid) which is a weak acid, therefore it is a basic salt.
    When sodium hydrogen carbonate (Baking soda) is heated sodium carbonate is formed which on crystallisation forms washing soda: 
    NaHCO₃    →       Na₂CO₃+ H₂O + CO₂
    Na₂CO₃ + 10H₂O   →      Na₂CO₃.10H₂O
99. A white powder is added while baking breads and while making cakes to make them soft and fluffy. 
    (i) What is the name of that powder? 
    The powder is baking powder. 
    (ii) What are the main ingredients in it? 
    It consist of sodium hydrogen carbonate and tartaric acid.
    (iii) What are the functions of each ingredient? 
    NaHCO₃ gives CO₂ gas on heating which makes the bread cake soft and fluffy. 
    Tartaric acid neutralises Na₂CO₃ which is bitter in taste. 
100. What is chlor-alkali process? Write a balanced chemical equation for the reaction involved in this process, to justify your answer. 
     When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
     2NaCl + 2H₂O   →   2NaOH + Cl₂ + H₂ 
101. 15 mL of water and 10 mL of sulphuric acid are to be mixed in a beaker :
     (i) State the method that should be followed with reason. 
     The process of dissolving an acid  in water is a highly exothermic process. Therefore  sulphuric acid must always be added slowly to water with constant stirring.
     (ii)What is this process called?
     This process is called dilution. 
102. A compound ‘X’ of sodium is commonly used for making crispy pakoras. It is also used for curing acidity in the stomach. Identify ‘X’. Write the formula and its chemical name. State the reaction which takes place when it is heated. 
      ‘X’ is sodium hydrogen carbonate. 
     Chemical name = Sodium  hydrogen carbonate 
     Chemical formula = NaHCO₃
     It is used in cooking and for curing acidity in stomach. 
     2NaHCO₃    →       Na₂CO₃+ H₂O + CO₂
103. Write the chemical name and  chemical formula of Plaster of paris. Write a chemical equation to show the reaction between Plaster of paris and water.
     Chemical name = calcium sulphate hemihydrate 
     Chemical formula = CaSO₄.½H₂O
     Plaster of Paris is a white powder. on mixing with water, it becomes solid and hard immediately and changes to gypsum 
     CaSO₄.½H₂O +1½H₂O   →   CuSO₄.2H₂O
104. What is the colour of litmus in a solution of ammonium hydroxide? 
     Ammonium hydroxide is a base. So when red litmus paper is dipped in a solution of ammonium hydroxide it will turn blue.
     The pH of soil ‘A’ is 7.5, while that of soil “B is 4.5. Which of the two soils A or B should be treated with powdered chalk to adjust the pH and why? 
     Soil ‘B’ is acidic, therefore it needs to be treated with powdered chalk to adjust its pH because chalk(CaCO₃) is basic salt, which will make soil neutral. 
105. You have been provided with three test tubes. One of them contains distilled water and the other two contains an acidic solution and a basic solution respectively. If you are given only red litmus, how will you identify the contents  of each test tube? 
     Add red litmus to each test tubes. The test tube in which it turns blue contains the base. 
     Use this blue litmus paper to test the solutions in other two test tubes.
     Add this blue litmus to the remaining two test tubes. The one in which it turns red contains the acid. 
     The third test tube in which blue litmus and red litmus do not change contains distilled water.
106. Five solutions A, B, C, D, and E showed pH as 4, 7, 1, 11 and 9 respectively when tested with universal indicator. Which solution is (i) Neutral, (ii) Strongly alkaline, (iii) Strongly acidic, (iv) Weakly acidic, (v) Weakly alkaline. Arrange the pH in increasing order of H⁺ ion concentration. 
     (i) Solution ‘B’ is neutral, 
     (ii) Solution D is strongly alkaline, 
     (iii) Solution ‘C’ is strongly acidic, 
     (iv) Solution A is weakly acidic, 
     (v) Solution ‘E’ is  weakly basic. 
     D<E<B<A<C is the increasing order of H⁺ ion concentration.
107. Give suitable reasons to justify the following statement:
     An aqueous solution of sodium chloride is neutral but an aqueous solution of sodium metal is basic.
     Sodium chloride is made up of a strong base, NaOH and a strong acid, HCl. Therefore, its aqueous solution is neutral in nature. 
     Sodium metal reacts with water to form NaOH(Base) and H₂ gas:
     Therefore, its aqueous solution is basic in nature.
108. What are strong acids and weak acids? In the following list of acids, separate strong acids from weak acids: hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid, Carbonic Acid
     Strong acid : Acids which are completely ionised in aqueous solution are called strong acids.
     Weak Acids : Acids which do not ionize completely in aqueous solution are called weak acids.
     Strong acid : Hydrochloric acid, Nitric acid, Sulphuric acid. 
     Weak acid: Citric acid, Acetic acid, Formic acid, Carbonic Acid
109. You have four solutions A, B, C and D. The pH of solution A is 6, B is 9, C is 12 and D is 7. 
     (i) Identify the most acidic and most basic solutions respectively.
     With pH = 6 solution ‘A’ is most acidic and With pH = 12, solution ‘C’ is most basic
     (ii) Arrange the above four solutions in the increasing order of H⁺ ion concentration. 
      C < B < D < A is the increasing order of H⁺ ion concentration.
     (iii) State the change in colour of pH paper on dipping in solution C and D. 
      pH paper will turn blue in solution ‘C’  and  green in solution  'D'
110. (a) A salt is produced by reaction between an acid and a base. Identify the acid and base from which the following salts have been formed:  (i) Na₂ SO₄ , (ii) NH₄ Cl, (Hi) KNO₃ , (iv) NaCl 
     (i)  Na₂SO₄ is formed from NaOH and H₂SO₄
     (ii) NH₄Cl is formed by NH₄OH and HCl. 
     (iii) KNO₃ is formed by KOH and HNO₃ .
     (iv) NaOH is formed by NaOH and HCl. 
     (b) Which one of these will have pH less than 7 and why? 
      NH₄Cl has pH less than 7 because it is a salt of weak base NH₄OH and strong acid, HCl, therefore the salt is acidic.
111. (i) How does the concentration of H₃O⁺ ions change when a solution of an acid is diluted? 
     When an acid solution is diluted, the concentration of H₃O⁺ ions decreases.
     (ii) Which one has a higher pH, a concentrated or a dilute solution of hydrochloric acid? 
     Dilute solution of hydrochloric acid has higher pH than concentrated.
     
112. (i) State the colour of phenolphthalein in soap solution.
     Soap solution is basic, so phenophthalein turns pink in soap solution.
     (ii) Name the by-product of chlor-alkali process which is used for the manufacture of bleaching powder. 
     Chlorine is the by-product of chlor-alkali process which is used in the manufacture of bleaching powder. 
     
113. (i) Solution ‘A’ gives pink colour when a drop of phenolphthalein indicator is added to it. Solution ‘B’ gives a red colour when a drop of methyl orange is added to it. What type of solutions are ‘A’ and ‘B’ and which of these will have higher pH? 
     Solution  ‘A’ is basic in nature and solution ‘B’ is acidic in nature.
     Solution ‘A’ will have higher pH than ‘B’. 
     (ii) Name one salt whose solution has pH greater than 7 and one salt with pH less than 7. 
     Na₂ CO₃ is the salt whose pH is more than and CuSO₄ is the salt whose pH is less than 7.
114. What is bleaching powder chemically? Give a reaction for its preparation. State two of its use. 
     Bleaching powder is chemically calcium oxychloride [CaOCl₂]
     Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂].
     Ca(OH)₂ + Cl₂ →    CaOCl₂ + H₂O
     Uses 1. As a bleaching agent in the textile and paper industries
               2. To to make drinking water free from germs.
115. Crystals of a substance changes their colour on heating in a closed vessel but regained it after sometime, when they were allowed to cool down. (i) Name one such substance. (ii) Explain the phenomenon involved.
     (i) CuSO₄.5H₂O (Hydrated copper sulphate) 
     (ii) The phenomenon involved is water of crystallization .
     The fix number of water molecules present in the unit formula of a salt is called water of crystallization.
     When we heat the crystals of hydrated copper sulphate this water is removed and the salt turns white. But when it is allowed to cool down in open air then it regains its lost water of crystallization and hence regains its colour also.
     CuSO₄ .5H₂O  →   CuSO₄ + 5H₂O                
             Blue              (Dirty white) 
116. What is colour of FeSO₄ .7H₂O crystals? How does this colour change upon heating? Give a balanced chemical equation for the change.
     The colour of FeSO₄ .7H₂O is pale/Light green.On heating crystals of FeSO₄.7H₂O loses of water of crystallisation. So it colour change from light green to white due to the formation of anhydrous FeSO₄. When  FeSO₄ is heated it gives ferric oxide, supher dioxide and supher trioxide
     FeSO₄ .7H₂O   → Heat→           FeSO₄ + 7H₂O 
     If it is heated strongly 
     2FeSO₄ →       Heat→        Fe₂O₃ + SO₂ + SO₃ 
117. State the reason for the following statements: 
     (i) Tap water conducts electricity whereas distilled water does not. 
     Tap water contains ions of dissolved salts and minerals which makes it a good conductor whereas distilled water does not contain any ions. 
     (ii) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does. 
     Dry HCl gas does not dissociate into ions, so it has no effect on the litmus. Hydrochloric acid form ions, so it turns blue litmus red. 
     (iii) During summer season, a milkman usually adds a very small amount of baking soda to fresh milk.
     Fresh milk gets soured in summer forming lactic acid. Baking soda being basic in nature neutralises lactic acid and prevents souring of milk.
     (iv) Ammonia is a base but it does not contain hydroxyl group.  
     NH₃ dissolves in H₂O forming NH₄OH, therefore it acts as base
     NH₃ + H₂O  ⟶ NH₄OH 
118. Describe an activity with diagram to illustrate that the reaction of metal carbonates or metal bicarbonates with acid produces carbon dioxide. Write the relevant equations of all the reactions that take place. Name any two forms in which calcium carbonate is found in nature. 
     Activity: To show reaction of metal carbonates and metal hydrogen carbonates with dilute acids. 
     Take two test tubes, label them as A and B.
     Take about 0.5 g of sodium carbonate  in test tube A and about 0.5 g of sodium hydrogencarbonate in test tube B.
     Add about 2 mL of dilute HCl to both the test tubes.
     Pass the gas produced in each case through lime water as shown in diagram.
     Observation: Lime water turns milky.  
     Conclusion: Metal carbonates and Metal hydrogen carbonates react with dilute acids to liberate carbon dioxide. 
     Test tube A: NaHCO₃ + HCl  →  NaCl + H₂O + CO₂
     Test tube B: Na₂CO₃ + 2HCl  →  2NaCl + H₂O + CO₂
     
     
119. You are provided with magnesium ribbon and sulphur powder. Explain with the help of activity that metal oxides are basic and oxides of non-metals are acidic in nature.  
     Burn magnesium ribbon with the help of tongs to form white ash . 
     Dissolve the ash [MgO] in water 
     Dip red litmus paper in the solution 
     which turns blue, showing that MgO is a basic oxide. 
     Solution of magnesium oxide  turns red litmus blue showing MgO is an basic oxide.
     Burn sulphur in the presence of air 
     Dissolve the oxide[ formed in water.
     Dip blue litmus paper into the solution 
     Solution of sulphur oxide  turns blue litmus red showing SO₂ is an acidic oxide.
     aaaa
120. State the observations you would make on adding sodium hydroxide to an aqueous solution of (i) ferrous sulphate, (ii) aluminium chloride. 
     (i) When ferrous sulphate reacts with sodium hydroxide solution, a green precipitate of ferrous hydroxide is formed.
     FeSO₄ +2NaOH  →   Na₂SO₄   +Fe(OH)₂
                                                     green ppt
     (ii) When aluminium chloride reacts with sodium hydroxide solution, a white precipitate of aluminium hydroxide is formed.
     AlCl₃  + 3NH₄OH → Al(OH)₃ +3NH₄Cl 
                                           White ppt
121. (i) Define pH scale. Draw a figure showing variation of pH with change in concentration of H⁺(aq) and OH⁻(aq) ions.
     A scale for measuring hydrogen ion concentration in a solution, called pH scale On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline).
     
     (ii) Mention the pH of acidic, basic and neutral solutions respectively. 
     pH less than 7 → on the pH acidic solution.
     pH more than 7 → on the pH basic solution.
     pH = 7 → neutral solution
122. Write chemical equations when zinc granules react with (i) Sulphuric acid, (ii) Hydrochloric acid, (iii) Aluminium chloride, (iv) Sodium hydroxide,(v) Nitric acid 
     (i) Zinc granules reacts with H₂SO₄ to produce zinc sulphate and  H₂ gas.
     H₂SO₄ + Zn → ZnSO₄ + H₂↑
     (ii) Zinc granules reacts with HCl to produce zinc chloride and H₂ gas.
      Zn   +  2HCl    →  ZnCl₂ + H₂ 
     (iii)  No reaction occurs when zinc reacts with aluminium chloride.
     (iv) Zinc granules reacts with  NaOH to form sodium zincate and H₂ gas.
     Zn  +   2NaOH  → Na₂ZnO₂ + H₂↑
     (v) Zinc granules reacts with nitric acid to form zinc nitrate and H₂ gas. 
     Zn  +   2HNO₃  → Zn(NO₃)₂ + H₂↑
123. Equal length of magnesium ribbon are taken in two test tubes A and B. H₂SO₄ is added to test tube ‘A’ and H₂CO₃ is added in test tube ‘B’ in equal amounts: 
     (i) Identify the test tube showing vigorous reaction. 
     Test tube ‘A’ will show vigorous reaction.
     (ii) Give reason to support your answer.
      H₂SO₄ is a strong acid, it reacts faster than H₂CO₃ , a weak acid.
     (iii) Name the gas liberated in both the test tubes. How will you prove its liberation? 
     H₂ gas  will liberate in both the test tubes
     If we bring a burning matchstick near the gas, it will burn with ‘pop’ sound.
     (iv) Write chemical equations for both the reactions. 
     Mg + H₂SO₄     ⟶  MgSO₄ + H₂ 
     Mg + H₂CO₃     ⟶  MgCO₃ + H₂ 
     (v) Out of two acids taken above, which one will have lower pH value and lower. H⁺ ion concentration respectively? 
     H₂SO₄ will have lower pH.
     H₂CO₃ will have lower H⁺ ion concentration.
124. The metal salt ‘A’ is blue in colour. When salt ‘A’ is heated strongly over a burner, then a substance ‘B’ present in it is eliminated and a white powder ‘C’ is left behind. When a few drops of a liquid ‘D’ is added to powder ‘C’, it becomes blue again. 
     (i)  Identify A, B, C and D. 
     ‘A’ is CuSO₄.5H₂O
     ‘B’ is H₂O
     ‘C’ is CuSO₄
     ‘D’ is H₂O
     (ii) Write the chemical equations involved. 
     FeSO₄. 5H₂O   → Heat→           FeSO₄ + 7H₂O 
     FeSO₄ + 7H₂O  →  FeSO₄ .5H₂O
     (iii) Give an example of the salt which also shows the above property.
     FeSO₄.7H₂O is a salt which also shows this property. 
125. How would you show that copper sulphate crystals contains water of crystallisation.? Describe an activity.
     When copper sulphate is heated its colour changes from blue to white and water droplets are formed. 
     CuSO₄.5H₂O  →  CuSO₄ + 5H₂O 
     This proves that copper sulphate crystals contain water of crystallisation.
     Activity: To study the effect of heat on hydrated crystalline salts. 
     Take 2 g of CuSO₄.5H₂O  in a test tube. which initial colour  is green
     Heat the test tube carefully as shown in the diagram. 
     Blue colour of CuSO₄.5H₂O is changed to dirty white anhydrous CuSO₄ and water droplets were formed.
     Cool the crystals and add few drops of water. 
     On adding water, blue colour of salt was restored. 
     Conclusion: CuSO₄.5H₂O  is a hydrated salt.When we heat the crystals of hydrated copper sulphate this water is removed and the salt turns white. But when it is allowed to cool down in open air then it regains its lost water of crystallization and hence regains its colour also.