Chemical Reactions and Equations Class 10 Notes | EduRev

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Class 10 : Chemical Reactions and Equations Class 10 Notes | EduRev

 Page 1


7 
 
CHEMICAL REACTIONS AND EQUATIONS 
GIST OF THE LESSON 
         1) Chemical reaction—   Chemical changes or chemical reactions are the changes in  
which one or more new substances are formed. 
        2 )Chemical Equations – Representation of a chemical reaction in terms of symbols and    
                        formulae of the reactants and products is known as chemical equation. 
        3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different  
                         elements is same on both sides of the arrow is called balanced chemical equation. 
        4) The chemical reactions can be classified into different types such as— 
a) Combination reaction – The reactions in which two or more substances combine to form a 
new substance are called combination reaction. For example, 
       2Mg(s) + O
2
 (g)  ?  2 MgO (s) 
b) Decomposition reaction - The reaction in which a single compound breaks up into two or 
more simpler substances are called decomposition reactions. For example, 
2Pb (NO
3
)
2
 (s) ? 2PbO (s) + 4NO
2
 (g) +O
2
 (g) 
The decomposition of a substance by passing electric current through it is known as 
electrolysis. 
The decomposition of a substance on heating is known as thermal decomposition. 
The decomposition of a substance by absorbing light energy is called photochemical 
decomposition. 
c) Displacement reactions -The chemical reactions in which a more reactive element 
displaces a less reactive element from a compound are known as displacement reactions. For 
example, 
i) Zn (s) + CuSO
4 
(aq)  ? ZnSO
4
 (aq) + Cu (s). 
ii) Cu (s) + 2AgNO
3
 (aq) ? Cu (NO
3
)
2 
(aq) +2Ag (s). 
d) Double Displacement Reactions - The chemical reactions in which compounds react to 
form two different compounds by mutual exchange of ions are called double displacement 
reactions. 
These reactions take place in solution two common types of this reaction are precipitation 
reactions and neutralization reactions 
i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed 
whereby   
Some salts precipitate due to mutual exchange of ions between the two salts. For example 
                          AgNO
3
 (aq) + NaCI (aq) -----> AgCl(s) + NaNO
3
. 
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt 
and water by exchange of ions. 
                                       NaOH (aq) + HCl (aq) ? NaCl (aq) + H
2
O. 
 
 
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Page 2


7 
 
CHEMICAL REACTIONS AND EQUATIONS 
GIST OF THE LESSON 
         1) Chemical reaction—   Chemical changes or chemical reactions are the changes in  
which one or more new substances are formed. 
        2 )Chemical Equations – Representation of a chemical reaction in terms of symbols and    
                        formulae of the reactants and products is known as chemical equation. 
        3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different  
                         elements is same on both sides of the arrow is called balanced chemical equation. 
        4) The chemical reactions can be classified into different types such as— 
a) Combination reaction – The reactions in which two or more substances combine to form a 
new substance are called combination reaction. For example, 
       2Mg(s) + O
2
 (g)  ?  2 MgO (s) 
b) Decomposition reaction - The reaction in which a single compound breaks up into two or 
more simpler substances are called decomposition reactions. For example, 
2Pb (NO
3
)
2
 (s) ? 2PbO (s) + 4NO
2
 (g) +O
2
 (g) 
The decomposition of a substance by passing electric current through it is known as 
electrolysis. 
The decomposition of a substance on heating is known as thermal decomposition. 
The decomposition of a substance by absorbing light energy is called photochemical 
decomposition. 
c) Displacement reactions -The chemical reactions in which a more reactive element 
displaces a less reactive element from a compound are known as displacement reactions. For 
example, 
i) Zn (s) + CuSO
4 
(aq)  ? ZnSO
4
 (aq) + Cu (s). 
ii) Cu (s) + 2AgNO
3
 (aq) ? Cu (NO
3
)
2 
(aq) +2Ag (s). 
d) Double Displacement Reactions - The chemical reactions in which compounds react to 
form two different compounds by mutual exchange of ions are called double displacement 
reactions. 
These reactions take place in solution two common types of this reaction are precipitation 
reactions and neutralization reactions 
i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed 
whereby   
Some salts precipitate due to mutual exchange of ions between the two salts. For example 
                          AgNO
3
 (aq) + NaCI (aq) -----> AgCl(s) + NaNO
3
. 
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt 
and water by exchange of ions. 
                                       NaOH (aq) + HCl (aq) ? NaCl (aq) + H
2
O. 
 
 
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8 
 
 
e) Redox reaction: Chemical reaction which shows both oxidation and reduction reaction. 
 
     Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen. 
 
     Reduction: Reaction that shows the loss of oxygen or gain of hydrogen. 
 
    Both oxidation and reduction take place simultaneously and hence called redox reaction. 
                                                 ZnO + C ? Zn + CO 
                                                ZnO reduce to Zn ---- reduction 
                                                C oxidize to CO ------oxidation 
f)  Exothermic reaction and endothermic reaction:  On the basis of energy changes during chemical   
                                                                                        reaction, they can be classified as 
i) Exothermic reaction: A chemical reaction in which heat energy is produced. 
 
                             C + O
2
 ? CO
2
 (g) + heat 
ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed. 
 
                  CaCO
3
 + Heat ? CaO + CO
2 
 
       5 Corrosion – The process of slow conversion of metals into their undesirable compounds due to 
their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion. 
Rusting – Iron when reacts with oxygen and moisture forms red substance called rust. 
 
      6 Rancidity – The taste and odour of food materials containing fat and oil changes when they are 
left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat and oil 
present in food material. 
It can be prevented by using various methods such as by adding antioxidants to the food materials, 
Storing food in air tight container and by flushing out air with nitrogen. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 NCERT BOOKS, Motivational Videos, Notes for All Classes and Many More... 
Page 3


7 
 
CHEMICAL REACTIONS AND EQUATIONS 
GIST OF THE LESSON 
         1) Chemical reaction—   Chemical changes or chemical reactions are the changes in  
which one or more new substances are formed. 
        2 )Chemical Equations – Representation of a chemical reaction in terms of symbols and    
                        formulae of the reactants and products is known as chemical equation. 
        3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different  
                         elements is same on both sides of the arrow is called balanced chemical equation. 
        4) The chemical reactions can be classified into different types such as— 
a) Combination reaction – The reactions in which two or more substances combine to form a 
new substance are called combination reaction. For example, 
       2Mg(s) + O
2
 (g)  ?  2 MgO (s) 
b) Decomposition reaction - The reaction in which a single compound breaks up into two or 
more simpler substances are called decomposition reactions. For example, 
2Pb (NO
3
)
2
 (s) ? 2PbO (s) + 4NO
2
 (g) +O
2
 (g) 
The decomposition of a substance by passing electric current through it is known as 
electrolysis. 
The decomposition of a substance on heating is known as thermal decomposition. 
The decomposition of a substance by absorbing light energy is called photochemical 
decomposition. 
c) Displacement reactions -The chemical reactions in which a more reactive element 
displaces a less reactive element from a compound are known as displacement reactions. For 
example, 
i) Zn (s) + CuSO
4 
(aq)  ? ZnSO
4
 (aq) + Cu (s). 
ii) Cu (s) + 2AgNO
3
 (aq) ? Cu (NO
3
)
2 
(aq) +2Ag (s). 
d) Double Displacement Reactions - The chemical reactions in which compounds react to 
form two different compounds by mutual exchange of ions are called double displacement 
reactions. 
These reactions take place in solution two common types of this reaction are precipitation 
reactions and neutralization reactions 
i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed 
whereby   
Some salts precipitate due to mutual exchange of ions between the two salts. For example 
                          AgNO
3
 (aq) + NaCI (aq) -----> AgCl(s) + NaNO
3
. 
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt 
and water by exchange of ions. 
                                       NaOH (aq) + HCl (aq) ? NaCl (aq) + H
2
O. 
 
 
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8 
 
 
e) Redox reaction: Chemical reaction which shows both oxidation and reduction reaction. 
 
     Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen. 
 
     Reduction: Reaction that shows the loss of oxygen or gain of hydrogen. 
 
    Both oxidation and reduction take place simultaneously and hence called redox reaction. 
                                                 ZnO + C ? Zn + CO 
                                                ZnO reduce to Zn ---- reduction 
                                                C oxidize to CO ------oxidation 
f)  Exothermic reaction and endothermic reaction:  On the basis of energy changes during chemical   
                                                                                        reaction, they can be classified as 
i) Exothermic reaction: A chemical reaction in which heat energy is produced. 
 
                             C + O
2
 ? CO
2
 (g) + heat 
ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed. 
 
                  CaCO
3
 + Heat ? CaO + CO
2 
 
       5 Corrosion – The process of slow conversion of metals into their undesirable compounds due to 
their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion. 
Rusting – Iron when reacts with oxygen and moisture forms red substance called rust. 
 
      6 Rancidity – The taste and odour of food materials containing fat and oil changes when they are 
left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat and oil 
present in food material. 
It can be prevented by using various methods such as by adding antioxidants to the food materials, 
Storing food in air tight container and by flushing out air with nitrogen. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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9 
 
MIND MAP 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TYPE OF 
CHEMICAL 
REACTION 
CHEMICAL 
REACTION 
EXOTHERMIC-release heat 
ENDOTHERMIC-absorb heat 
 
 
REDOX 
REACTION 
Show both oxidation 
and reduction 
COMBINATION 
Two more 
substance to 
give single 
product 
A + B ? C 
DECOMPOSITION 
Single substances 
decomposes to 
give two or more 
new substances 
A ? B + C 
 
DISPLACEMENT 
More reactive 
element displaces 
less reactive 
element 
A + BC ? AC + 
B 
DOUBLE 
DISPLACEMENT 
Two different 
atoms or group of 
atoms displaces 
each other 
AB + CD ? AC + 
BD 
RANCIDIT
Y 
Oxidation 
of fats and 
oil 
COROSSION 
Eating away of 
metals 
CHEMICAL 
EQUATION 
WRITIN
G 
BALANCI
NG 
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Page 4


7 
 
CHEMICAL REACTIONS AND EQUATIONS 
GIST OF THE LESSON 
         1) Chemical reaction—   Chemical changes or chemical reactions are the changes in  
which one or more new substances are formed. 
        2 )Chemical Equations – Representation of a chemical reaction in terms of symbols and    
                        formulae of the reactants and products is known as chemical equation. 
        3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different  
                         elements is same on both sides of the arrow is called balanced chemical equation. 
        4) The chemical reactions can be classified into different types such as— 
a) Combination reaction – The reactions in which two or more substances combine to form a 
new substance are called combination reaction. For example, 
       2Mg(s) + O
2
 (g)  ?  2 MgO (s) 
b) Decomposition reaction - The reaction in which a single compound breaks up into two or 
more simpler substances are called decomposition reactions. For example, 
2Pb (NO
3
)
2
 (s) ? 2PbO (s) + 4NO
2
 (g) +O
2
 (g) 
The decomposition of a substance by passing electric current through it is known as 
electrolysis. 
The decomposition of a substance on heating is known as thermal decomposition. 
The decomposition of a substance by absorbing light energy is called photochemical 
decomposition. 
c) Displacement reactions -The chemical reactions in which a more reactive element 
displaces a less reactive element from a compound are known as displacement reactions. For 
example, 
i) Zn (s) + CuSO
4 
(aq)  ? ZnSO
4
 (aq) + Cu (s). 
ii) Cu (s) + 2AgNO
3
 (aq) ? Cu (NO
3
)
2 
(aq) +2Ag (s). 
d) Double Displacement Reactions - The chemical reactions in which compounds react to 
form two different compounds by mutual exchange of ions are called double displacement 
reactions. 
These reactions take place in solution two common types of this reaction are precipitation 
reactions and neutralization reactions 
i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed 
whereby   
Some salts precipitate due to mutual exchange of ions between the two salts. For example 
                          AgNO
3
 (aq) + NaCI (aq) -----> AgCl(s) + NaNO
3
. 
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt 
and water by exchange of ions. 
                                       NaOH (aq) + HCl (aq) ? NaCl (aq) + H
2
O. 
 
 
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8 
 
 
e) Redox reaction: Chemical reaction which shows both oxidation and reduction reaction. 
 
     Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen. 
 
     Reduction: Reaction that shows the loss of oxygen or gain of hydrogen. 
 
    Both oxidation and reduction take place simultaneously and hence called redox reaction. 
                                                 ZnO + C ? Zn + CO 
                                                ZnO reduce to Zn ---- reduction 
                                                C oxidize to CO ------oxidation 
f)  Exothermic reaction and endothermic reaction:  On the basis of energy changes during chemical   
                                                                                        reaction, they can be classified as 
i) Exothermic reaction: A chemical reaction in which heat energy is produced. 
 
                             C + O
2
 ? CO
2
 (g) + heat 
ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed. 
 
                  CaCO
3
 + Heat ? CaO + CO
2 
 
       5 Corrosion – The process of slow conversion of metals into their undesirable compounds due to 
their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion. 
Rusting – Iron when reacts with oxygen and moisture forms red substance called rust. 
 
      6 Rancidity – The taste and odour of food materials containing fat and oil changes when they are 
left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat and oil 
present in food material. 
It can be prevented by using various methods such as by adding antioxidants to the food materials, 
Storing food in air tight container and by flushing out air with nitrogen. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 NCERT BOOKS, Motivational Videos, Notes for All Classes and Many More... 
9 
 
MIND MAP 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TYPE OF 
CHEMICAL 
REACTION 
CHEMICAL 
REACTION 
EXOTHERMIC-release heat 
ENDOTHERMIC-absorb heat 
 
 
REDOX 
REACTION 
Show both oxidation 
and reduction 
COMBINATION 
Two more 
substance to 
give single 
product 
A + B ? C 
DECOMPOSITION 
Single substances 
decomposes to 
give two or more 
new substances 
A ? B + C 
 
DISPLACEMENT 
More reactive 
element displaces 
less reactive 
element 
A + BC ? AC + 
B 
DOUBLE 
DISPLACEMENT 
Two different 
atoms or group of 
atoms displaces 
each other 
AB + CD ? AC + 
BD 
RANCIDIT
Y 
Oxidation 
of fats and 
oil 
COROSSION 
Eating away of 
metals 
CHEMICAL 
EQUATION 
WRITIN
G 
BALANCI
NG 
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 NCERT BOOKS, Motivational Videos, Notes for All Classes and Many More... 
10 
 
FA I 
CHEMICAL REACTIONS AND EQUATIONS 
FORMATIVE ASSESSMENT I 
Q. PAPER 
 
MARKS-30                                                                          TIME- 70 MINUTES 
 
Instructions: 
? Questions : 1 to 5 – 1 Mark each 
? Questions : 6 to 9 – 2 Marks each 
? Questions : 10 to 13 – 3 Marks each 
? Question 14 – 5 Marks 
 
1. On what chemical law, balancing of chemical equation is based? 
2. Identify the compound oxidized in the following reaction: 
H
2
S 
(g)
 + Cl
2
 ? S
 (s)
 + HCl 
(g)
 
3. Give an example of photochemical reaction. 
4. Name the reaction which forms insoluble salts. 
5. Name the product obtained and type of reaction given below: 
Na
2
SO
4
 + BaCl
2
 ? _________ + ________ 
6. Explain the following in terms of gain or loss of oxygen with one example: 
a. Oxidation 
b. Reduction 
7. A copper coin is kept in a solution of silver nitrate for some time, what will happen to the coin and 
the colour of the solution? 
8. Why do we apply paint on iron articles? 
9. What happens chemically when quicklime is added to water? 
10. What is rancidity? Write the common methods to prevent it. 
11. What is corrosion? State the conditions necessary for rusting of iron. How rusting is harmful? 
12. Name the type of reactions in the following cases: 
a. Garbage producing foul smell 
b. Burning of natural gas. 
c. Carbon dioxide gas passed through lime water. 
13. Blue crystals of copper sulphate on heating in a dry test tube become colourless. Give reasons. 
 
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Page 5


7 
 
CHEMICAL REACTIONS AND EQUATIONS 
GIST OF THE LESSON 
         1) Chemical reaction—   Chemical changes or chemical reactions are the changes in  
which one or more new substances are formed. 
        2 )Chemical Equations – Representation of a chemical reaction in terms of symbols and    
                        formulae of the reactants and products is known as chemical equation. 
        3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different  
                         elements is same on both sides of the arrow is called balanced chemical equation. 
        4) The chemical reactions can be classified into different types such as— 
a) Combination reaction – The reactions in which two or more substances combine to form a 
new substance are called combination reaction. For example, 
       2Mg(s) + O
2
 (g)  ?  2 MgO (s) 
b) Decomposition reaction - The reaction in which a single compound breaks up into two or 
more simpler substances are called decomposition reactions. For example, 
2Pb (NO
3
)
2
 (s) ? 2PbO (s) + 4NO
2
 (g) +O
2
 (g) 
The decomposition of a substance by passing electric current through it is known as 
electrolysis. 
The decomposition of a substance on heating is known as thermal decomposition. 
The decomposition of a substance by absorbing light energy is called photochemical 
decomposition. 
c) Displacement reactions -The chemical reactions in which a more reactive element 
displaces a less reactive element from a compound are known as displacement reactions. For 
example, 
i) Zn (s) + CuSO
4 
(aq)  ? ZnSO
4
 (aq) + Cu (s). 
ii) Cu (s) + 2AgNO
3
 (aq) ? Cu (NO
3
)
2 
(aq) +2Ag (s). 
d) Double Displacement Reactions - The chemical reactions in which compounds react to 
form two different compounds by mutual exchange of ions are called double displacement 
reactions. 
These reactions take place in solution two common types of this reaction are precipitation 
reactions and neutralization reactions 
i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed 
whereby   
Some salts precipitate due to mutual exchange of ions between the two salts. For example 
                          AgNO
3
 (aq) + NaCI (aq) -----> AgCl(s) + NaNO
3
. 
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt 
and water by exchange of ions. 
                                       NaOH (aq) + HCl (aq) ? NaCl (aq) + H
2
O. 
 
 
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8 
 
 
e) Redox reaction: Chemical reaction which shows both oxidation and reduction reaction. 
 
     Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen. 
 
     Reduction: Reaction that shows the loss of oxygen or gain of hydrogen. 
 
    Both oxidation and reduction take place simultaneously and hence called redox reaction. 
                                                 ZnO + C ? Zn + CO 
                                                ZnO reduce to Zn ---- reduction 
                                                C oxidize to CO ------oxidation 
f)  Exothermic reaction and endothermic reaction:  On the basis of energy changes during chemical   
                                                                                        reaction, they can be classified as 
i) Exothermic reaction: A chemical reaction in which heat energy is produced. 
 
                             C + O
2
 ? CO
2
 (g) + heat 
ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed. 
 
                  CaCO
3
 + Heat ? CaO + CO
2 
 
       5 Corrosion – The process of slow conversion of metals into their undesirable compounds due to 
their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion. 
Rusting – Iron when reacts with oxygen and moisture forms red substance called rust. 
 
      6 Rancidity – The taste and odour of food materials containing fat and oil changes when they are 
left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat and oil 
present in food material. 
It can be prevented by using various methods such as by adding antioxidants to the food materials, 
Storing food in air tight container and by flushing out air with nitrogen. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 NCERT BOOKS, Motivational Videos, Notes for All Classes and Many More... 
9 
 
MIND MAP 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TYPE OF 
CHEMICAL 
REACTION 
CHEMICAL 
REACTION 
EXOTHERMIC-release heat 
ENDOTHERMIC-absorb heat 
 
 
REDOX 
REACTION 
Show both oxidation 
and reduction 
COMBINATION 
Two more 
substance to 
give single 
product 
A + B ? C 
DECOMPOSITION 
Single substances 
decomposes to 
give two or more 
new substances 
A ? B + C 
 
DISPLACEMENT 
More reactive 
element displaces 
less reactive 
element 
A + BC ? AC + 
B 
DOUBLE 
DISPLACEMENT 
Two different 
atoms or group of 
atoms displaces 
each other 
AB + CD ? AC + 
BD 
RANCIDIT
Y 
Oxidation 
of fats and 
oil 
COROSSION 
Eating away of 
metals 
CHEMICAL 
EQUATION 
WRITIN
G 
BALANCI
NG 
www.ncerthelp.com
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 NCERT BOOKS, Motivational Videos, Notes for All Classes and Many More... 
10 
 
FA I 
CHEMICAL REACTIONS AND EQUATIONS 
FORMATIVE ASSESSMENT I 
Q. PAPER 
 
MARKS-30                                                                          TIME- 70 MINUTES 
 
Instructions: 
? Questions : 1 to 5 – 1 Mark each 
? Questions : 6 to 9 – 2 Marks each 
? Questions : 10 to 13 – 3 Marks each 
? Question 14 – 5 Marks 
 
1. On what chemical law, balancing of chemical equation is based? 
2. Identify the compound oxidized in the following reaction: 
H
2
S 
(g)
 + Cl
2
 ? S
 (s)
 + HCl 
(g)
 
3. Give an example of photochemical reaction. 
4. Name the reaction which forms insoluble salts. 
5. Name the product obtained and type of reaction given below: 
Na
2
SO
4
 + BaCl
2
 ? _________ + ________ 
6. Explain the following in terms of gain or loss of oxygen with one example: 
a. Oxidation 
b. Reduction 
7. A copper coin is kept in a solution of silver nitrate for some time, what will happen to the coin and 
the colour of the solution? 
8. Why do we apply paint on iron articles? 
9. What happens chemically when quicklime is added to water? 
10. What is rancidity? Write the common methods to prevent it. 
11. What is corrosion? State the conditions necessary for rusting of iron. How rusting is harmful? 
12. Name the type of reactions in the following cases: 
a. Garbage producing foul smell 
b. Burning of natural gas. 
c. Carbon dioxide gas passed through lime water. 
13. Blue crystals of copper sulphate on heating in a dry test tube become colourless. Give reasons. 
 
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11 
 
14.  
a. Why can not a chemical change be normally reversed? 
b. Why is it always essential to balance a chemical equation? 
c. What happens when CO
2
 gas is passed through lime water and why does it disappear on 
passing excess CO
2
? 
d. Can rusting of iron take place in distilled water? 
 
HOTS QUESTIONS (SOLVED) 
 
Q.1. A water insoluble substance =X‘ on reacting with dilute H
2
SO
4
 released a colourless and 
odourless gas accompanied by brisk effervescence. When the gas was passed through water, 
the solution obtained turned blue litmus red. On bubbling the gas through lime water, it 
initially became milky and milkyness disappeared when the gas was passed in excess. 
Identify the substance =X‘. Write its chemical equations of the reactions involved. 
Ans. The water insoluble substance =X‘ is metal carbonate CaCO
3
. 
CaCO
3 (S)
 + H
2
SO
4 (Aq) 
? CaSO
4(Aq)
 + H
2
O 
(Aq) 
+ O
2 (G)
 
Ca(OH)
2
 + CO
2 (G)
?CaCO
3 (S)
 + H
2
O 
(L)
 
    (milky) 
CaCO
3 (S)
 + CO
2 (G)
+ H
2
O 
(Aq)
?Ca(OH)
2 
    
       (milkiness)  
 
Q.2. Ahmad took a magnesium ribbon (cleaned) and burned it on a flame. The white powder 
formed was taken in a test tube and water was added to it. He then tested the solution formed 
with red and blue litmus paper. What change was seen? Why? 
Ans. Red litmus paper turned blue. 
Blue litmus paper remained blue. 
 This is because the magnesium ribbon on burning in air forms the white magnesium 
oxide. Which dissolved in water, it forms magnesium hydroxide, which is Basic in nature.
  
Q.3. Give one example of a combination reaction in which an element combines with a compound 
to give you a new compound. 
Ans. O
2
 + 2SO
2
? 2SO
3
 
8NH
3
 + 3Cl
2
? 6NH
4
Cl 
 
Q.4. Marble statues often slowly get corroded when kept in open for a long time. Assign a suitable 
explanation. 
  
Q.5. Mohan tool pure water for the electrolytic decomposition of water but did not see any bubbles 
near the electrodes. Explain why? 
Q. 6 Rancidity is a process used for spoiling of cooked food materials like vegetables, etc. When 
kept for long time in open.  How can you prevent such process to proceed?  Give an example. 
Q. 7  A substance =X‘ displaces =Y‘ from its solution in water. It is called displacement reaction.  
What other chemical name can be given to such type of reactions?  Explain, giving an 
example? 
Q. 8 A grey coloured metal =Z‘ (Atomic weight=65) is used in making dry cell.  It reacts with dil. 
HCl to liberate a gas.  What is the gas evolved?  Calculate the minimum amount of =Z‘ 
required to produce 100 l 0f gas? 
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