1. Chemical Reactions and Equations

Chemical reaction - The process in which a chemical change take place is known as chemical reaction.

Example : Photosynthesis and Burning of fuel.

Some observation that helps us to determine whether a chemical reaction takes place :-

• Change in state

• Change in colour

• Evolution of a gas

• Change in temperature

• Formation of ppt

Chemical Equation : Representation of chemical reaction using symbols and formulas of the substances is called Chemical Equation.

Example : When a magnesium ribbon is burntin persence of oxygen, it gets converted to magnesium oxide.the above word-equation can be written as –

2Mg + O₂ → 2MgO

Reactants : The substances which participate in a chemical reaction, are called reactants. The reactants are written on left hand side in chemical reactions.

Products : The substances which are formed in a chemical reaction are called products. The Products are written on right hand side in chemical reactions.

Skeletal chemical equation : A chemical equation in which the total number of atoms of each element on the reactant side is not equal to the total number of atoms of the same element on the product side is called a skeletal equation.

Law of Conservation of mass : Mass can neither be created nor destroyed in a chemical reaction.

Balanced Chemical Equations : A chemical equation in which the total number of atoms of each element remains same before and after chemical reaction in known as balanced chemical equation.

2Mg + O₂ → 2MgO

Hit-and-Trial method for balancing chemical reactions :
Fe + H₂O ➡ Fe₃O₄ + H₂
Step I : Draw boxes around each formula. Do not change anything inside the boxes
Hit-and-Trial method for balancing chemical reactions :
Fe + H₂O ➡ Fe₃O₄ + H₂
Step I : Draw boxes around each formula. Do not change anything inside the boxes

H₂O

→

Fe₃O₄

H₂

Step II : To Find out the number of atoms of each element in reactants and products.

Element	Number of atoms in reactants	Number of atoms in products
Fe	1	3
H	2	2
O	1	4

Step III : To Find the atom with the maximum number in the reactants or products and balance it. Here Fe₃O₄ has maximum number of atoms. So we select oxygen atoms in it.

Atoms of oxygen	in reactants	in products
Initial	1	4
To balance	1 x 4 = 4	4

Now the partly balanced equation becomes –

H₂O

➡

Fe₃O₄

H₂

Step IV : Now we balance the remaining atom of the partially balanced equation. Now we balance hydrogen and Ferus atoms in the partially balanced equations.

Atoms of hydrogen	in reactants	products
Initial	8	2
To balance	8	2 x 4 =8

Now the partly balanced equation becomes –

H₂O

➡

Fe₃O₄

H₂

Atoms of Ferus	in reactants	products
Initial	1	3
To balance	1 x = 3	3

Now the partly balanced equation becomes –

H₂O

➡

Fe₃O₄

H₂

Step V : Finally, we check the correctness of the balanced equation

3Fe + 4H₂O ➡ Fe₃O₄ + 4H₂

Step VI : To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulas.

For gaseous State → (g)

For liquid state → (l)

For solid state → (s)

For aqueous state/ solution in water → (aq)

Finally the balanced equation becomes :

3Fe(s) + 4H₂O(g) ➡ Fe₃O₄(s) + 4H₂(g)

Types of Chemical Reactions

1. Combination Reaction :– Such reactions in which two or more reactants combine to form a product are called combination reactions.

A + B → AB

Example -

[i] Reaction of calcium oxide with water :

Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide)

CaO + H₂O → Ca(OH)₂ + Heat

(Quick lime) (Slaked lime)

A solution of slaked lime [Calcium hydroxide] is used for whitewashing walls. When walls are whitewashed, Calcium hydroxide reacts slowly with the carbon dioxide in air and form a thin layer of calcium carbonate on the walls and gives a shiny finish to the walls

Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O(l)

(Calciumhydroxide) (Calcium carbonate)

[ii] Formation of water from H₂ and O₂

2H₂(g) + O₂(g) → 2H₂O(l)

(iii) Burning of coal

C(s) + O₂(g)→ CO₂(g)

Exothermic chemical reactions: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Examples of exothermic reactions are –

(i) Burning of natural gas

CH₄(g) + 2O₂ (g) → CO₂(g) + 2H₂O(g) + Heat

(ii) Rrespiration is an exothermic process :

We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose.

This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration,

C₆H₁₂O₆ (aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy

(iii) The decomposition of vegetable matter into compost

2. Decomposition Reaction

A reaction in which a single reactant breaks down to form two or more products is called decomposition or decomposition reaction.

AB ➡ A + B

Ferrous sulphate crystals contain 7 water molecules When it is heated, it loses its 7 water molecules and creates an anhydrous ferrous sulphate molecule and the green colour of the ferrous sulphate crystals is changed.(White) When anhydrous sulphate is heated to a high temperature, it decompose to ferric oxide(Brown), sulphur dioxide, and sulphur trioxde

FeSO₄.7H₂O → Heat → FeSO₄

Green White

FeSO₄ → Heat → Fe₃O₄ + SO₂ +SO₃

White Reddish Brown

FeSO₄.7H₂O →Heat → Fe₃O₄ + SO₂ +SO₃

There are three types of decomposition

(1) Thermal decomposition – When a decomposition reaction is carried out by heating, it is called thermal decomposition

Decomposition of calcium carbonate [formation of lime]

Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction used in various industries. When calcium carbonate is heated it breaks down into calcium oxide and carbon dioxide.

Calcium oxide is called lime or quick lime. Calcium oxide is used in the manufacture of cement.

CaCO₃(s) →Heat→ CaO(s)+CO₂(g)

(2) Electrolytic decomposition reaction

When a decomposition reaction is carried out by Elcctricity, it is called Electrolytic decomposition.

Electrolysis of Water

When electric current is passed through the water, it decomposes in oxygen and hydrogen it is called electrolysis of water.

2H₂O → Electricity → 2H₂ + O₂

In this reaction two molecules of water are broken down to form two molecules of hydrogen and one molecule of oxygen.

(3) Photolytic Decomposition Reaction:

When a decomposition reaction is carried out by light, it is called Photolytic decomposition.

Decomposition of silver chloride into silver and chlorine by light.

2AgCl → (Sunlight) → 2Ag + Cl₂

Decomposition of silver bromide into silver and bromide by light.

2AgBr → (Sunlight) → 2Ag + Br₂

This reactions is used in black and white photography

Endothermic chemical reactions.

Reactions in which heat is absorbed are called endothermic chemical reactions. The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants so all decomposition reactions are endothermic reactions.

3. Displacment reaction

A reaction in which a more reactive element displaces a less reactive element from its compound is called displacement reaction.

AB + C → AC + B

(i) Reaction of zinc with copper sulphate

Zn + CuSO₄ → ZnSO₄ + Cu

In this reaction more reactive Zn displaces the less reactive Cu from its compound copper sulphate

(ii) Rreaction of iron with copper sulphate

When an iron nail is immersed in copper sulphate solution, iron displaces copper from copper sulphate because iron is more reactive than copper As a result, the blue colour of the copper sulphate solution changes to light green and Iron nail becomes brownish in colour due to the deposition of copper.

Fe + CuSO₄ → FeSO₄ + Cu

(iii) Reaction of Lead with copper chloride

Pb + CuCl₂ → PbCl₂ + Cu

4. Double Displacement Reaction :

The reaction in which an ion or a group of ions is exchanged between two reactants is called a double displacement reaction.

AB + CD → AD + BC

Example

Reaction of sodium sulfate with barium chloride

Na₂SO₄ + BaCl₂ → BaSo₄ ⬇ + 2NaCl

When sodium sulfate reacts with barium chloride, a white precipitate of barium sulfate is obtained. In this reaction, a precipitate which is insoluble in water, is formed. so this reaction is called a precipitation reaction.

Oxidation : Such a reaction in which the addition of oxygen to the substance or the removal of hydrogen from the substance loss of electrons is called oxidation.

Addition of Oxygen
Remual of Hydrgen
Loss of electrons.

2Cu + O₂ → 2 CuO

In this reaction oxygen is added to copper and copper oxide is formed. i.e., Cu is oxidise in CuO

Reduction - Such a reaction in which the addition of hydrogen to the substance or the removal of oxygen from the substance is called reduction.

CuO + H₂ → Cu + H₂O

5. Redox reaction :

Those reaction in which oxidation and reduction take place simultaneously is called redox reaction.

CuO + H₂ → Cu + H₂O

In this reaction H₂ is oxidised to H₂O and CuO is reduced to Cu

MnO₂ + 4 HCl → MnCl₂ + Cl₂ + H₂O

In this reaction HCl is oxidised to Cl₂ whereas MnO₂ is reduced to MnCl₂

ZnO + C → Zn + CO

In this reaction carbon is oxidised to CO and ZnO is reduced to Zn

Effect of Oxidation

1. Corrosion : When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

Example

Rusting of iron.

Black coating on silver

Green coating on copper

2. Rancidity : Oxidation of Fats or oils in foods items result in change of test and smell. Which makes food items rancid. this process is called rancidity

Prevention of rancidity

Adding antioxidants to food containing fats and oils.

Keeping food in air tight containers

Replacing oxygen in the containers with another gas .

For example chips manufacturers flush bags of chips with nitogen gas to prevent the chips from getting oxidised

Which one is a chemical change Rusting of iron or melting of iron?
Rusting of iron is a chemical change.
Which one is a chemical change fermentation of fruit juice or diluting fruit juice?
Fermentation of fruit juice is a chemical change.
Which one of the following is a chemical change? Give reason also.
(a) Burning of wax (b) Melting of wax
Burning of wax is a chemical change because new products with new properties will be formed on burning.
State one basic difference between a physical change and a chemical change.
In a physical change, no new substance is formed. In a chemical change, new substance with new properties is formed.
Define a balanced chemical equation.
Balanced chemical equation is a equation in which number of atoms of various elements are equal on both sides of the equation.
What is meant by a chemical reaction?
The process in which a chemical change take place is known as chemical reaction.
Example : Photosynthesis and Burning of fuel.
Write observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.

(i) Change in colour FeSO₄.7H₂O → Heat → FeSO₄
Green White
(ii) Evolution of gas Zn + H₂SO₄ → ZnSO₄ + H₂ ↑
Hydrogen gas
(iii) Formation of precipitate. Pb(NO₃)₂ + 2HCl → PbCl₂ + 2HNO₃
White ppt
(iv) Change in temperature CaO + H₂O → Ca(OH)₂ + Heat
(v) Change in state - AgNO₃(aq) + NaCl(aq) → Heat → AgCl(s) + NaNO₃(aq)
Dil. HCl is added to Zn granules.” How will you prove that chemical change has taken place here ? Support your response with three arguments.
Bubbles of gas/ Evolution of gas
Change in colour (Zn - silvery grey to black)
Change in temperature
On what basis is a chemical reaction balanced?
Chemical equation is balanced on the basis of law of conservation of mass.
Consider the above mentioned two chemical equations with two different kinds of arrows (↑ and ↓ ) along with the product. What do these two different arrows indicate ?
↑ shows evolution of gas, ↓ represents a precipitate
Name and state the law which is kept in mind when we balance chemical equations.
Law of conservation of mass - Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants must be equal to total mass of products.
Why is it necessary to balance a chemical equation? Or
“We need to balance a skeleton chemical equation”. Give reason to justify the statement.
Chemical equation must be balanced so as to follow the law of conservation of mass. The total mass of reactants must be equal to the total mass of products, that is why all reactions should he balanced.
Observe the two chemical equations given below.
(i) Ca(OH)2 + HNO₃→ Ca(NO₃)2+ H2O
(ii) KBr + AgNO₃→ KNO₃ + AgBr
(a) Explain how a balanced equation can be identified.
The number of atoms of each element should be the same on the reactants' side and the products' side.
(b) Which of the two equations is/are NOT balanced? Balance the equation(s) by rewriting.
Eqution (i) is NOT balanced
Balanced equation: Ca(OH)₂ + 2HNO₃ → Ca(NO₃ )₂ + 2 H₂O
It has been found that marble of Taj is getting corroded due to development of industrial areas around it. Explain this fact giving a chemical equation
Taj is made up of CaCO₃ (marbal) which reacts with acid formed by pollution of SO₂ and NO₂ from Mathura refinery and other industries.
CaCO₃ + 2H₂SO₄→ CaSO₄ + CO₂ + 2H₂O
CaCO₃ + 2HNO₃→ Ca(NO₃)₂ + H₂O + CO₂
When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced. Why?
Mg is getting oxidised because it is gaining oxygen to form magnesium oxide.
When magnesium ribbon(wire) buns in air or oxygen, a product is formed. State the type of chemical reaction and name the product formed in the reaction. Write balanced chemical equation for the reaction.
It is a combination reaction/ Exothermic reaction/ Oxidation reaction
The product formed is magnesium oxide
2Mg + O₂ → 2MgO
A metal ’X’ is found in form of ribbons. It burns in presence of oxygen to form ‘Y" after cleaning it with sand paper. Identify the metal ‘X’ and nature of ‘Y’ formed. Write balanced chemical equation and observation.
‘X" is magnesium. It forms Y which is MgO on burning in presence of oxygen.
2Mg + O₂ → 2MgO + Heat + Light
It burns with dazzling white light. The reaction is highly exothermic.
Why should a magnesium ribbon be cleaned before burning in the air?
Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer which is a very stable compound. This layer hinders the burning of magnesium. That's why the magnesium ribbon is cleaned before burning in the air.
A shining metal ‘M’ on burning gives a dazzling white flame & changes to a white powder ‘N’.
(i) Identify ‘M’ & ‘N’.
M is Magnesium (Mg) and, N is Magnesium oxide
(ii) Does ‘M’ undergo oxidation or reduction in this reaction ? Justify.
Magnesium undergo oxidation because oxygen is added to it.
Translate the following statement into chemical equation and then balance it. “A metal in the form of ribbon burns with a dazzling white flame and changes into white powder.”
2Mg + O₂ → Burning → 2MgO + Light + Heat
Following changes take place in this reaction
Change in tem-perature
2Mg + O₂ → 2MgO + Heat
Can combination reaction be an oxidation reaction?
Yes, combination reaction can be called a oxidation reaction.
Define combination reaction. Give one example of a combination reaction which is also exothermic.
Such reactions in which two or more reactants combine to form a product are called combination reactions.
Example:
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide)
CaO + H₂O → Ca(OH)₂ + Heat
(Quick lime) (Slaked lime)
It is an exothermic and combination reaction.
Solid calcium oxide was taken in a container and water was added slowly to it.Write the observations.
The container becomes hot and hissing sound is produced.
The chemical formula of the product formed is Ca(OH)₂
A solution of substance X is used for white washing. What is substance X? Write the chemical reaction of X with water.
‘X’ is quick lime or Calcium Oxide and its formula is CaO.
Chemical reaction of X with water.
CaO + H₂O → Ca(OH)₂ + Heat
(i) What happens chemically when quick lime (Solid calcium oxide) is added to water filled in a bucket?
Calcium hydroxide (Slaked lime) is formed with evolution of heat and hissing sound.
CaO + H₂O → Ca(OH)₂
(ii) Write the chemical name and formula of the product formed.
CaO + H₂O → Ca(OH)₂
Chemical name and formula of the product formed is calcium hydroxide/ Ca(OH)₂
(iii) What changes take place in this reaction :
Change in temperature CaO + H₂O → Ca(OH)₂ + Heat
Define the term decomposition reaction. Give one example of a combination reaction which is also endothermic.
A reaction in which a single reactant breaks down to form two or more products is called decomposition or decomposition reaction.
AB ➡ A + B
Example
CaCO₃ → Heat→ CaO + CO₂
Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Give one example each of thermal decomposition, photolytic decomposition and electrolytic decomposition reactions.
Thermal decomposition
CaCO₃  →   Heat→     CaO + CO₂
Photolytic decomposition
2AgBr → sunlight → 2Ag + Br₂
Electrolytic decomposition
2H₂O  →   Electricity→    2H₂  + O
Why are decomposition reactions called the opposite of combination reactions? Give chemical equations for these reactions.
In decomposition reactions, a compound is broken down into one or more elements or compounds whease In combination reactions, two or more elements or compounds combine to form a new compound.
For Example
Decomposition reaction
CaCO₃ →Heat→ CaO + CO₂
Combination reactions
CaO + H₂O → Ca(OH)₂ + Heat
Therefore, decomposition reactions are opposite to combination reactions
What happens when carbon dioxide is bubbled through lime water
(i) In small amount (ii) In excess?
(i) Lime water turn milky when small amount of CO₂ is passed through it
Ca(OH)₂ + CO₂→ CaCO₃ + H₂O
(ii) If excess of CO₂ is passed milkiness disappears
CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂
Write balanced chemical equation and identify the type of reaction
(i) Calcium carbonate → Heat→ Calcium oxide + Carbon dioxide
CaCO₃ → Heat→ CaO + CO₂
decomposition reaction/Endothermic reaction
(ii) Calcium oxide + Water → Calcium carbonate + Heat
CaO + H₂O → Ca(OH)₂ + Heat
Combination reaction/Exothermic reaction
State the main difference between endothermic reaction and an exothermic reaction.
In endothermic reaction, heat is absorbed. While in exothermic reaction, heat is evolved.
Write the essential condition for the following reaction to take place
2AgBr → 2Ag + Br₂ Write application of this reaction.
The reaction will take place in presence of sunlight. This reaction is used in black and white photography.
Why do we store silver chloride in dark coloured bottle? Explain in brief.
AgCl is photosensitive, it will decompose to Ag and Cl₂ in the presence of sunlight. Therefore, AgCl is stored in dark coloured bottle to protect it from sunlight.
2AgCl → sunlight→ 2Ag + Cl₂
What change in colour is observed when white silver chloride is left exposed to sunlight? State the type of chemical reaction in this change.
When silver chloride is left exposed to sunlight, it decomposes into Ag and Cl₂ in the presence of sunlight and it becomes gray
2AgCl → sunlight→ 2Ag + Cl₂
It is photochemical decomposition reaction.
Why solution of slaked lime is used for white washing?
When walls are whitewashed with a solution of slaked lime [Calcium hydroxide] , Calcium hydroxide reacts slowly with the carbon dioxide in air and form a thin layer of calcium carbonate on the walls and gives a shiny finish to the walls
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O(l)
In the electrolysis of water.

(i) Name the gas collected at the cathode and anode respectively.(i) Name the gas collected at the cathode and anode respectively.
Hydrogen is collected at the cathode, oxygen is collected at the anode.
(ii) Why is volume of gas collected at one electrode double than that at the other?
It is because H₂O contains hydrogen and oxygen in the ratio 2 : 1 that is why volume of H₂ is double than that of oxygen.
(iii) What would happen if dilute H₂SO₄ is not added to water?
dilute H₂SO₄ only increases the speed of the reaction. Hence, if we do not add sulphuric acid to water at the time of electrolysis, the conductivity will decrease and the process becomes slow.
(iv) How will you test this gas?
Bring a burning matchstick near the gas, if the gas burns with ‘pop’ sound, the gas is H₂
Why is respiration considered as exothermic process ?
Respiration is an exothermic process because energy is released, in respiration.
When do desert plant take up carbon dioxide and perform photosynthesis.
Desert plant take up carbon dioxide and perform photosynthesis at night.
A green coloured hydrated metallic salt on heating loses water of crystallisation molecules and gives a gas with suffocating smell. Identify the salt and write the chemical equation for the reaction.
The salt is FeSO₄.7H₂O (Hydrated ferrous sulphate)
FeSO₄.7H₂O → Heat→ FeSO₄ +7H₂O
2FeSO₄ → Heat → Fe₂O₃ + SO₂ + SO₂
(i) What is colour of ferrous sulphate crystals? How does this colour change after heating ?
FeSO₄. 7H₂O crystals are pale green in colour. They become dirty white on heating.
(ii) Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change?
Ferric oxide, sulphur dioxide and sulphur trioxide are formed
2FeSO₄ → Heat→ Fe₂O₃ + SO₂ + SO₃
Complete the following equation for the chemical reaction
FeSO₄→ Heat→ Fe₂O₃ + ___ + ___
2FeSO₄→ Heat→ Fe₂O₃ + SO₂ + SO₃
A white salt on heating decomposes to give brown fumes and a yellow residue is left behind. Name the salt and write the decomposition reaction.
The given white salt is lead nitrate. It decomposes to give brown fumes(NO₂)
The decomposition reaction.
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(White salt) Heat (Residue) (Brown fumes)
Why does not silver evolve hydrogen on reacting with dil H₂SO₄
Silver do not evolve hydrogen on reacting with dil. H₂SO₄ as silver is less reactive metal than hydrogen
Write balanced chemical equations and type of reaction for the following reactions.
(i) Silver bromide on exposure to sunlight decomposes into silver and bromine.
2AgBr → sunlight → 2Ag+ Br₂
Photolytic decomposition
(ii) Silver chloride → sunlight→ Silver + Chlorine
2AgCl → sunlight→ 2Ag + Cl₂
Photolytic decomposition
Write balanced chemical equations and type of reaction for the following reactions.
(i) During respiration, glucose combines with oxygen and forms carbon dioxide and water along with the release of energy.(The process of respiration.)
C₆H₁₂O₆ (aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy
Exothermic Reaction
(ii) Natural gas burns in air to form carbon dioxide and water.
CH₄ + 2O₂ → CO₂ + 2H₂O
Exothermic reactions/ Oxidation reaction
What is observed after about 1 hour of adding the strips of copper and aluminium separately to ferrous sulphate solution filled in two beakers? Name the reaction if any change in colour is noticed. Also,write chemical equation for the reaction.
When we add the strips of copper to ferrous sulphate solution, there is no change in colour because copper is less reactive than iron. so, it can’t displace iron from its salt.
Cu (s) + FeSO₄ → No change will be appear.
When we add the strips of aluminium to ferrous sulphate solution, there is aluminium displaces iron from its salt solution because aluminium is high reactive than iron.
2Al (S) + 3FeSO₄ (aq) → Al₂(SO₄)₃ + 3Fe (S)
Here, Displacement reaction will takes place.
Why does the colour of copper sulphate solution changes when an iron pin is dipped in it ?
More reactive Iron displaces the less reactive Cu from copper sulphate (Blue) solution to form iron (II) sulphate (Green) solution and iron pin becomes brownish in colour
Fe + CuSO₄→ FeSO₄ + Cu
List two observations that are noticed when an iron nail is put inside copper sulphate solution. Write the chemical equation for the reaction that occurs.
The blue coloured solution will become pale green.
Reddish brown metal will get deposited.
Fe   +  CuSO₄   →   FeSO₄       +  Cu
    Blue           PaleGreen    Reddishbrown
A zinc plate was put into solution of copper sulphate kept in a glass container. It was found that blue colour of the solution gets fader and fader with passage of time. After few days when zinc plate was taken out of the solution, a number of holes were observed on it. State the reason for the changes observed on zinc plate. and write the chemical equation for the reaction involved
Zinc displaces copper from copper sulphate solution to form colourless ZnSO₄ and copper metal is deposited. Zinc gets consumed due to which holes are formed.
Chemical equation
Zn + CuSO₄→ ZnSO₄ + Cu
What is rancidity? Mention any two ways by which rancidity can be prevented.

Oxidation of Fats or oils in foods items result in change of test and smell. Which makes food items rancid. this process is called rancidity
Prevention of rancidity
Adding antioxidants to food containing fats and oils.
Keeping food in air tight containers.
Replacing oxygen in the containers with another gas
Identify the type of reaction, giving justification in each case.
(i) 2KI + Cl₂→ 2KCl + I₂
Displacement reaction because Cl₂ is displacing I₂ from KI solution.
(ii) 2K + Cl₂→ 2KCl
Combination reaction because K reacts with Cl₂ to form potassium chloride.
Identify the type of each of the following reactions:
(i) A reaction in which a single product is formed from two or more reactants.
Combination reaction
(ii) The reaction mixture becomes warm.
Exothermic reaction
(iii) An insoluble substance is formed.
Precipitation reaction (Double displacement reaction)
(iv) External surface of the container in which reaction takes place becomes cold.
Endothermic reaction
Balance the following reactions
◈BaCl₂ + H₂SO₄→ BaSO₄ + HCl
BaCl₂ + H₂SO₄→ BaSO₄ + 2HCl
◈Fe + H₂O → Fe₃O₄ + H₂
3Fe + 4H₂O → Fe₃O₄ + 4H₂
◈MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O
MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O
◈Al + CuCl₂ → AlCl₃ + Cu
2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
◈FeSO₄ → Heat→ Fe₂O₃ + SO₂ + SO₃
2FeSO₄ → Heat→ Fe₂O₃ + SO₂ + SO₃
◈Pb(NO₃)₂ → PbO + NO₂ + O₂
2Pb(NO₃)₂ → Heat→ 2PbO + 4NO₂ + O₂
◈Ca(OH)₂ + HNO₃→ Ca(NO₃ )₂ + H₂O
Ca(OH)₂ + 2HNO₃→ Ca(NO₃)₂ + 2H₂O
Write chemical equations for the reactions taking place when:
(i) Iron reacts with steam
3Fe + 4H₂O → Fe₃O₄ + 4H₂
(ii) Electric current is passed through water.
2H₂O(l) → 2H₂ + O₂
(iii) Copper is heated in air
2Cu + O₂ → 2CuO
(iv) Ammonia and hydrogen chloride gases are mixe
NH₃ + HCl → NH₄Cl
Write balanced equation for the reaction between Mg and hydrochloric acid. Name the product obtained, identify the type of reaction.
Mg + 2HCl (dil) → MgCl₂ + H₂
Magnesium chloride and hydrogen gas are formed in this reaction. It is a displacement reaction.
What can be seen when a strip of copper metal is placed in a solution of silver nitrate?
The solution will become blue, shiny silver metal will get deposited.
Cu + 2AgNO₃→ Cu(NO₃ )₂ + 2Ag
Which of the following statement is correct and why?
Copper can displace silver from silver nitrate solution and Silver can displace copper from copper sulphate solution.
Copper can displace Ag from AgNO₃ solution because Cu is more reactive than Ag
Cu + AgNO₃→ CuNO₃ + Ag
Reema took 5ml of Lead Nitrate solution in a beaker and added approximately 4ml of Potassium Iodide solution to it. What would she observe?
Yellow precipitate is formed.
PbNO₃ + KI→ PbI₂ + KNO₃
Consider the following chemical equation.
X + Barium chloride → Y + Sodium chloride (White ppt)
Identify X, Y and the type of reaction.
X is silver nitrate and Y is silver chloride.
2AgNO₃ + BaCl₂→ 2AgCl₂ + Ba(NO₃)₂
Whiteppt
The reaction is an example of double displacement (precipitation) reaction.
An aqueous solution of metal nitrate ‘P’ reacts with sodium bromide solution to form yellow precipitate ‘Q’ which is used in photography. ‘Q’ on exposure to sunlight undergoes decomposition to form metal present along with a reddish brown gas. Identify ‘P’ and ‘Q’ write the balanced chemical equation for the chemical reaction. List the two categories in which reaction can be placed.
‘P’ is silver nitrate (AgNO₃ ) and ‘Q’ is AgBr
AgNO₃ + NaBr → AgBr + NaNO₃
(P) (Q)
This reaction is categorised as double displacement as well as precipitation reaction.
A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of reaction.
AgNO₃ + KCl → AgCl + KNO₃
(White ppt.)
It is a double displacement reaction.
What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the reaction and name the type of reaction.
White precipitate of BaSO₄ is formed.
2NaSO₄(aq) + BaCl₂ (aq) → BaSO₄(s) + 2NaCl (aq)
It is a double displacement reaction.
If reactants are taken in solid state, products will not be formed.
Write a balanced chemical equation for the reaction between sodium chloride and silver nitrate indicating the physical state of the reactants and the products.
AgNO₃(aq) + NaCl(aq) → Heat → AgCl(s) + NaNO₃(aq)
Write a balanced equation for the chemical reaction that can be characterised as precipitation reaction.
AgNO₃ + NaCl → AgCl + NaNO₃
It is a precipitation reaction.
Distinguish between a displacement reaction and a double displacement reaction. Identify the displacement and the double displacement reaction from the following reactions
A displacement reaction is one in which a more reactive element displaces a less reactive element from its compound.
A double displacement reaction is one in which an ion or a group of ions is exchanged between two reactants.
(i) HCl + NaOH → NaCl + H₂O
Double displacement reaction.
(ii) Fe + CuSO₄→ FeSO₄ + Cu
Displacement reaction
Write a complete balanced chemical equation for the following reaction.
Sodium hydroxide + Sulphuric acid →
2NaOH + H₂SO₄→ Na₂SO₄ + 2H₂O
Write balanced chemical equations and type of reaction for the following reactions.
(i) Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate.
BaCl₂ + ZnSO₄→ BaSO₄⬇ + ZnCl₂
Double displacement reaction/precipitation reaction.
(ii) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
3BaCl₂ + Al₂(SO₄)₃→ 3BaSO₄⬇ + 2AlCl₃
Double displacement reaction/precipitation reaction.
(iii) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and solution of Sodium chloride.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)⬇ + 2NaCl(aq)
Double displacement reaction/precipitation reaction.
(iv) Barium chloride solution is mixed with copper sulphate solution and white precipitate is formed.
BaCl₂ + CuSO₄ → BaSO₄⬇ + CuCl₂
Double displacement reaction/precipitation reaction.
State one industrial application of reduction process.
It is used in the extraction of metals
ZnO + C →Heat→ Zn + CO₂
Define reduction reaction.
Such a reaction in which the addition of hydrogen to the substance or the removal of oxygen from the substance or gain of electrons is called reduction.
Addition of Hydrogen
Remual of Oxygen
Gain of electrons.
If copper metal is heated over a flame it develops a coating. What is the colour and composition of coating?
Black coloured coating is formed. It is a due to formation of copper oxide.
On heating copper powder in air, the surface of copper powder becomes coated with black CuO. How can this black coating be converted into brown copper? Write chemical equation for the reaction that occurs during the colour change.
2Cu + O₂→ Heat→ 2CuO
(Black)
It is an oxidation Reaction
Copper oxide on heating with H₂ will change back to reddish brown copper metal.
CuO + H₂ → Heat → Cu + H₂O
When the powder of common metal is heated in open china dish its colour turns black. However, when hydrogen is passed over the hot black substance so formed, it regains its original colour. Based on the above information answer the following questions.
(i) What type of chemical reaction takes place in each of the two given steps?
Oxidation reaction
(ii) Name the metal initially taken in powdered form. Write balanced equations for both the reactions.
Copper metal was present initially taken in powdered form
Cu + O₂ → CuO
(Black)
CuO + H₂→ Cu + H₂O
brown
What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction?
Such a reaction in which the addition of oxygen to the substance or the removal of hydrogen from the substance or loss of electrons is called oxidation.
Addition of Oxygen
Remual of Hydrgen
Loss of electrons.
Example : 2Cu + O₂→ 2 CuO
Oxidation is an exothermic reaction.
Name the reducing agent in the following reaction 3MnO₂ + 4A1 → 3Mn + Al₂O₃. State which is more reactive, Mn or Al and Why?
Al is the reducing agent.
Al is more reactive than Mn. Reason It is because Al is displacing Mn from MnO₂.
What is redox reaction?
Those reaction in which oxidation and reduction take place simultaneously is called redox reaction.
Identify the substances oxidised and the substance reduced in the following reactions. Name the oxidising and reducing agent in the following reaction
(i) MnO₂ + 4HCl → MnCl₂ + Cl₂ + H₂O
MnO₂ is oxidising agent HCl is reducing agent.
HCl is getting oxidised, MnO₂ is getting reduced.
(ii) CuO + H₂→ Cu + H₂O
CuO is oxidising agent H₂ is reducing agent.
H₂ is getting oxidised, CuO is getting reduced.
(iii) CuO + Zn → heat→ Cu + ZnO
CuO is oxidising agent Zn is reducing agent.
Zn is getting oxidised, CuO is getting reduced.
(iv) ZnO + C → Heat→ Zn + CO
ZnO is oxidising agent C is reducing agent.
C is getting oxidised, ZnO is getting reduced.
“Oxidation and reduction processes occur simultaneously”. Justify this statement with the help of example.
Those reaction in which oxidation and reduction take place simultaneously is called redox reaction.
Example : CuO + H₂ → Cu + H₂O

It shows oxidation and reduction occur simultaneously
Name the process due to which food materials like butter gets spoiled due to oxidation at room temperature if kept for long time. What happens to its taste and smell?
The process is called Rancidity of butter. It leads to change in taste i.e. becomes sour in taste and gives foul smell.
Identify the type of chemical reaction in the following statement and define each of them:
(i) Digestion of food in our body.
Decomposition reaction: It is a process in which a compound is broken down into simple substances.
(ii) Rusting of iron.
Redox : Oxidation and Reduction are taking place simultaneously in rusting of iron.
Those reaction in which oxidation and reduction take place simultaneously is called redox reaction.
(iii) Heating of manganese dioxide with aluminium powder.
Displacement reaction: The reaction in which a more reactive element can displace a less reactive element from its salt solution.
(iv) Blue colour of copper sulphate solution disappears when iron filings are added to it.
Displacement reaction: The reaction in which a more reactive element can displace a less reactive element.
Write balanced chemical equations and type of reaction for the following reactions.
(i) Iron nails when left dipped in blue copper sulphate solution become reddish brown in colour and the blue colour of copper sulphate fades away.
Fe + CuSO₄ → FeSO₄ + Cu
Blue PaleGreen Reddishbrown
Displacement reaction
(ii) Hydrogen gas combines with nitrogen to form ammonia.
3H₂ + N₂→ 2NH₃
Combination reaction.
(iii) On heating green coloured ferrous sulphate crystals, raddish brown solid is left and smell of a gas having odour of burning sulphur is experienced.
2FeSO₄ → Heat → Fe₂O₃ + SO₂ + SO₂
Thermal Decomposition reaction
(v) Quick lime reacts vigorously with water releasing a large amount of heat
CaO + H₂O → Ca(OH)₂ + Heat
(Quick lime) (Slaked lime)
Combination reaction.
Write the balanced chemical equations for the following chemical reactions:
Potassium bromide + Barium iodide → Barium bromide + Potassium Iodide
2KBr + BaI₂→ BaBr₂ + 2KI
Hydrogen + Chlorine → Hydrogen chloride
H₂ + Cl₂→ sunlight→ 2HCl.
Lead + Copper Chloride → Lead chloride + Copper
Pb + CuCl₂ → PbCl₂ + Cu
Zinc oxide + Carbon → Zinc + Carbon Monoxide
ZnO + C → Zn + CO
OH
Name the type of reactions represented by the following equations

C + O₂ → CO₂
Combination Reaction
2H₂ + O₂→ 2H₂O
Combination reaction.
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
Double displacement reaction
CuSO₄ + Zn → ZnSO₄ + Cu
Double displacement reaction.
2H₂O(l) → 2H₂ + O₂
Decomposition reaction
NH₃ + HCl → NH₄Cl
Combination reaction
A student has been collecting antique silver utensils for her research.One day she observed a black coating on silver utensils.Which chemical phenomenon is responsible for this change?Writethe chemicalname of black coating.

The phenomenon is called corrosion (oxidation). Blackcoating on silver utensils is silver sulphide.
2 Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g)
silver sulphide
Write balanced chemical equations for the following reactions.
Sodium metal reacts with water to form sodium hydroxide and hydrogen gas.
2Na + 2H₂O → 2NaOH + H₂
Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
2H₂S + 3O₂→ 2H₂O + 2SO₂
Potassium reacts with water to give potassium hydroxide and hydrogen gas.
2K + 2H₂O(l) → 2KOH + H₂
NaOH solution is heated with zinc granules.
Zn + 2NaOH → Na₂ZnO₂ + H₂
Dilute sulphuric acid is added to sodium carbonate.
Na₂CO₃ + H₂SO₄→ Na₂SO₄ + H₂O + CO₂
Egg shell is dropped in hydrochloric acid,
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Copper (II) oxide reacts with dilute hydrochloric acid.
CuO + 2HC1 → CuCl₂ + H₂O
Phosphorus burns in the presence of chlorine to form phosphorus pentachloride.
2P + 5Cl₂→ 2PCl₅
Carbon monoxide reacts with hydrogen gas at 340 atm to form methyl alcohol.
CO + 2H₂→ atm340→ CH₃
When an iron nail is dipped in copper sulphate solution,a brown coating of copper is formed on the surface of iron and the colour of copper sulphate solution changes from blue to light green.
Why does the colour of copper sulphate change when an iron nailis kept in it?Justify your answer
When an iron nail immersed in the solution of copper sulphate than iron displaces copper from the solution of copper sulphate because iron is more reactive than copper. Therefore copper sulphate solution colour changes from blue to pale green.
Reaction:
CuSO₄(aq)+Fe(s)→FeSO₄(aq)+Cu(s)
Dark room is used to process photographic film, to make print sand to carry out other associated tasks.It is a room that can be made completely dark to allow the processing of the light- sensitive photographic materials, including film and photographic paper. Silver bromide is a light- sensitive compound that is it gets decomposed when exposed to light.So when silver bromide is exposed to sunlight, it gets decomposed to give silver metal and bromine gas is liberated.The reactionis called a photolysis reaction.
a. During decomposition reactions……….is required.
b. Reactions in which energy is absorbed are known as….......................reactions
(i) energy
(ii) endothermic
Rashi whitewashed her home. She used a white powder for whitewashing. It produces calcium hydroxide(CaOH)₂ when it reacts with water(H₂O) and absorbs carbondioxide(CO₂) from the environment, as a result, it produces calcium carbonate (CaCO₃) which creates a hard coating on the walls and after 2-3 days the walls start shining.
(i) Which is slaked lime in above reaction?
Ca(OH)₂ calcium hydroxide
(ii) Why is slaked lime is used for whitewashing?
A solution of slaked lime [Calcium hydroxide] is used for whitewashing walls because When walls are whitewashed, Calcium hydroxide reacts slowly with the carbon dioxide in air and form a thin layer of calcium carbonate on the walls and gives a shiny finish to the walls
(iii) Why walls start shining after 2-3 days?
When walls are whitewashed, Calcium hydroxide reacts slowly with the carbon dioxide in air and form a thin layer of calcium carbonate on the walls and gives a shiny finish to the walls
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O(l)
(Calcium hydroxide) (Calcium carbonate)

Calcium oxide can be reduced to calcium, by heating with sodium metal. Which compound would act as an oxidizing agent in the above process?
[A] Sodium
[B] sodium oxide
[C] calcium
[D] calcium oxide [D]
The chemical reaction between MnO₂ and HCl is an example of:
[A] displacement reaction
[B] combination reaction
[C] redox reaction
[D] decomposition reaction [C]
Identify the correct statement from the following:
[A] MnO₂ is getting reduced whereas HCl is getting oxidized
[B] MnO₂ is getting oxidized whereas HCl is getting reduced.
[C] MnO₂ and HCl both are getting reduced.
[D] MnO₂ and HCl both are getting oxidized [A]
Which of the following gases is used to store fat and oil-containing foods for a long time?
[A] Carbon dioxide
[B] Oxygen
[C] Nitrogen
[D] Neon [C]
The chemical reaction between Hydrogen sulphide and iodine to give Hydrogen iodide and sulphur is given below:
H₂S + I₂→ 2HI + S.
The reducing and oxidising agents involved in this redox reaction are:
[A] Iodine and sulphur, respectively
[B] Iodine and hydrogen sulphide, respectively
[C] Sulphur and iodine, respectively
[D] Hydrogen sulphide and sulphur, respectively [B]
A substance which oxidizes itself and reduces other is known as
[A] Oxidising agent
[B] Reducing agent
[C] Both (a) and (b)
[D] None of these. [B]
Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during the electrolysis of water is
[A] 1:1
[B] 2:1
[C] 4:1
[D] 1:2 [B]
MnO₂ +xHCl → MnCl₂ +yH₂0 +zCl₂In order to balance the above chemical equation, the values of x, y and z respectively are
[A] 6, 2, 2
[B] 4, 2, 1
[C] 4, 1,2
[D] 2, 2, 1 [B]
Which of the following is an example of endothermic process ?
[A] Formation of slaked lime
[B] Decomposition of vegetable matter into compost
[C] Dissolution of ammonium chloride in water
[D] Digestion of food in our body []
Which among the following is (are) double displacement reaction(s)?
(t) Pb + CuCl → PbCl₂ + Cu
(ii) Na₂SO₄ + BaCl₂ → BaSO₄ +2NaCl
(iii) C +O₂ → CO₂
(iv) CH₄ + 2O₂ → CO₂ +2H₂O
[A] (i) and(iv)
[B] (ii)only
[C] (i) and(ii)
[D] (iii) and(iv) [B]
Which option denotes a double displacement reaction?
[A] A + B + C
[B] A+B → C
[C] AC +BD → AD+ BC
[D] AC + B→ AB + C [C]
Which of the following is correct balanced equation:-
[A] Fe +H₂O→ Fe₃O₄ +H₂↑
[B] 2Fe+4H₂O→Fe₃O₄+4H₂↑
[C] 3Fe+4H₂O→Fe₃O₄+4H₂↑
[D] 3Fe+ H₂O→Fe₃O₄+ H₂↑ [C]
A reactionin which a single product is formed from two or more reactants is known as a.............reaction.
[A] combination
[B] doubledisplacement
[C] decompositionreaction
[D] displacementreaction [A]
What is chemical formula for marble ?
]A] CaO
[B] Ca(OH)₂
[C] CaCO₃
[D] CaCl₂ [C]
What is the chemical formula for quick lime?
[A] CaO
[B] Ca(OH)₂
[C] CaCO₃
[D] CaCl₂ [A]
What is observed when a solution of potassium iodide is added to silver nitrate solution?
[A] No reaction takes place
[B] White precipitate of silver iodide is formed
[C] yellow precipitate of Agl is formed
[D] Agl is soluble in water. [B]
The following reaction is an example of a 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)
(i) displacement reaction
(ii) combination reaction
(iii)redoxr eaction
(iv) neutralisation reaction
[A] (i) and (iv)
[B] (ii) and(iii)
[C] (i) and(iii)
[D] (iii) and(iv) [C]
Which of the following reactions is not correct:
[A] Zn+ CuSO₄ → ZnSO₄ +Cu
[B] 2Ag+Cu(NO₃)₂→2AgNO₃+Cu
[C] Fe+CuSO₄→FeSO₄+Cu
[D] Mg+ 2HCl→ MgCl₂ +H₂
If substance loses oxygen or gains hydrogen during are action it is said to be………………
[A] Oxidised
[B]Reduced
[C]tarnished
[D] corroded [B]
Respiration is a/an:
[A] endothermic reaction
[B] exothermic reaction
[C] decomposition reaction
[D] displacement reaction [B]
Some metals tarnished by substances around them such as moisture when left for some time what is this process called as?
[A]Corrosion
[B]Rancidity
[C]Galvanization
[D] Reduction [A]
Keeping food in air tight containers helps to slow down:
[A] reduction
[B] displacement
[C] oxidation
[D] all of the above [C]
When a single reactant breaks down to give simple rproducts the reaction is known as:
[A] endothermic reaction
[B] exothermic reaction
[C] decomposition reaction
[D] oxidation reaction [C]
We use a solution of slaked lime to whitewash the walls. In the beginning the wall looks rough and coarse, but after 2 - 3 days the wall appears smooth and shiny. This is due to the formation of:
[A] Calcium hydroxide
[B] Calcium oxide
[C] Calcium chloride
[D] Calcium carbonate [D]

1. Chemical Reactions and Equations

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