Textbook: Chemical Reactions and Equations | Science and Technology Class 10 (Maharashtra SSC Board) PDF Download

 Page 1


30
1. What are the types of molecules of elements and compounds?
2. What is meant by valency of elements?
3. Chemical Reactions and Equations
Ø		 Chemical reactions                 Ø		 Rules of writing chemical reaction
Ø		 Balancing a chemical equation Ø 	 	 Types of chemical reactions
3. What is the requirement for writing molecular formulae of different compounds? How 
are the molecular formulae of the compounds written?
 In earlier standards we have seen how compounds are formed by chemical 
combination of elements. We have also learnt that the driving force behind formation of a 
chemical bond is to attain an electronic configuration with a complete octet.  The atoms 
attain a complete octet by giving, taking or sharing of electrons with each other.
Chemical Reaction
Some of the scientists of the 18
th
 and 19
th
 century carried out fundamental experiments 
on chemical reactions. They proved from their experiments that  during chemical reactions 
composition of the matter changes and that change remains permanent. On the contrary 
during physical change only the state of matter changes and this change is often  temporary 
in nature.
Identify physical and chemical changes from the phenomena given in the following table.
3.1 Some common phenomenon
Note :  Do the following activities in a group of friends. Take help of your teacher 
wherever necessary.  
Apparatus:  Thermometer, evaporating dish, tripod stand, 
funnel, Bunsen burner, etc.
Phenomenon Physical 
change                
Chemical
change   
1. Transformation of ice into water.
2. Cooking of food.
3. Ripening of fruit.
4. Milk turned in to curd.
5. Evaporation of water.
6. Digestion of food in the stomach.
7. Size reduction of naphtha balls exposed to air.
8. Staining of Shahbad or Kadappa tile by lemon juice.
9. Breaking of a glass object on falling from a height.
Chemicals : Lime stone powder, copper sulphate, calcium chloride, potassium chromate, 
zinc dust, sodium carbonate, phthalic anhydride, etc.
Procedure :  Carry out the  activities 1 to 5 as given below. Read and record the temperatures 
in the activities 2 to 4. 
Can you recall?
Try this.
Page 2


30
1. What are the types of molecules of elements and compounds?
2. What is meant by valency of elements?
3. Chemical Reactions and Equations
Ø		 Chemical reactions                 Ø		 Rules of writing chemical reaction
Ø		 Balancing a chemical equation Ø 	 	 Types of chemical reactions
3. What is the requirement for writing molecular formulae of different compounds? How 
are the molecular formulae of the compounds written?
 In earlier standards we have seen how compounds are formed by chemical 
combination of elements. We have also learnt that the driving force behind formation of a 
chemical bond is to attain an electronic configuration with a complete octet.  The atoms 
attain a complete octet by giving, taking or sharing of electrons with each other.
Chemical Reaction
Some of the scientists of the 18
th
 and 19
th
 century carried out fundamental experiments 
on chemical reactions. They proved from their experiments that  during chemical reactions 
composition of the matter changes and that change remains permanent. On the contrary 
during physical change only the state of matter changes and this change is often  temporary 
in nature.
Identify physical and chemical changes from the phenomena given in the following table.
3.1 Some common phenomenon
Note :  Do the following activities in a group of friends. Take help of your teacher 
wherever necessary.  
Apparatus:  Thermometer, evaporating dish, tripod stand, 
funnel, Bunsen burner, etc.
Phenomenon Physical 
change                
Chemical
change   
1. Transformation of ice into water.
2. Cooking of food.
3. Ripening of fruit.
4. Milk turned in to curd.
5. Evaporation of water.
6. Digestion of food in the stomach.
7. Size reduction of naphtha balls exposed to air.
8. Staining of Shahbad or Kadappa tile by lemon juice.
9. Breaking of a glass object on falling from a height.
Chemicals : Lime stone powder, copper sulphate, calcium chloride, potassium chromate, 
zinc dust, sodium carbonate, phthalic anhydride, etc.
Procedure :  Carry out the  activities 1 to 5 as given below. Read and record the temperatures 
in the activities 2 to 4. 
Can you recall?
Try this.
31
A physical change takes place due to change in the parameters such as temperature, 
pressure. Often a physical change in reversible. The composition of matter remains the 
same in a physical change. For example, ice is transformed into water on heating and 
water is transformed into ice on cooling. On the contrary, if the composition of matter 
changes during a process then it is called a chemical change. When we call a particular 
process or phenomenon as a chemical change, some chemical reactions are taking place 
in the concerned matter. 
A chemical reaction is a process in which some substances undergo bond breaking 
and are transformed into new substances by formation of new bonds. The substances 
taking part in chemical reaction are called reactants, whereas the substances formed as a 
result of a chemical reaction by formation of new bonds are called products. For example, 
formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In 
this reaction coal (carbon) and oxygen (from air) are the reactants while carbon dioxide 
is the product. A chemical reaction is represented by writing a chemical equation.
Activity Colour 
change 
(if present)
Gas released 
(yes/no)
Temperature 
change
 (if present)
Nature of change 
(chemical /physical)
1
2
3
4
5
Find out 
Observe and keep a record of the physical and chemical changes 
that you experience in your daily life.
3.3 Observation table
1. Take a spoonful of lime stone powder in  an 
evaporating dish. Heat it strongly on a high blue 
flame. 
2. Add zinc (Zn) dust into the copper sulphate 
(CuSO
4
) solution.
3. Add potassium chromate (K
2
CrO
4
) solution to      
barium sulphate (BaSO
4
) solution. 
4. Add sodium carbonate (Na
2
CO
3
) solution to the  
calcium  chloride (CaCl
2
) solution.
5. Take phthalic anhydride in the evaporating dish.  
      Close  the end of the stem of a funnel with a 
cotton  plug. Keep this funnel inverted on the 
evaporating dish. Heat the  evaporating  dish on 
a tripod stand slowly on a low flame. What did 
you observe in the funnel during heating?
 Record the observation of all the activities. What did you find?
   Complete the following observation table with reference to the activities 1 to 5.
3.2 To heat lime stone powder
Lime stone 
powder
Bunsen 
burner
Page 3


30
1. What are the types of molecules of elements and compounds?
2. What is meant by valency of elements?
3. Chemical Reactions and Equations
Ø		 Chemical reactions                 Ø		 Rules of writing chemical reaction
Ø		 Balancing a chemical equation Ø 	 	 Types of chemical reactions
3. What is the requirement for writing molecular formulae of different compounds? How 
are the molecular formulae of the compounds written?
 In earlier standards we have seen how compounds are formed by chemical 
combination of elements. We have also learnt that the driving force behind formation of a 
chemical bond is to attain an electronic configuration with a complete octet.  The atoms 
attain a complete octet by giving, taking or sharing of electrons with each other.
Chemical Reaction
Some of the scientists of the 18
th
 and 19
th
 century carried out fundamental experiments 
on chemical reactions. They proved from their experiments that  during chemical reactions 
composition of the matter changes and that change remains permanent. On the contrary 
during physical change only the state of matter changes and this change is often  temporary 
in nature.
Identify physical and chemical changes from the phenomena given in the following table.
3.1 Some common phenomenon
Note :  Do the following activities in a group of friends. Take help of your teacher 
wherever necessary.  
Apparatus:  Thermometer, evaporating dish, tripod stand, 
funnel, Bunsen burner, etc.
Phenomenon Physical 
change                
Chemical
change   
1. Transformation of ice into water.
2. Cooking of food.
3. Ripening of fruit.
4. Milk turned in to curd.
5. Evaporation of water.
6. Digestion of food in the stomach.
7. Size reduction of naphtha balls exposed to air.
8. Staining of Shahbad or Kadappa tile by lemon juice.
9. Breaking of a glass object on falling from a height.
Chemicals : Lime stone powder, copper sulphate, calcium chloride, potassium chromate, 
zinc dust, sodium carbonate, phthalic anhydride, etc.
Procedure :  Carry out the  activities 1 to 5 as given below. Read and record the temperatures 
in the activities 2 to 4. 
Can you recall?
Try this.
31
A physical change takes place due to change in the parameters such as temperature, 
pressure. Often a physical change in reversible. The composition of matter remains the 
same in a physical change. For example, ice is transformed into water on heating and 
water is transformed into ice on cooling. On the contrary, if the composition of matter 
changes during a process then it is called a chemical change. When we call a particular 
process or phenomenon as a chemical change, some chemical reactions are taking place 
in the concerned matter. 
A chemical reaction is a process in which some substances undergo bond breaking 
and are transformed into new substances by formation of new bonds. The substances 
taking part in chemical reaction are called reactants, whereas the substances formed as a 
result of a chemical reaction by formation of new bonds are called products. For example, 
formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In 
this reaction coal (carbon) and oxygen (from air) are the reactants while carbon dioxide 
is the product. A chemical reaction is represented by writing a chemical equation.
Activity Colour 
change 
(if present)
Gas released 
(yes/no)
Temperature 
change
 (if present)
Nature of change 
(chemical /physical)
1
2
3
4
5
Find out 
Observe and keep a record of the physical and chemical changes 
that you experience in your daily life.
3.3 Observation table
1. Take a spoonful of lime stone powder in  an 
evaporating dish. Heat it strongly on a high blue 
flame. 
2. Add zinc (Zn) dust into the copper sulphate 
(CuSO
4
) solution.
3. Add potassium chromate (K
2
CrO
4
) solution to      
barium sulphate (BaSO
4
) solution. 
4. Add sodium carbonate (Na
2
CO
3
) solution to the  
calcium  chloride (CaCl
2
) solution.
5. Take phthalic anhydride in the evaporating dish.  
      Close  the end of the stem of a funnel with a 
cotton  plug. Keep this funnel inverted on the 
evaporating dish. Heat the  evaporating  dish on 
a tripod stand slowly on a low flame. What did 
you observe in the funnel during heating?
 Record the observation of all the activities. What did you find?
   Complete the following observation table with reference to the activities 1 to 5.
3.2 To heat lime stone powder
Lime stone 
powder
Bunsen 
burner
32
Chemical equations
 Let us first look at a chemical reaction. In the activity 2, a colourless solution of zinc 
sulphate (ZnSO
4
) is formed on addition of zinc dust to the blue solution of copper sulphate 
(CuSO
4
). This chemical reaction can be written in brief as follows.
Aqueous solution of  copper sulphate + zinc dust  Aqueous solution of zinc  
                sulphate + copper .......... (1)
 This simple way of representing a chemical reaction in words is called a ‘Word Equation’. 
 A word equation can be written in a further condensed form  by using chemical formulae 
as follows.
    CuSO
4
 + Zn     ZnSO
4
  + Cu.................(2)
 The representation of a chemical reaction in a condensed form using chemical formulae 
is called as the chemical equation. In the above equation copper sulphate (CuSO
4
) and zinc 
(Zn) are the reactants. They react with each other to form copper particles (Cu) and a 
solution of the colourless zinc sulphate (ZnSO
4
) as the products having totally different 
properties. The ionic bond in the reactant CuSO
4
 breaks and the ionic bond in the product 
ZnSO
4
 is formed during the reaction.
Writing a Chemical Equation
Let us now see the conventions followed while writing a chemical equation.
1. In a chemical equation the reactants are written on the left hand side while the 
products on the right hand  side. An arrow heading towards the products is drawn in between 
them. This arrow indicates the direction of the reaction.
2. If the reactants or products are two or more, they are linked with a plus sign (+) in 
between them. For example, in the equation (2) a plus sign (+) is drawn in between the 
reactants CuSO
4
 and Zn. Similarly, a plus sign (+) is drawn in between the products ZnSO
4
 
and Cu.
3. To make the chemical equation more informative the physical states of the reactants 
are indicated in the equation. Their gaseous, liquid and solid states are indicated by  writing 
the letters (g), (l) and (s), respectively in the brackets. Moreover, if the product is gaseous, 
instead of (g) it can be indicated by an arrow  pointing upwards. If the product formed is 
insoluble solid, in the form of a precipitate, then instead of (s) it can be indicated by an 
arrow  pointing downwards. When reactants and products are in the form of solution in 
water, they are said to be present in aqueous solution state. This state is indicated by putting 
the letters aq in brackets after their formula. Thus, the equation (2) is rewritten as equation 
(3) shown below.
CuSO
4
 (aq) + Zn (s)  ZnSO
4
 (aq) + Cu (s) ..................... (3)
4. When heat is to be given from outside to bring about a reaction, it is indicated by the 
sign rwritten above the arrow that indicates the direction of the reaction. For example, the 
reaction in which slaked lime is formed on heating lime stone is written as follows.
         CaCO
3 
(s)  
D
   CaO (s) + CO
2
  ........................ (4)
Similarly, the fact that heat is released during the reaction between the aqueous solution 
of copper sulphate and zinc dust is indicated as follows.
CuSO
4
 (aq) + Zn (s)  ZnSO
4
 (aq) + Cu (s) + Heat ..................... (5)
5. It is necessary to fulfill certain conditions like specific temperature, pressure, catalyst, 
etc. to bring about some reactions. These conditions are indicated below or above the arrow 
indicating the direction of the reaction. For example, the reaction of a vegetable oil takes 
place at the temperature of 60 
0
C with hydrogen gas in presence of the Ni catalyst and is 
written as follows.
Page 4


30
1. What are the types of molecules of elements and compounds?
2. What is meant by valency of elements?
3. Chemical Reactions and Equations
Ø		 Chemical reactions                 Ø		 Rules of writing chemical reaction
Ø		 Balancing a chemical equation Ø 	 	 Types of chemical reactions
3. What is the requirement for writing molecular formulae of different compounds? How 
are the molecular formulae of the compounds written?
 In earlier standards we have seen how compounds are formed by chemical 
combination of elements. We have also learnt that the driving force behind formation of a 
chemical bond is to attain an electronic configuration with a complete octet.  The atoms 
attain a complete octet by giving, taking or sharing of electrons with each other.
Chemical Reaction
Some of the scientists of the 18
th
 and 19
th
 century carried out fundamental experiments 
on chemical reactions. They proved from their experiments that  during chemical reactions 
composition of the matter changes and that change remains permanent. On the contrary 
during physical change only the state of matter changes and this change is often  temporary 
in nature.
Identify physical and chemical changes from the phenomena given in the following table.
3.1 Some common phenomenon
Note :  Do the following activities in a group of friends. Take help of your teacher 
wherever necessary.  
Apparatus:  Thermometer, evaporating dish, tripod stand, 
funnel, Bunsen burner, etc.
Phenomenon Physical 
change                
Chemical
change   
1. Transformation of ice into water.
2. Cooking of food.
3. Ripening of fruit.
4. Milk turned in to curd.
5. Evaporation of water.
6. Digestion of food in the stomach.
7. Size reduction of naphtha balls exposed to air.
8. Staining of Shahbad or Kadappa tile by lemon juice.
9. Breaking of a glass object on falling from a height.
Chemicals : Lime stone powder, copper sulphate, calcium chloride, potassium chromate, 
zinc dust, sodium carbonate, phthalic anhydride, etc.
Procedure :  Carry out the  activities 1 to 5 as given below. Read and record the temperatures 
in the activities 2 to 4. 
Can you recall?
Try this.
31
A physical change takes place due to change in the parameters such as temperature, 
pressure. Often a physical change in reversible. The composition of matter remains the 
same in a physical change. For example, ice is transformed into water on heating and 
water is transformed into ice on cooling. On the contrary, if the composition of matter 
changes during a process then it is called a chemical change. When we call a particular 
process or phenomenon as a chemical change, some chemical reactions are taking place 
in the concerned matter. 
A chemical reaction is a process in which some substances undergo bond breaking 
and are transformed into new substances by formation of new bonds. The substances 
taking part in chemical reaction are called reactants, whereas the substances formed as a 
result of a chemical reaction by formation of new bonds are called products. For example, 
formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In 
this reaction coal (carbon) and oxygen (from air) are the reactants while carbon dioxide 
is the product. A chemical reaction is represented by writing a chemical equation.
Activity Colour 
change 
(if present)
Gas released 
(yes/no)
Temperature 
change
 (if present)
Nature of change 
(chemical /physical)
1
2
3
4
5
Find out 
Observe and keep a record of the physical and chemical changes 
that you experience in your daily life.
3.3 Observation table
1. Take a spoonful of lime stone powder in  an 
evaporating dish. Heat it strongly on a high blue 
flame. 
2. Add zinc (Zn) dust into the copper sulphate 
(CuSO
4
) solution.
3. Add potassium chromate (K
2
CrO
4
) solution to      
barium sulphate (BaSO
4
) solution. 
4. Add sodium carbonate (Na
2
CO
3
) solution to the  
calcium  chloride (CaCl
2
) solution.
5. Take phthalic anhydride in the evaporating dish.  
      Close  the end of the stem of a funnel with a 
cotton  plug. Keep this funnel inverted on the 
evaporating dish. Heat the  evaporating  dish on 
a tripod stand slowly on a low flame. What did 
you observe in the funnel during heating?
 Record the observation of all the activities. What did you find?
   Complete the following observation table with reference to the activities 1 to 5.
3.2 To heat lime stone powder
Lime stone 
powder
Bunsen 
burner
32
Chemical equations
 Let us first look at a chemical reaction. In the activity 2, a colourless solution of zinc 
sulphate (ZnSO
4
) is formed on addition of zinc dust to the blue solution of copper sulphate 
(CuSO
4
). This chemical reaction can be written in brief as follows.
Aqueous solution of  copper sulphate + zinc dust  Aqueous solution of zinc  
                sulphate + copper .......... (1)
 This simple way of representing a chemical reaction in words is called a ‘Word Equation’. 
 A word equation can be written in a further condensed form  by using chemical formulae 
as follows.
    CuSO
4
 + Zn     ZnSO
4
  + Cu.................(2)
 The representation of a chemical reaction in a condensed form using chemical formulae 
is called as the chemical equation. In the above equation copper sulphate (CuSO
4
) and zinc 
(Zn) are the reactants. They react with each other to form copper particles (Cu) and a 
solution of the colourless zinc sulphate (ZnSO
4
) as the products having totally different 
properties. The ionic bond in the reactant CuSO
4
 breaks and the ionic bond in the product 
ZnSO
4
 is formed during the reaction.
Writing a Chemical Equation
Let us now see the conventions followed while writing a chemical equation.
1. In a chemical equation the reactants are written on the left hand side while the 
products on the right hand  side. An arrow heading towards the products is drawn in between 
them. This arrow indicates the direction of the reaction.
2. If the reactants or products are two or more, they are linked with a plus sign (+) in 
between them. For example, in the equation (2) a plus sign (+) is drawn in between the 
reactants CuSO
4
 and Zn. Similarly, a plus sign (+) is drawn in between the products ZnSO
4
 
and Cu.
3. To make the chemical equation more informative the physical states of the reactants 
are indicated in the equation. Their gaseous, liquid and solid states are indicated by  writing 
the letters (g), (l) and (s), respectively in the brackets. Moreover, if the product is gaseous, 
instead of (g) it can be indicated by an arrow  pointing upwards. If the product formed is 
insoluble solid, in the form of a precipitate, then instead of (s) it can be indicated by an 
arrow  pointing downwards. When reactants and products are in the form of solution in 
water, they are said to be present in aqueous solution state. This state is indicated by putting 
the letters aq in brackets after their formula. Thus, the equation (2) is rewritten as equation 
(3) shown below.
CuSO
4
 (aq) + Zn (s)  ZnSO
4
 (aq) + Cu (s) ..................... (3)
4. When heat is to be given from outside to bring about a reaction, it is indicated by the 
sign rwritten above the arrow that indicates the direction of the reaction. For example, the 
reaction in which slaked lime is formed on heating lime stone is written as follows.
         CaCO
3 
(s)  
D
   CaO (s) + CO
2
  ........................ (4)
Similarly, the fact that heat is released during the reaction between the aqueous solution 
of copper sulphate and zinc dust is indicated as follows.
CuSO
4
 (aq) + Zn (s)  ZnSO
4
 (aq) + Cu (s) + Heat ..................... (5)
5. It is necessary to fulfill certain conditions like specific temperature, pressure, catalyst, 
etc. to bring about some reactions. These conditions are indicated below or above the arrow 
indicating the direction of the reaction. For example, the reaction of a vegetable oil takes 
place at the temperature of 60 
0
C with hydrogen gas in presence of the Ni catalyst and is 
written as follows.
33
Apparatus: Test tube, conical flask, balance, etc.
Chemicals : Sodium chloride and silver nitrate.
Silver nitrate is used in the voters-ink. 
Procedure :
1. Take sodium chloride solution in a conical flask and silver nitrate solution in a test tube.
2. Tie a thread to the test tube and insert it carefully into the conical flask. Make the  
conical flask air tight by fitting a rubber cork.
3.  Weigh the conical flask with the help of a balance.
4.  Now tilt the conical flask and mix the solution present in the test tube with the solution 
in the conical flask.
5.  Weigh the conical flask again.
 Which changes did you find? Did any  insoluble substance form? Was there any 
change in the weight? 
A word equation is written for the above activity as shown below.
 Silver nitrate + Sodium chloride      Silver chloride + Sodium nitrate
The above word equation is represented by the following chemical equation.
(Concentrated) 
However, on reaction with dilute nitric acid, the product formed is nitric oxide gas.
3Cu(s) + 8HNO
3
(aq)  3Cu(NO
3
)
2
(aq) + 2NO(g) + 4H
2
O(l) ...........(8)
(dilute)
6 . Special information or names of reactants/ products are written below their 
formulae. For example, copper on reaction with concentrated nitric acid gives reddish 
coloured poisonous nitrogen di oxide gas.
Cu(s) + 4 HNO
3
(aq)    Cu(NO
3
)
2
(aq) + 2NO
2
(g) + 2H
2
O(l) ...........(7)
What are the other uses of silver nitrate in everyday life? Find out
Vegetable oil  (l) + H
2
(g)       Vanaspathi ghee (s)............. (6)
Try this.
Sodium chloride 
solution
 Silver nitrate solution
Conical flask
Test tube
Balance
3.4  The reaction of sodium chloride with silver nitrate
Do you know ?
60
0 
C
Ni Catalyst
AgNO
3
(aq)+ NaCl(aq)  AgCl   + NaNO
3
(aq) ........ (9)
                          (white)
Page 5


30
1. What are the types of molecules of elements and compounds?
2. What is meant by valency of elements?
3. Chemical Reactions and Equations
Ø		 Chemical reactions                 Ø		 Rules of writing chemical reaction
Ø		 Balancing a chemical equation Ø 	 	 Types of chemical reactions
3. What is the requirement for writing molecular formulae of different compounds? How 
are the molecular formulae of the compounds written?
 In earlier standards we have seen how compounds are formed by chemical 
combination of elements. We have also learnt that the driving force behind formation of a 
chemical bond is to attain an electronic configuration with a complete octet.  The atoms 
attain a complete octet by giving, taking or sharing of electrons with each other.
Chemical Reaction
Some of the scientists of the 18
th
 and 19
th
 century carried out fundamental experiments 
on chemical reactions. They proved from their experiments that  during chemical reactions 
composition of the matter changes and that change remains permanent. On the contrary 
during physical change only the state of matter changes and this change is often  temporary 
in nature.
Identify physical and chemical changes from the phenomena given in the following table.
3.1 Some common phenomenon
Note :  Do the following activities in a group of friends. Take help of your teacher 
wherever necessary.  
Apparatus:  Thermometer, evaporating dish, tripod stand, 
funnel, Bunsen burner, etc.
Phenomenon Physical 
change                
Chemical
change   
1. Transformation of ice into water.
2. Cooking of food.
3. Ripening of fruit.
4. Milk turned in to curd.
5. Evaporation of water.
6. Digestion of food in the stomach.
7. Size reduction of naphtha balls exposed to air.
8. Staining of Shahbad or Kadappa tile by lemon juice.
9. Breaking of a glass object on falling from a height.
Chemicals : Lime stone powder, copper sulphate, calcium chloride, potassium chromate, 
zinc dust, sodium carbonate, phthalic anhydride, etc.
Procedure :  Carry out the  activities 1 to 5 as given below. Read and record the temperatures 
in the activities 2 to 4. 
Can you recall?
Try this.
31
A physical change takes place due to change in the parameters such as temperature, 
pressure. Often a physical change in reversible. The composition of matter remains the 
same in a physical change. For example, ice is transformed into water on heating and 
water is transformed into ice on cooling. On the contrary, if the composition of matter 
changes during a process then it is called a chemical change. When we call a particular 
process or phenomenon as a chemical change, some chemical reactions are taking place 
in the concerned matter. 
A chemical reaction is a process in which some substances undergo bond breaking 
and are transformed into new substances by formation of new bonds. The substances 
taking part in chemical reaction are called reactants, whereas the substances formed as a 
result of a chemical reaction by formation of new bonds are called products. For example, 
formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In 
this reaction coal (carbon) and oxygen (from air) are the reactants while carbon dioxide 
is the product. A chemical reaction is represented by writing a chemical equation.
Activity Colour 
change 
(if present)
Gas released 
(yes/no)
Temperature 
change
 (if present)
Nature of change 
(chemical /physical)
1
2
3
4
5
Find out 
Observe and keep a record of the physical and chemical changes 
that you experience in your daily life.
3.3 Observation table
1. Take a spoonful of lime stone powder in  an 
evaporating dish. Heat it strongly on a high blue 
flame. 
2. Add zinc (Zn) dust into the copper sulphate 
(CuSO
4
) solution.
3. Add potassium chromate (K
2
CrO
4
) solution to      
barium sulphate (BaSO
4
) solution. 
4. Add sodium carbonate (Na
2
CO
3
) solution to the  
calcium  chloride (CaCl
2
) solution.
5. Take phthalic anhydride in the evaporating dish.  
      Close  the end of the stem of a funnel with a 
cotton  plug. Keep this funnel inverted on the 
evaporating dish. Heat the  evaporating  dish on 
a tripod stand slowly on a low flame. What did 
you observe in the funnel during heating?
 Record the observation of all the activities. What did you find?
   Complete the following observation table with reference to the activities 1 to 5.
3.2 To heat lime stone powder
Lime stone 
powder
Bunsen 
burner
32
Chemical equations
 Let us first look at a chemical reaction. In the activity 2, a colourless solution of zinc 
sulphate (ZnSO
4
) is formed on addition of zinc dust to the blue solution of copper sulphate 
(CuSO
4
). This chemical reaction can be written in brief as follows.
Aqueous solution of  copper sulphate + zinc dust  Aqueous solution of zinc  
                sulphate + copper .......... (1)
 This simple way of representing a chemical reaction in words is called a ‘Word Equation’. 
 A word equation can be written in a further condensed form  by using chemical formulae 
as follows.
    CuSO
4
 + Zn     ZnSO
4
  + Cu.................(2)
 The representation of a chemical reaction in a condensed form using chemical formulae 
is called as the chemical equation. In the above equation copper sulphate (CuSO
4
) and zinc 
(Zn) are the reactants. They react with each other to form copper particles (Cu) and a 
solution of the colourless zinc sulphate (ZnSO
4
) as the products having totally different 
properties. The ionic bond in the reactant CuSO
4
 breaks and the ionic bond in the product 
ZnSO
4
 is formed during the reaction.
Writing a Chemical Equation
Let us now see the conventions followed while writing a chemical equation.
1. In a chemical equation the reactants are written on the left hand side while the 
products on the right hand  side. An arrow heading towards the products is drawn in between 
them. This arrow indicates the direction of the reaction.
2. If the reactants or products are two or more, they are linked with a plus sign (+) in 
between them. For example, in the equation (2) a plus sign (+) is drawn in between the 
reactants CuSO
4
 and Zn. Similarly, a plus sign (+) is drawn in between the products ZnSO
4
 
and Cu.
3. To make the chemical equation more informative the physical states of the reactants 
are indicated in the equation. Their gaseous, liquid and solid states are indicated by  writing 
the letters (g), (l) and (s), respectively in the brackets. Moreover, if the product is gaseous, 
instead of (g) it can be indicated by an arrow  pointing upwards. If the product formed is 
insoluble solid, in the form of a precipitate, then instead of (s) it can be indicated by an 
arrow  pointing downwards. When reactants and products are in the form of solution in 
water, they are said to be present in aqueous solution state. This state is indicated by putting 
the letters aq in brackets after their formula. Thus, the equation (2) is rewritten as equation 
(3) shown below.
CuSO
4
 (aq) + Zn (s)  ZnSO
4
 (aq) + Cu (s) ..................... (3)
4. When heat is to be given from outside to bring about a reaction, it is indicated by the 
sign rwritten above the arrow that indicates the direction of the reaction. For example, the 
reaction in which slaked lime is formed on heating lime stone is written as follows.
         CaCO
3 
(s)  
D
   CaO (s) + CO
2
  ........................ (4)
Similarly, the fact that heat is released during the reaction between the aqueous solution 
of copper sulphate and zinc dust is indicated as follows.
CuSO
4
 (aq) + Zn (s)  ZnSO
4
 (aq) + Cu (s) + Heat ..................... (5)
5. It is necessary to fulfill certain conditions like specific temperature, pressure, catalyst, 
etc. to bring about some reactions. These conditions are indicated below or above the arrow 
indicating the direction of the reaction. For example, the reaction of a vegetable oil takes 
place at the temperature of 60 
0
C with hydrogen gas in presence of the Ni catalyst and is 
written as follows.
33
Apparatus: Test tube, conical flask, balance, etc.
Chemicals : Sodium chloride and silver nitrate.
Silver nitrate is used in the voters-ink. 
Procedure :
1. Take sodium chloride solution in a conical flask and silver nitrate solution in a test tube.
2. Tie a thread to the test tube and insert it carefully into the conical flask. Make the  
conical flask air tight by fitting a rubber cork.
3.  Weigh the conical flask with the help of a balance.
4.  Now tilt the conical flask and mix the solution present in the test tube with the solution 
in the conical flask.
5.  Weigh the conical flask again.
 Which changes did you find? Did any  insoluble substance form? Was there any 
change in the weight? 
A word equation is written for the above activity as shown below.
 Silver nitrate + Sodium chloride      Silver chloride + Sodium nitrate
The above word equation is represented by the following chemical equation.
(Concentrated) 
However, on reaction with dilute nitric acid, the product formed is nitric oxide gas.
3Cu(s) + 8HNO
3
(aq)  3Cu(NO
3
)
2
(aq) + 2NO(g) + 4H
2
O(l) ...........(8)
(dilute)
6 . Special information or names of reactants/ products are written below their 
formulae. For example, copper on reaction with concentrated nitric acid gives reddish 
coloured poisonous nitrogen di oxide gas.
Cu(s) + 4 HNO
3
(aq)    Cu(NO
3
)
2
(aq) + 2NO
2
(g) + 2H
2
O(l) ...........(7)
What are the other uses of silver nitrate in everyday life? Find out
Vegetable oil  (l) + H
2
(g)       Vanaspathi ghee (s)............. (6)
Try this.
Sodium chloride 
solution
 Silver nitrate solution
Conical flask
Test tube
Balance
3.4  The reaction of sodium chloride with silver nitrate
Do you know ?
60
0 
C
Ni Catalyst
AgNO
3
(aq)+ NaCl(aq)  AgCl   + NaNO
3
(aq) ........ (9)
                          (white)
34
Balancing a Chemical Equation
Complete the table aside on the 
basis of the equation (9).
It is seen that the number of atoms 
of the elements in the reactants in this 
equation is same as the number of 
atoms of those elements in the products. 
Such an equation is called a ‘balanced 
equation’. If the number of atoms of 
each element is not the same on the two 
sides of an equation, it is called an 
‘unbalanced equation’.
Reactants
(Left side) 
Products
(Right side)
Element Number of 
atoms
Number of 
atoms
Ag
N
O
Na
Cl
3.5 Details of equation (9)
In any reaction, the total mass of each of the elements in the 
reactants is same as the total mass of each of the respective 
elements in the products. This is in accordance with the  law of 
conservation of mass that you studied in the previous standard.
Steps in balancing a chemical reaction 
A chemical equation is balanced step by step. A trial and error method is used for this 
purpose. Consider the following equation as an example : 
Sodium hydroxide + Sulphuric acid  Sodium sulphate + water. 
STEP I. Write the chemical equation from the given word equation. 
NaOH + H
2
SO
4
   Na
2
SO
4
+ H
2
O ..............(10)
STEP III : It is convenient to start balancing an equation from the compound which 
contains the maximum number of atoms. Moreover it is convenient to first consider that 
element in this compound, which has unequal number of atoms on the two sides. 
Reactants 
(Left side)
Products 
(Right side)
Element Number of atoms Number of atoms
Na 1 2
O 5 5
H 3 2
S 1 1
Number of 
sodium 
atoms
In the 
Reactants 
(in NaOH)
In the 
Products  
(in Na
2
SO
4
)
Initially 1 2
To balance 1 x 2 2
Always remember
STEP II. Check whether the 
equation (10) is balanced or not by 
comparing the  number of atoms of 
the various elements present on the 
two sides of the equation. 
It is seen that the number of 
atoms of all the elements on the two  
sides are not the same. It means that 
the equation (10) is not balanced.
(i) In the equation (10), there are two 
components Na
2
SO
4
 and H
2
SO
4
, which contain 
the maximum number that is seven atoms each. 
Any one of them can be selected. Select the 
compound Na
2
SO
4
. Further select sodium for 
balancing as the number of atoms of  sodium 
in this compound is unequal on the two sides. It 
should be remembered that, the formula of a compound cannot be changed while balancing 
the number of atoms. It means that, here to make the number of sodium atoms in the 
reactants as ‘2’ the formula NaOH cannot be changed to Na
2
OH. Instead a factor of ‘2’ 
will have to be applied to NaOH. Write down the resulting equation (10)
/ 
on doing this.