Selina Textbook Solutions: Chemical Changes and Reactions | Chemistry Class 9 ICSE PDF Download

 Page 1


Chemical Changes and Reactions 
 
 
Solution 1: 
1. A chemical reaction is the process of breaking the chemical bonds of the reacting 
substances (reactants) and making new bonds to form new substances 
(products). 
2. Conditions necessary for a chemical change or reaction are 
? Evolution of gas 
? Change of colour 
? Formation of precipitate 
? Change of state 
Solution 2: 
? A chemical bond is the force which holds the atoms of a molecule together as in a 
compound. 
? Formation of gas bubbles in a liquid during a reaction is called effervescence. 
? Chemical reactions which are characterised by the formation of insoluble solid 
substances are called precipitates. 
Solution 3: 
 
Page 2


Chemical Changes and Reactions 
 
 
Solution 1: 
1. A chemical reaction is the process of breaking the chemical bonds of the reacting 
substances (reactants) and making new bonds to form new substances 
(products). 
2. Conditions necessary for a chemical change or reaction are 
? Evolution of gas 
? Change of colour 
? Formation of precipitate 
? Change of state 
Solution 2: 
? A chemical bond is the force which holds the atoms of a molecule together as in a 
compound. 
? Formation of gas bubbles in a liquid during a reaction is called effervescence. 
? Chemical reactions which are characterised by the formation of insoluble solid 
substances are called precipitates. 
Solution 3: 
 
Solution 4: 
 
Solution 5: 
 
Solution 6: 
 
Page 3


Chemical Changes and Reactions 
 
 
Solution 1: 
1. A chemical reaction is the process of breaking the chemical bonds of the reacting 
substances (reactants) and making new bonds to form new substances 
(products). 
2. Conditions necessary for a chemical change or reaction are 
? Evolution of gas 
? Change of colour 
? Formation of precipitate 
? Change of state 
Solution 2: 
? A chemical bond is the force which holds the atoms of a molecule together as in a 
compound. 
? Formation of gas bubbles in a liquid during a reaction is called effervescence. 
? Chemical reactions which are characterised by the formation of insoluble solid 
substances are called precipitates. 
Solution 3: 
 
Solution 4: 
 
Solution 5: 
 
Solution 6: 
 
Solution 7: 
? Silver nitrate solution is kept in brown bottles in the laboratory because it 
decomposes in the presence of light. 
? Molybdenum increases the efficiency of the catalyst iron used in the manufacture 
of ammonia. 
? This is because the blue colour of the copper sulphate solution fades and 
eventually turns into light green due to the formation of ferrous sulphate. 
PAGE NO: 27 
Solution 1: 
? Displacement 
? Double decomposition 
? Accelerate, decelerate, unaffected 
Solution 2: 
Combination 
Decomposition 
Solution 3: 
 
Solution 4: 
A reaction in which two or more substances combine together to form a single 
substance is called a synthesis or combination reaction. 
A + B ? AB 
Page 4


Chemical Changes and Reactions 
 
 
Solution 1: 
1. A chemical reaction is the process of breaking the chemical bonds of the reacting 
substances (reactants) and making new bonds to form new substances 
(products). 
2. Conditions necessary for a chemical change or reaction are 
? Evolution of gas 
? Change of colour 
? Formation of precipitate 
? Change of state 
Solution 2: 
? A chemical bond is the force which holds the atoms of a molecule together as in a 
compound. 
? Formation of gas bubbles in a liquid during a reaction is called effervescence. 
? Chemical reactions which are characterised by the formation of insoluble solid 
substances are called precipitates. 
Solution 3: 
 
Solution 4: 
 
Solution 5: 
 
Solution 6: 
 
Solution 7: 
? Silver nitrate solution is kept in brown bottles in the laboratory because it 
decomposes in the presence of light. 
? Molybdenum increases the efficiency of the catalyst iron used in the manufacture 
of ammonia. 
? This is because the blue colour of the copper sulphate solution fades and 
eventually turns into light green due to the formation of ferrous sulphate. 
PAGE NO: 27 
Solution 1: 
? Displacement 
? Double decomposition 
? Accelerate, decelerate, unaffected 
Solution 2: 
Combination 
Decomposition 
Solution 3: 
 
Solution 4: 
A reaction in which two or more substances combine together to form a single 
substance is called a synthesis or combination reaction. 
A + B ? AB 
In the above reaction, substances A and B combine to give a molecule of a new 
substance, AB. 
Carbon burns in oxygen to form a gaseous compound, carbon dioxide. 
C + O 2 ? CO 2 
Solution 5: 
 
Solution 6: 
The reaction between an acid and a base which forms salt and water only is referred to 
as reaction of neutralisation. 
Applications of neutralisation reactions: 
? When someone is stung by a bee, formic acid enters the skin and causes pain, 
which can be relieved by rubbing the spot with slaked lime or baking soda, both of 
which are bases. 
? Acid which accidentally spills on to our clothes can be neutralised with ammonia 
solution. 
? If the soil is somewhat acidic and thus unfavourable for growing of certain crops, 
then slaked lime is added to neutralise the excess acid. 
Solution 7: 
Hydrolysis is the process in which a salt and water react to form an acidic or basic 
solution. 
In the process of hydrolysis, only those salts hydrolyse which are formed by the reaction 
of 
? strong base and weak acid 
? strong acid and weak base 
This happens because a salt formed due to a strong base and a weak acid on 
dissolving in water will form a basic solution. 
A basic solution turns red litmus blue. 
Na 2CO 3 + 2H 2O ? 2NaOH + H 2CO 3 
However, the salt formed due to a strong acid and a weak base on dissolving in water 
will make an acidic solution. Acidic solutions turn blue litmus red. 
FeCl 3 + 3H 2O ? Fe(OH) 3 + 3HCl 
Page 5


Chemical Changes and Reactions 
 
 
Solution 1: 
1. A chemical reaction is the process of breaking the chemical bonds of the reacting 
substances (reactants) and making new bonds to form new substances 
(products). 
2. Conditions necessary for a chemical change or reaction are 
? Evolution of gas 
? Change of colour 
? Formation of precipitate 
? Change of state 
Solution 2: 
? A chemical bond is the force which holds the atoms of a molecule together as in a 
compound. 
? Formation of gas bubbles in a liquid during a reaction is called effervescence. 
? Chemical reactions which are characterised by the formation of insoluble solid 
substances are called precipitates. 
Solution 3: 
 
Solution 4: 
 
Solution 5: 
 
Solution 6: 
 
Solution 7: 
? Silver nitrate solution is kept in brown bottles in the laboratory because it 
decomposes in the presence of light. 
? Molybdenum increases the efficiency of the catalyst iron used in the manufacture 
of ammonia. 
? This is because the blue colour of the copper sulphate solution fades and 
eventually turns into light green due to the formation of ferrous sulphate. 
PAGE NO: 27 
Solution 1: 
? Displacement 
? Double decomposition 
? Accelerate, decelerate, unaffected 
Solution 2: 
Combination 
Decomposition 
Solution 3: 
 
Solution 4: 
A reaction in which two or more substances combine together to form a single 
substance is called a synthesis or combination reaction. 
A + B ? AB 
In the above reaction, substances A and B combine to give a molecule of a new 
substance, AB. 
Carbon burns in oxygen to form a gaseous compound, carbon dioxide. 
C + O 2 ? CO 2 
Solution 5: 
 
Solution 6: 
The reaction between an acid and a base which forms salt and water only is referred to 
as reaction of neutralisation. 
Applications of neutralisation reactions: 
? When someone is stung by a bee, formic acid enters the skin and causes pain, 
which can be relieved by rubbing the spot with slaked lime or baking soda, both of 
which are bases. 
? Acid which accidentally spills on to our clothes can be neutralised with ammonia 
solution. 
? If the soil is somewhat acidic and thus unfavourable for growing of certain crops, 
then slaked lime is added to neutralise the excess acid. 
Solution 7: 
Hydrolysis is the process in which a salt and water react to form an acidic or basic 
solution. 
In the process of hydrolysis, only those salts hydrolyse which are formed by the reaction 
of 
? strong base and weak acid 
? strong acid and weak base 
This happens because a salt formed due to a strong base and a weak acid on 
dissolving in water will form a basic solution. 
A basic solution turns red litmus blue. 
Na 2CO 3 + 2H 2O ? 2NaOH + H 2CO 3 
However, the salt formed due to a strong acid and a weak base on dissolving in water 
will make an acidic solution. Acidic solutions turn blue litmus red. 
FeCl 3 + 3H 2O ? Fe(OH) 3 + 3HCl 
Solution 8: 
Iron(III) chloride is a salt prepared from strong acid HCl and a weak base Fe(OH) 3. 
Fe(OH) 3 + 3HCl? FeCl 3 + 3H 2O 
On the other hand, sodium carbonate is a salt prepared from a strong base NaOH and a 
weak acid H 2CO 3. 
Fe(OH) 3 + 3HCl ? FeCl 3 + 3H 2O 
Solution 9: 
Decomposition is the breaking up of a compound either into elements or simpler 
compounds such that these products do not combine to form the original compound. 
Decomposition may occur in the presence of heat or light or by the passage of an 
electric current. 
Example: Mercuric oxide when heated decomposes to form two elements-mercury and 
oxygen. 
 
Solution 10: 
? Cl 2 + 2KBr ? 2KCl + Br 2 
Displacement 
? Fe + CuSO 4 ? FeSO 4 + Cu 
Displacement 
? 2H 2O ? 2Hg + O 2 
Decomposition 
? PbO 2 + SO 2 ? PbSO 4 
Combination 
? AgNO 3 + NaCl ? AgCl + NaNO 3 
Double decomposition 
? 2KClO 3 ? 2KCl + 3O 2 
Decomposition 
? 2H 2O 2 ? 2H 2O + O 2 
Decomposition 
? KNO 3 + H 2SO 4 ? HNO 3 + KHSO 4 
Double decomposition 
? CuO + H 2 ? Cu + H 2O 
Displacement 
? CaCO 3 ? CaO + CO 2 
Decomposition 
? NH 4Cl ? NH 3 + HCl 
Decomposition