Physical and Chemical Changes
1. INTRODUCTION
All substances around us undergo changes. In some cases, the changes are small
and difficult to detect. In other cases, the changes are obvious and easy to detect.
These changes generally get accelerated, if we heat the substances.
Most of these changes can be classified under two headings:
(i) Physical changes
(ii) Chemical changes
Physical changes are generally temporary in nature and no new substances are
formed. Chemical changes are generally permanent in nature and new substances are
formed, which have entirely new properties.
2. PHYSICAL CHANGES
The changes are temporary in nature and can be reversed. Such changes are called
physical changes. Let’s see few experiments showing physical changes.
Experiment 1
Take a dry hard glass test tube and put in it about 2 g of candle wax. Meat the test
tube gently on a Bunsen flame. What do you observe?
The wax melts to form a colourless liquid. Now cool the tube by holding it in cold
water. What do you observe?
The molten wax solidifies. Thus, we can say that on heating wax melts and on
cooling, the liquid wax solidifies, but no new products are formed.
Similarly, ice, ghee, butter, etc., melt on heating and solidify on cooling.
Experiment 2
Zinc oxide is a white powder. Place about 2 g of zinc oxide in a dry test tube. Heat the
test tube strongly. What do you notice after 2 minutes of heating?
The zinc oxide changes to yellow colour. Cool the test tube. In a few minutes, the
colour of zinc oxide changes to white colour.
Thus, we can say that the change in zinc oxide was temporary in nature.
Similarly, if we heat lead monoxide powder, which is yellow in colour, its colour
changes to reddish brown. However, on cooling, its colour changes back to yellow.
Experiment 3
Take about 50 Cm3
of water in a beaker. Add a spoonful of common salt in it and stir. You will observe
that salt dissolves in water. Now evaporate the salt solution on heated sand bath. It is
observed that water disappears leaving behind common salt. Furthermore, the mass
of common salt remains same.
Thus, we can say that the above changes are
(i) temporary in nature,
(ii) no new products are formed, and
(iii) there is no change in weight when change takes place.
1. INTRODUCTION
All substances around us undergo changes. In some cases, the changes are small
and difficult to detect. In other cases, the changes are obvious and easy to detect.
These changes generally get accelerated, if we heat the substances.
Most of these changes can be classified under two headings:
(i) Physical changes
(ii) Chemical changes
Physical changes are generally temporary in nature and no new substances are
formed. Chemical changes are generally permanent in nature and new substances are
formed, which have entirely new properties.
2. PHYSICAL CHANGES
The changes are temporary in nature and can be reversed. Such changes are called
physical changes. Let’s see few experiments showing physical changes.
Experiment 1
Take a dry hard glass test tube and put in it about 2 g of candle wax. Meat the test
tube gently on a Bunsen flame. What do you observe?
The wax melts to form a colourless liquid. Now cool the tube by holding it in cold
water. What do you observe?
The molten wax solidifies. Thus, we can say that on heating wax melts and on
cooling, the liquid wax solidifies, but no new products are formed.
Similarly, ice, ghee, butter, etc., melt on heating and solidify on cooling.
Experiment 2
Zinc oxide is a white powder. Place about 2 g of zinc oxide in a dry test tube. Heat the
test tube strongly. What do you notice after 2 minutes of heating?
The zinc oxide changes to yellow colour. Cool the test tube. In a few minutes, the
colour of zinc oxide changes to white colour.
Thus, we can say that the change in zinc oxide was temporary in nature.
Similarly, if we heat lead monoxide powder, which is yellow in colour, its colour
changes to reddish brown. However, on cooling, its colour changes back to yellow.
Experiment 3
Take about 50 Cm3
of water in a beaker. Add a spoonful of common salt in it and stir. You will observe
that salt dissolves in water. Now evaporate the salt solution on heated sand bath. It is
observed that water disappears leaving behind common salt. Furthermore, the mass
of common salt remains same.
Thus, we can say that the above changes are
(i) temporary in nature,
(ii) no new products are formed, and
(iii) there is no change in weight when change takes place.
