“Facts are not science — as the dictionary is not literature.”
Martin H. Fischer
CHAPTER 1
Chemical Reactions
and Equations
C onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn the
magnesium ribbon keeping it
away as far as possible from
your eyes. Figure 1.1
n What do you observe? Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
2024-25
, You must have observed that magnesium ribbon burns with a
dazzling white flame and changes into a white powder. This powder is
magnesium oxide. It is formed due to the reaction between magnesium
and oxygen present in the air.
Activity 1.2 Activity 1.3
n Take lead nitrate n Take a few zinc granules in a conical flask or a test tube.
solution in a test n Add dilute hydrochloric acid or sulphuric acid to this
tube. (Fig. 1.2).
n Add potassium CAUTION: Handle the acid with care.
iodide solution n Do you observe anything happening around the zinc
to this. granules?
n What do you n Touch the conical flask or test tube. Is there any change in
observe? its temperature?
From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
n change in state
n change in colour
n evolution of a gas
n change in temperature.
As we observe the changes around us, we can see
that there is a large variety of chemical reactions taking
place around us. We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter.
Figure 1.2 1.1 CHEMIC
CHEMICAL EQUATIONS
AL EQUATIONS
Formation of hydrogen
Activity 1.1 can be described as – when a magnesium ribbon is burnt in
gas by the action of
dilute sulphuric acid on oxygen, it gets converted to magnesium oxide. This description of a
zinc chemical reaction in a sentence form is quite long. It can be written in a
shorter form. The simplest way to do this is to write it in the form of a
word-equation.
The word-equation for the above reaction would be –
Magnesium + Oxygen → Magnesium oxide (1.1)
(Reactants) (Product)
The substances that undergo chemical change in the reaction (1.1),
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
A word-equation shows change of reactants to products through an
arrow placed between them. The reactants are written on the left-hand
side (LHS) with a plus sign (+) between them. Similarly, products are
written on the right-hand side (RHS) with a plus sign (+) between them.
The arrowhead points towards the products, and shows the direction of
the reaction.
2 Science
2024-25
, 1.1.1 Writing a Chemical Equation
Is there any other shorter way for representing chemical equations?
Chemical equations can be made more concise and useful if we use
chemical formulae instead of words. A chemical equation represents a
chemical reaction. If you recall formulae of magnesium, oxygen and
magnesium oxide, the above word-equation can be written as –
Mg + O2 → MgO (1.2)
Count and compare the number of atoms of each element on the
LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If yes, then the equation is balanced. If not,
then the equation is unbalanced because the mass is not the same on
both sides of the equation. Such a chemical equation is a skeletal
chemical equation for a reaction. Equation (1.2) is a skeletal chemical
equation for the burning of magnesium in air.
1.1.2 Balanced Chemical Equations
Recall the law of conservation of mass that you studied in Class IX; mass
can neither be created nor destroyed in a chemical reaction. That is, the
total mass of the elements present in the products of a chemical reaction
has to be equal to the total mass of the elements present in the reactants.
In other words, the number of atoms of each element remains the
same, before and after a chemical reaction. Hence, we need to balance a
skeletal chemical equation. Is the chemical Eq. (1.2) balanced? Let us
learn about balancing a chemical equation step by step.
The word-equation for Activity 1.3 may be represented as –
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
The above word-equation may be represented by the following
chemical equation –
Zn + H2SO4 → ZnSO4 + H2 (1.3)
Let us examine the number of atoms of different elements on both
sides of the arrow.
Element Number of atoms in Number of atoms
reactants (LHS) in products (RHS)
Zn 1 1
H 2 2
S 1 1
O 4 4
As the number of atoms of each element is the same on both sides of
the arrow, Eq. (1.3) is a balanced chemical equation.
Let us try to balance the following chemical equation –
Fe + H2O → Fe3O4 + H2 (1.4)
Chemical Reactions and Equations 3
2024-25
Martin H. Fischer
CHAPTER 1
Chemical Reactions
and Equations
C onsider the following situations of daily life and think what happens
when –
n milk is left at room temperature during summers.
n an iron tawa/pan/nail is left exposed to humid atmosphere.
n grapes get fermented.
n food is cooked.
n food gets digested in our body.
n we respire.
In all the above situations, the nature and the identity of the initial
substance have somewhat changed. We have already learnt about physical
and chemical changes of matter in our previous classes. Whenever a chemical
change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a
chemical reaction. How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions.
Activity 1.1
CAUTION: This Activity needs
the teacher’s assistance. It
would be better if students
wear suitable eyeglasses.
n Clean a magnesium ribbon
about 3-4 cm long by rubbing
it with sandpaper.
n Hold it with a pair of tongs.
Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig. 1.1. Burn the
magnesium ribbon keeping it
away as far as possible from
your eyes. Figure 1.1
n What do you observe? Burning of a magnesium ribbon in air and collection of magnesium
oxide in a watch-glass
2024-25
, You must have observed that magnesium ribbon burns with a
dazzling white flame and changes into a white powder. This powder is
magnesium oxide. It is formed due to the reaction between magnesium
and oxygen present in the air.
Activity 1.2 Activity 1.3
n Take lead nitrate n Take a few zinc granules in a conical flask or a test tube.
solution in a test n Add dilute hydrochloric acid or sulphuric acid to this
tube. (Fig. 1.2).
n Add potassium CAUTION: Handle the acid with care.
iodide solution n Do you observe anything happening around the zinc
to this. granules?
n What do you n Touch the conical flask or test tube. Is there any change in
observe? its temperature?
From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
n change in state
n change in colour
n evolution of a gas
n change in temperature.
As we observe the changes around us, we can see
that there is a large variety of chemical reactions taking
place around us. We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter.
Figure 1.2 1.1 CHEMIC
CHEMICAL EQUATIONS
AL EQUATIONS
Formation of hydrogen
Activity 1.1 can be described as – when a magnesium ribbon is burnt in
gas by the action of
dilute sulphuric acid on oxygen, it gets converted to magnesium oxide. This description of a
zinc chemical reaction in a sentence form is quite long. It can be written in a
shorter form. The simplest way to do this is to write it in the form of a
word-equation.
The word-equation for the above reaction would be –
Magnesium + Oxygen → Magnesium oxide (1.1)
(Reactants) (Product)
The substances that undergo chemical change in the reaction (1.1),
magnesium and oxygen, are the reactants. The new substance is
magnesium oxide, formed during the reaction, as a product.
A word-equation shows change of reactants to products through an
arrow placed between them. The reactants are written on the left-hand
side (LHS) with a plus sign (+) between them. Similarly, products are
written on the right-hand side (RHS) with a plus sign (+) between them.
The arrowhead points towards the products, and shows the direction of
the reaction.
2 Science
2024-25
, 1.1.1 Writing a Chemical Equation
Is there any other shorter way for representing chemical equations?
Chemical equations can be made more concise and useful if we use
chemical formulae instead of words. A chemical equation represents a
chemical reaction. If you recall formulae of magnesium, oxygen and
magnesium oxide, the above word-equation can be written as –
Mg + O2 → MgO (1.2)
Count and compare the number of atoms of each element on the
LHS and RHS of the arrow. Is the number of atoms of each element the
same on both the sides? If yes, then the equation is balanced. If not,
then the equation is unbalanced because the mass is not the same on
both sides of the equation. Such a chemical equation is a skeletal
chemical equation for a reaction. Equation (1.2) is a skeletal chemical
equation for the burning of magnesium in air.
1.1.2 Balanced Chemical Equations
Recall the law of conservation of mass that you studied in Class IX; mass
can neither be created nor destroyed in a chemical reaction. That is, the
total mass of the elements present in the products of a chemical reaction
has to be equal to the total mass of the elements present in the reactants.
In other words, the number of atoms of each element remains the
same, before and after a chemical reaction. Hence, we need to balance a
skeletal chemical equation. Is the chemical Eq. (1.2) balanced? Let us
learn about balancing a chemical equation step by step.
The word-equation for Activity 1.3 may be represented as –
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
The above word-equation may be represented by the following
chemical equation –
Zn + H2SO4 → ZnSO4 + H2 (1.3)
Let us examine the number of atoms of different elements on both
sides of the arrow.
Element Number of atoms in Number of atoms
reactants (LHS) in products (RHS)
Zn 1 1
H 2 2
S 1 1
O 4 4
As the number of atoms of each element is the same on both sides of
the arrow, Eq. (1.3) is a balanced chemical equation.
Let us try to balance the following chemical equation –
Fe + H2O → Fe3O4 + H2 (1.4)
Chemical Reactions and Equations 3
2024-25
