Chemical Reactions & Equations - Notes | Study Science for Class 10 - Short Notes - Class 10

 Page 1


Chapter 1 Chemical Reactions and Equations 
 
Chemical reaction – A process by which two or more substances react with each other to produce a new 
substance by the rearrangement of their atoms, ions or molecules. 
Examples: 
? Burning of magnesium in air to form magnesium oxide. 
? Cooking of food 
? Rusting of iron 
? Burning of paper, wood, cloth, candle etc. 
? Digestion of food 
? Respiration 
? Metals react with acids to produce hydrogen gas. (candle or matchstick burns with a “pop up” sound in 
the presence of hydrogen) 
Characteristics of chemical reactions: 
a. The substance that reacts are known as reactants.  
b. The new substances formed after the chemical reaction are known as products. Burning of 
magnesium in the presence of oxygen to form magnesium oxide. Reactants – Magnesium and 
oxygen 
Product – Magnesium oxide 
c. A chemical reaction is accompanied by change in color, odor, temperature, state and absorption of 
release of energy in form of heat or light.  
Chemical equations 
 Representation of a chemical reaction in form of symbols of the substances involved; wherein the 
reactants are written on the left-hand side and the product on the right-hand side of an arrow showing the 
direction of the reaction.  
 Reactants      Products 
Magnesium reacts with oxygen to produce magnesium oxide 
   OR 
 Mg + O2    MgO 
          (Reactants)       (Product)  
Word equation 
A form of chemical equation in which the name of the reactants and products are written instead of their 
symbols.  
 Magnesium + Oxygen     Magnesium oxide 
        (Reactants)          (Product) 
Balancing a chemical reaction 
According to the law of conservation of mass matter can neither be created nor destroyed. Therefore, the 
number of atoms in the reactants and the products has to be exactly the same. 
Page 2


Chapter 1 Chemical Reactions and Equations 
 
Chemical reaction – A process by which two or more substances react with each other to produce a new 
substance by the rearrangement of their atoms, ions or molecules. 
Examples: 
? Burning of magnesium in air to form magnesium oxide. 
? Cooking of food 
? Rusting of iron 
? Burning of paper, wood, cloth, candle etc. 
? Digestion of food 
? Respiration 
? Metals react with acids to produce hydrogen gas. (candle or matchstick burns with a “pop up” sound in 
the presence of hydrogen) 
Characteristics of chemical reactions: 
a. The substance that reacts are known as reactants.  
b. The new substances formed after the chemical reaction are known as products. Burning of 
magnesium in the presence of oxygen to form magnesium oxide. Reactants – Magnesium and 
oxygen 
Product – Magnesium oxide 
c. A chemical reaction is accompanied by change in color, odor, temperature, state and absorption of 
release of energy in form of heat or light.  
Chemical equations 
 Representation of a chemical reaction in form of symbols of the substances involved; wherein the 
reactants are written on the left-hand side and the product on the right-hand side of an arrow showing the 
direction of the reaction.  
 Reactants      Products 
Magnesium reacts with oxygen to produce magnesium oxide 
   OR 
 Mg + O2    MgO 
          (Reactants)       (Product)  
Word equation 
A form of chemical equation in which the name of the reactants and products are written instead of their 
symbols.  
 Magnesium + Oxygen     Magnesium oxide 
        (Reactants)          (Product) 
Balancing a chemical reaction 
According to the law of conservation of mass matter can neither be created nor destroyed. Therefore, the 
number of atoms in the reactants and the products has to be exactly the same. 
This is made sure by balancing the chemical reaction 
Example 1:  
In the above reaction of Magnesium oxide there are two atoms of oxygen in the reactants however only one in 
the products. This means that the above reaction is incomplete.  
Mg + O2    MgO 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 1 
O 2 1 
 
Balancing a chemical reaction means equalizing the number of all the elements involved in a chemical reaction 
on both the sides.  
Step 1 – Start by balancing the element, which has the greatest number of atoms in the chemical reaction. In 
this example Oxygen has the maximum number of atoms so we start by balancing oxygen first.  
 Now if multiply the product (MgO) by 2 the number of oxygen atom on both sides will become 2, however the 
number of magnesium atom will become 2 in the product but remain 1 in the reactants.  
 
 Mg + O2    2MgO 
 
 
 
 
Step 2 – If we multiply 2 on the Mg of the reactants the number of magnesium will become 2 on both the sides 
without effecting the oxygen atoms on either of the side. 
 2Mg + O2    2MgO 
 
 
 
Now the number of magnesium atoms is same (2) on both sides  
The number of oxygen (2) atoms is also same in both reactants and products 
Therefore, we can say that the chemical reaction is now balanced. 
Step 3 – Write the physical states (s – solid; l – liquid; g - gas) of all the molecules  
Magnesium ribbon is solid; oxygen present in air is in gaseous form and magnesium oxide (smoke) produced in 
in gaseous form  
 2Mg (s) + O2 (g)    2MgO(g) 
 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 2 
O 2 2 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 2 2 
O 2 2 
Page 3


Chapter 1 Chemical Reactions and Equations 
 
Chemical reaction – A process by which two or more substances react with each other to produce a new 
substance by the rearrangement of their atoms, ions or molecules. 
Examples: 
? Burning of magnesium in air to form magnesium oxide. 
? Cooking of food 
? Rusting of iron 
? Burning of paper, wood, cloth, candle etc. 
? Digestion of food 
? Respiration 
? Metals react with acids to produce hydrogen gas. (candle or matchstick burns with a “pop up” sound in 
the presence of hydrogen) 
Characteristics of chemical reactions: 
a. The substance that reacts are known as reactants.  
b. The new substances formed after the chemical reaction are known as products. Burning of 
magnesium in the presence of oxygen to form magnesium oxide. Reactants – Magnesium and 
oxygen 
Product – Magnesium oxide 
c. A chemical reaction is accompanied by change in color, odor, temperature, state and absorption of 
release of energy in form of heat or light.  
Chemical equations 
 Representation of a chemical reaction in form of symbols of the substances involved; wherein the 
reactants are written on the left-hand side and the product on the right-hand side of an arrow showing the 
direction of the reaction.  
 Reactants      Products 
Magnesium reacts with oxygen to produce magnesium oxide 
   OR 
 Mg + O2    MgO 
          (Reactants)       (Product)  
Word equation 
A form of chemical equation in which the name of the reactants and products are written instead of their 
symbols.  
 Magnesium + Oxygen     Magnesium oxide 
        (Reactants)          (Product) 
Balancing a chemical reaction 
According to the law of conservation of mass matter can neither be created nor destroyed. Therefore, the 
number of atoms in the reactants and the products has to be exactly the same. 
This is made sure by balancing the chemical reaction 
Example 1:  
In the above reaction of Magnesium oxide there are two atoms of oxygen in the reactants however only one in 
the products. This means that the above reaction is incomplete.  
Mg + O2    MgO 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 1 
O 2 1 
 
Balancing a chemical reaction means equalizing the number of all the elements involved in a chemical reaction 
on both the sides.  
Step 1 – Start by balancing the element, which has the greatest number of atoms in the chemical reaction. In 
this example Oxygen has the maximum number of atoms so we start by balancing oxygen first.  
 Now if multiply the product (MgO) by 2 the number of oxygen atom on both sides will become 2, however the 
number of magnesium atom will become 2 in the product but remain 1 in the reactants.  
 
 Mg + O2    2MgO 
 
 
 
 
Step 2 – If we multiply 2 on the Mg of the reactants the number of magnesium will become 2 on both the sides 
without effecting the oxygen atoms on either of the side. 
 2Mg + O2    2MgO 
 
 
 
Now the number of magnesium atoms is same (2) on both sides  
The number of oxygen (2) atoms is also same in both reactants and products 
Therefore, we can say that the chemical reaction is now balanced. 
Step 3 – Write the physical states (s – solid; l – liquid; g - gas) of all the molecules  
Magnesium ribbon is solid; oxygen present in air is in gaseous form and magnesium oxide (smoke) produced in 
in gaseous form  
 2Mg (s) + O2 (g)    2MgO(g) 
 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 2 
O 2 2 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 2 2 
O 2 2 
Example 2 
Iron reacts with water to form iron oxide and hydrogen gas  
`Chemical reaction – Fe + H2O    Fe3O4 + H2 
  
 
 
 
Step 1 – Oxygen has the greatest number of atoms in the reaction so start by balancing oxygen first. 
If we multiply 4 with water (H2O) in the reactants the oxygen will get balanced  
Fe + 4H2O      Fe3O4 + H2 
 
 
 
 
Step 2 – Now hydron has the greatest number of atoms and is unbalanced so the next step is to balance 
hydrogen. 
If we multiply 4 with hydrogen on the products the number of hydrogen atoms will become equal (8) on both 
the sides.  
 Fe + 4H2O      Fe3O4 + 4H2 
 
 
 
 
Step 3 – Only iron is left unbalanced. There are three iron atoms on the right-hand side and only one on the left-
hand side 
If we multiply iron in the reactants with 3 it will get balanced  
3Fe + 4H2O      Fe3O4 + 4H2 
 
 
 
 
The number of atoms of iron (3), hydrogen (8) and oxygen (4) are equal in both reactants (left) and products 
(right). We can say that the chemical equation is balanced  
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 2 2 
O 1 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 2 8 
O 4 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 8 8 
O 4 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 3 3 
H 8 8 
O 4 4 
Page 4


Chapter 1 Chemical Reactions and Equations 
 
Chemical reaction – A process by which two or more substances react with each other to produce a new 
substance by the rearrangement of their atoms, ions or molecules. 
Examples: 
? Burning of magnesium in air to form magnesium oxide. 
? Cooking of food 
? Rusting of iron 
? Burning of paper, wood, cloth, candle etc. 
? Digestion of food 
? Respiration 
? Metals react with acids to produce hydrogen gas. (candle or matchstick burns with a “pop up” sound in 
the presence of hydrogen) 
Characteristics of chemical reactions: 
a. The substance that reacts are known as reactants.  
b. The new substances formed after the chemical reaction are known as products. Burning of 
magnesium in the presence of oxygen to form magnesium oxide. Reactants – Magnesium and 
oxygen 
Product – Magnesium oxide 
c. A chemical reaction is accompanied by change in color, odor, temperature, state and absorption of 
release of energy in form of heat or light.  
Chemical equations 
 Representation of a chemical reaction in form of symbols of the substances involved; wherein the 
reactants are written on the left-hand side and the product on the right-hand side of an arrow showing the 
direction of the reaction.  
 Reactants      Products 
Magnesium reacts with oxygen to produce magnesium oxide 
   OR 
 Mg + O2    MgO 
          (Reactants)       (Product)  
Word equation 
A form of chemical equation in which the name of the reactants and products are written instead of their 
symbols.  
 Magnesium + Oxygen     Magnesium oxide 
        (Reactants)          (Product) 
Balancing a chemical reaction 
According to the law of conservation of mass matter can neither be created nor destroyed. Therefore, the 
number of atoms in the reactants and the products has to be exactly the same. 
This is made sure by balancing the chemical reaction 
Example 1:  
In the above reaction of Magnesium oxide there are two atoms of oxygen in the reactants however only one in 
the products. This means that the above reaction is incomplete.  
Mg + O2    MgO 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 1 
O 2 1 
 
Balancing a chemical reaction means equalizing the number of all the elements involved in a chemical reaction 
on both the sides.  
Step 1 – Start by balancing the element, which has the greatest number of atoms in the chemical reaction. In 
this example Oxygen has the maximum number of atoms so we start by balancing oxygen first.  
 Now if multiply the product (MgO) by 2 the number of oxygen atom on both sides will become 2, however the 
number of magnesium atom will become 2 in the product but remain 1 in the reactants.  
 
 Mg + O2    2MgO 
 
 
 
 
Step 2 – If we multiply 2 on the Mg of the reactants the number of magnesium will become 2 on both the sides 
without effecting the oxygen atoms on either of the side. 
 2Mg + O2    2MgO 
 
 
 
Now the number of magnesium atoms is same (2) on both sides  
The number of oxygen (2) atoms is also same in both reactants and products 
Therefore, we can say that the chemical reaction is now balanced. 
Step 3 – Write the physical states (s – solid; l – liquid; g - gas) of all the molecules  
Magnesium ribbon is solid; oxygen present in air is in gaseous form and magnesium oxide (smoke) produced in 
in gaseous form  
 2Mg (s) + O2 (g)    2MgO(g) 
 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 2 
O 2 2 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 2 2 
O 2 2 
Example 2 
Iron reacts with water to form iron oxide and hydrogen gas  
`Chemical reaction – Fe + H2O    Fe3O4 + H2 
  
 
 
 
Step 1 – Oxygen has the greatest number of atoms in the reaction so start by balancing oxygen first. 
If we multiply 4 with water (H2O) in the reactants the oxygen will get balanced  
Fe + 4H2O      Fe3O4 + H2 
 
 
 
 
Step 2 – Now hydron has the greatest number of atoms and is unbalanced so the next step is to balance 
hydrogen. 
If we multiply 4 with hydrogen on the products the number of hydrogen atoms will become equal (8) on both 
the sides.  
 Fe + 4H2O      Fe3O4 + 4H2 
 
 
 
 
Step 3 – Only iron is left unbalanced. There are three iron atoms on the right-hand side and only one on the left-
hand side 
If we multiply iron in the reactants with 3 it will get balanced  
3Fe + 4H2O      Fe3O4 + 4H2 
 
 
 
 
The number of atoms of iron (3), hydrogen (8) and oxygen (4) are equal in both reactants (left) and products 
(right). We can say that the chemical equation is balanced  
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 2 2 
O 1 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 2 8 
O 4 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 8 8 
O 4 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 3 3 
H 8 8 
O 4 4 
Step 4 - Write the physical states (s – solid; l – liquid; g - gas) of all the molecules 
Iron is solid; water is in form of vapors and therefore in gaseous state; iron oxide or rust is in solid state and 
hydrogen is a gas.  
3Fe (s)+ 4H2O (g)          Fe3O4 (s) + 4H2 (g) 
 
 
Meaning of a balanced chemical equation 
a. 3Fe + 4H2O         Fe3O4 + 4H2 
b. 2Mg + O2          2MgO 
There are thousands of atoms present in a ribbon of magnesium or a piece of iron.  
3 molecules of iron react with 4 molecules of water to form 1 molecule of iron oxide and 4 molecules of 
hydrogen gas.                  Similarly, 2 molecules of magnesium 
react with one molecule of oxygen to produce 2 molecules of magnesium oxide.  
Can you guess how many molecules of magnesium will react with 4 molecules of oxygen? And how many 
molecules of magnesium oxide will be produced? 
 
The conditions require for a reaction to proceed are written on the arrow 
For example, the reaction of photosynthesis requires sunlight and chlorophyll, which are written above or below 
the arrow of the chemical equation. 
 
 6CO2 (aq.) + 6H2O (l)     C6H12O6 (aq.) + 6O2 (aq.)   
 
aq. means dissolved in water or in form of solution. 
 
Types of chemical reactions: 
1. Combination reaction 
A type of reaction in which two or more reactants combine to form one product is known as a combination 
reaction.  
e.g.  
i.   Calcium oxide or quick lime reacts with water to form calcium hydroxide or slaked lime 
 
  CaO (s) + H2O (l)       Ca(OH)2 (aq.) 
   (Calcium oxide   (water)    (Calcium hydorxide 
  Or Quick lime )      or slaked lime) 
Sunlight 
Chlorophyll 
Page 5


Chapter 1 Chemical Reactions and Equations 
 
Chemical reaction – A process by which two or more substances react with each other to produce a new 
substance by the rearrangement of their atoms, ions or molecules. 
Examples: 
? Burning of magnesium in air to form magnesium oxide. 
? Cooking of food 
? Rusting of iron 
? Burning of paper, wood, cloth, candle etc. 
? Digestion of food 
? Respiration 
? Metals react with acids to produce hydrogen gas. (candle or matchstick burns with a “pop up” sound in 
the presence of hydrogen) 
Characteristics of chemical reactions: 
a. The substance that reacts are known as reactants.  
b. The new substances formed after the chemical reaction are known as products. Burning of 
magnesium in the presence of oxygen to form magnesium oxide. Reactants – Magnesium and 
oxygen 
Product – Magnesium oxide 
c. A chemical reaction is accompanied by change in color, odor, temperature, state and absorption of 
release of energy in form of heat or light.  
Chemical equations 
 Representation of a chemical reaction in form of symbols of the substances involved; wherein the 
reactants are written on the left-hand side and the product on the right-hand side of an arrow showing the 
direction of the reaction.  
 Reactants      Products 
Magnesium reacts with oxygen to produce magnesium oxide 
   OR 
 Mg + O2    MgO 
          (Reactants)       (Product)  
Word equation 
A form of chemical equation in which the name of the reactants and products are written instead of their 
symbols.  
 Magnesium + Oxygen     Magnesium oxide 
        (Reactants)          (Product) 
Balancing a chemical reaction 
According to the law of conservation of mass matter can neither be created nor destroyed. Therefore, the 
number of atoms in the reactants and the products has to be exactly the same. 
This is made sure by balancing the chemical reaction 
Example 1:  
In the above reaction of Magnesium oxide there are two atoms of oxygen in the reactants however only one in 
the products. This means that the above reaction is incomplete.  
Mg + O2    MgO 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 1 
O 2 1 
 
Balancing a chemical reaction means equalizing the number of all the elements involved in a chemical reaction 
on both the sides.  
Step 1 – Start by balancing the element, which has the greatest number of atoms in the chemical reaction. In 
this example Oxygen has the maximum number of atoms so we start by balancing oxygen first.  
 Now if multiply the product (MgO) by 2 the number of oxygen atom on both sides will become 2, however the 
number of magnesium atom will become 2 in the product but remain 1 in the reactants.  
 
 Mg + O2    2MgO 
 
 
 
 
Step 2 – If we multiply 2 on the Mg of the reactants the number of magnesium will become 2 on both the sides 
without effecting the oxygen atoms on either of the side. 
 2Mg + O2    2MgO 
 
 
 
Now the number of magnesium atoms is same (2) on both sides  
The number of oxygen (2) atoms is also same in both reactants and products 
Therefore, we can say that the chemical reaction is now balanced. 
Step 3 – Write the physical states (s – solid; l – liquid; g - gas) of all the molecules  
Magnesium ribbon is solid; oxygen present in air is in gaseous form and magnesium oxide (smoke) produced in 
in gaseous form  
 2Mg (s) + O2 (g)    2MgO(g) 
 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 1 2 
O 2 2 
Element No. of atoms (Reactants) No. of atoms (Product) 
Mg 2 2 
O 2 2 
Example 2 
Iron reacts with water to form iron oxide and hydrogen gas  
`Chemical reaction – Fe + H2O    Fe3O4 + H2 
  
 
 
 
Step 1 – Oxygen has the greatest number of atoms in the reaction so start by balancing oxygen first. 
If we multiply 4 with water (H2O) in the reactants the oxygen will get balanced  
Fe + 4H2O      Fe3O4 + H2 
 
 
 
 
Step 2 – Now hydron has the greatest number of atoms and is unbalanced so the next step is to balance 
hydrogen. 
If we multiply 4 with hydrogen on the products the number of hydrogen atoms will become equal (8) on both 
the sides.  
 Fe + 4H2O      Fe3O4 + 4H2 
 
 
 
 
Step 3 – Only iron is left unbalanced. There are three iron atoms on the right-hand side and only one on the left-
hand side 
If we multiply iron in the reactants with 3 it will get balanced  
3Fe + 4H2O      Fe3O4 + 4H2 
 
 
 
 
The number of atoms of iron (3), hydrogen (8) and oxygen (4) are equal in both reactants (left) and products 
(right). We can say that the chemical equation is balanced  
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 2 2 
O 1 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 2 8 
O 4 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 1 3 
H 8 8 
O 4 4 
Element No. of atoms (Reactants) No. of atoms (Product) 
Fe 3 3 
H 8 8 
O 4 4 
Step 4 - Write the physical states (s – solid; l – liquid; g - gas) of all the molecules 
Iron is solid; water is in form of vapors and therefore in gaseous state; iron oxide or rust is in solid state and 
hydrogen is a gas.  
3Fe (s)+ 4H2O (g)          Fe3O4 (s) + 4H2 (g) 
 
 
Meaning of a balanced chemical equation 
a. 3Fe + 4H2O         Fe3O4 + 4H2 
b. 2Mg + O2          2MgO 
There are thousands of atoms present in a ribbon of magnesium or a piece of iron.  
3 molecules of iron react with 4 molecules of water to form 1 molecule of iron oxide and 4 molecules of 
hydrogen gas.                  Similarly, 2 molecules of magnesium 
react with one molecule of oxygen to produce 2 molecules of magnesium oxide.  
Can you guess how many molecules of magnesium will react with 4 molecules of oxygen? And how many 
molecules of magnesium oxide will be produced? 
 
The conditions require for a reaction to proceed are written on the arrow 
For example, the reaction of photosynthesis requires sunlight and chlorophyll, which are written above or below 
the arrow of the chemical equation. 
 
 6CO2 (aq.) + 6H2O (l)     C6H12O6 (aq.) + 6O2 (aq.)   
 
aq. means dissolved in water or in form of solution. 
 
Types of chemical reactions: 
1. Combination reaction 
A type of reaction in which two or more reactants combine to form one product is known as a combination 
reaction.  
e.g.  
i.   Calcium oxide or quick lime reacts with water to form calcium hydroxide or slaked lime 
 
  CaO (s) + H2O (l)       Ca(OH)2 (aq.) 
   (Calcium oxide   (water)    (Calcium hydorxide 
  Or Quick lime )      or slaked lime) 
Sunlight 
Chlorophyll 
Calcium hydroxide or slaked lime is used in white washing the walls of the buildings. Slaked lime reacts with 
the carbondioxide in the air to form a layer of calcium carbonate that gives the shiny appearance to the wals of 
the houses.  
  Ca(OH)2 (aq.) + CO2 (g)        CaCO3 (s) + H2O(l) 
  (Slaked lime) (Carbon dioxide)  (Calcium carbonate (shiny)) 
ii. Burining of coal 
 C (s) + O2 (g)             CO2 (g) 
        (Carbon)  (Oxygen)   (Carbon dioxide) 
iii. Water 
 2H2 + O2     2H2O 
 (Hydrogen) (Oxygen)               (Water) 
 
2. Decomposition reaction 
A type of reaction in which a reactant is broken down into two or more product is known as a 
decomposition reaction. 
e.g. 
 i. Ferrous sulphate when heated is broken down into ferric oxide, sulphur dioxide (pungent 
smell) and sulphur trioxide 
 
 2FeSO4 (s)
 Heat    
  Fe2O3 (s) + SO2 (g) + SO3 (g) 
          (Ferrous sulphate)   (Ferric oxide)   
 
ii.  Calcium carbonate or limestone when heated decomposes into quicklime and carbon dioxide 
 
CaCO3 (s)
 Heat    
 CaO (s) + CO2 (g) 
(calcium carbonate      (Calcium oxide 
Or Limestone)      or Quick lime) 
iii. Heating of lead nitrate produces lead oxide, nitrogen dioxide (brown fumes) and oxygen  
 
2Pb(NO3)2 (s)
  Heat   
       2PbO (s) + 4 NO2 + O2  
(Lead nitrate)         (Lead oxide) (Nitrogen  (Oxygen) 
            Dioxide) 
iv. Electrolysis of water 
     2H2O (l)
  
 
Electrolysis (passing electricity)
      2H2 + O2 
v. Photolysis of silver chloride (AgCl) and silver bromide (AgBr) 
2AgCl (s)
 Sunlight   
  2Ag (s) + Cl2 (g) 
(silver chloride)   (Silver)      (Chlorine) 
(White)    (grey)