PHYSICAL CHANGES
Activity 6.1
Cut a piece of paper in four square
pieces. Cut each square piece further into four square pieces. Lay these pieces on the floor or a table so that the pieces acquire the shape of the original piece of paper (Fig. 6.1).
Obviously, you cannot join the pieces back to make the original piece, but is there a change in the property of the paper?
Activity 6.2
Collect the chalk dust lying on the floor near the chalkboard in your classroom. Or, crush a small piece of chalk into dust. Add a little water to the dust to make a paste. Roll it into the shape of a piece of chalk. Let it dry.
Did you recover chalk from the dust?
Activity 6.3
Take some ice in a glass or plastic tumbler. Melt a small portion of ice by placing the tumbler in the sun. You have now a mixture of ice and water. Now place the tumbler in a freezing mixture (ice plus common salt).Does the water become solid ice once again?
Activity 6.4
Boil some water in a container. Do you see the steam rising from the surface of water? Hold an inverted pan by its handle over the steam at some distance from the boiling water. Observe the inner surface of the pan.
Do you see any droplet of water there?
Activity 6.5
Hold a used hack-saw blade with a pair of tongs. Keep the tip of the free end of the blade on the gas stove. Wait for a few minutes.
Does the colour of the tip of the blade change?
Remove the blade from the flame. Observe the tip once again after some time.Does it get back its original colour? In Activities 6.1 and 6.2 above, you saw that paper and a piece of chalk underwent changes in size. In Activities 6.3 and 6.4, water changed its state (from solid to liquid, or from gas to liquid). In Activity 6.5, the hack-saw blade changed colour on heating.
Properties such as shape, size, colour and state of a substance are called its physical properties. A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
Let us now consider the other kind of change