Changes around us

      GENERAL SCIENCE    

STD: 6                              Le – 6,       CHANGES AROUND US

Learning Objectives:

·         To differentiate reversible and irreversible changes

·         To analyze the factors that brings about changes in different substances

·         To classify the changes into physical and chemical changes

Answer The Following:

1. Differentiate between reversible and irreversible changes with examples.

The changes that be reversed easily to obtain a substance in its original form is called a reversible change.

Eg:- When we heat ice, it melts to form water. If we cool this water, it again changes into ice. So, the changing of ice into water has been reversed by cooling. Thus melting of ice is a reversible change.

The change that cannot be reversed to obtain a substance back in its original form is called an irreversible change.

Eg: Burning of paper, cooking of food.

2. What happens to a material’s size when it is (a) heated   (b) cooled?

Many materials increase in size or expand on heating and decrease in size or contract on cooling.

For example, metals like iron and aluminium expand on heating and contract on  cooling.

       

3. How heating and cooling of water brings changes in the state of matter?

When we boil water, it changes into steam. If we cool steam, it again changes into water. So, the changing of water into steam has been reversed by cooling. Thus, the boiling of water is a reversible change. 

     

4. Explain how a metal rim is fixed around the wooden wheel of cart.

In bullock carts wooden wheels are fitted with iron rims to make them last longer. The iron rim is made slightly smaller in size than the wooden wheel. The ironsmith heats this iron rim uniformly over a dry dung-cake fire. On heating the iron rim expands and becomes somewhat bigger in size. This is now easily put around the wooden wheel. Water is then poured over the hot iron rim to cool it. On cooling the hot iron rim contracts and fits tightly around the wooden wheel.

5. What are the kinds of changes caused by pressure?

Pressure can cause many kinds of changes. Some important ones are:

(1) Change in shape and size: If we press fluffy solids like a cushion or a ball of wool their shape changes. When the pressure is released, they get back to their original shape. A gas balloon changes its shape and size when filled with gas. When the balloon deflates, the shape and size again changes.

(2) Breaking objects: When pressure is applied on brittle substances like glass or biscuits, they break into pieces. It is an irreversible change.

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