Page 35 - ICSE Chemistry 8

Observa on: You will observe that water on freezing changes into ice cubes.
Conclusion: Thus, mel ng of ice is a physical change as it is reversible.

Activity 2.5

Aim: To understand that dissolu on of sugar in water is a physical
change sugar
Materials required: A beaker, some water, some sugar, a spoon, an
evapora ng dish, a burner
Procedure sugar solution
• Take a beaker half-fi lled with water.

• Add two spoonfuls of sugar to it and s r.
Observa on: You will observe that sugar dissolves in water.
Procedure
• Now transfer the sugar solu on from the beaker to an
evapora ng dish. Heat the solu on ll the en re water heating sugar sugar left at the bottom
solution of the dish
changes into steam.
Observa on: You will observe that the water evaporates and sugar crystals are le behind in the dish.

Conclusion: Thus, dissolu on of sugar in water is a physical change.

Categorising Physical Changes

Let’s study about diff erent categories to which a few physical changes belong.
Dissolving

Dissolving is a process in which a solid, liquid or gas mixes uniformly with another substance to form a
solu on. When we dissolve sugar in water, the sugar par cles spread out and form a solu on with water.
This process can be reversed by evapora ng water and collec ng the sugar.

Evaporation

Evapora on is a process by which a substance changes from its liquid state to its gaseous state at a
temperature below the boiling point. The evapora on of water is a physical change which is slow, natural,
desirable and reversible.

We have studied in the previous class that rate of evapora on depends on factors such as temperature,
surface area, humidity and speed of air. An increase in temperature increases the rate of evapora on. The
rate of evapora on is higher if the surface area is larger. Increase in humidity in the air decreases the rate
of evapora on. The rate of evapora on is higher when the speed of air over the liquid is high.

Boiling

Boiling is a process by which a substance changes from its liquid state to its gaseous state when heated to
its boiling point. Boiling of water is a physical change which is man-made, fast, reversible and desirable.

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