Reversible and Irreversible Changes Involving Same Materials

Changes Around Us of Class 6

Reversible and Irreversible changes involving same materials

Even the same material can undergo reversible change or irreversible change under different set of conditions. This will become more clear from the following examples.

1. Folding of Paper and Cutting of Paper

Take a sheet of paper and fold it to form a toy aeroplane. Now, when we unfold this toy aeroplane, we will get exactly the same sheet of paper. So, the folding of paper is a reversible change. In other words, the change which occurs during the folding of paper can be reversed. We again take a sheet of paper. This time we cut the sheet of paper into a number of small pieces with the help of scissors. Now, we cannot combine the various pieces of paper to get back exactly the original sheet of paper. So, the cutting of paper is an irreversible change. In other words, the change which occurs during the cutting of paper cannot be reversed.

2. Rolling a roti and Baking a Roti

Let us take some dough (kneaded flour) and make a ball of it. We roll this ball of dough into a circular roti by using a rolling pin (belan). Now, if we want, we can convert this rolled roti back into the ball of dough. This means that the rolling of roti out of dough is a change which can be reversed. So, the rolling of roti out of dough is a reversible change. Let us now bake the rolled out roti on a ‘tawa’ kept on a stove. We will get a baked roti, Now, the baked roti cannot be changed back into the original ball of dough. So, the baking of roti is a change which cannot be reversed. So, baking of roti is an irreversible change.

3. Shaping of wet clay into clay pot and baking a clay pot

Many of us have seen a potter working on his wheel called potter’s wheel. He uses wet clay to make pots of different shapes and sizes. The shaping of a lump of wet clay into a pot by a potter on his wheel is reversible change. This is because the wet clay pot can be converted back into the original clay. The clay pots are baked in an oven to make them strong.  The baking of clay pot in an oven is, however, as irreversible change. This is because the baked clay pot cannot be changed back into the original clay.

4. Inflating a balloon and bursting a balloon

We take a balloon and blow air into it. The shape and size of balloon changes. The balloon gets inflated. Let us now allow the air filled in the balloon to escape. We will find that when all the air escapes, the balloon comes back to its original shape and size. This means that inflating a balloon is a change which can be reversed. In other words, inflating a balloon is a reversible change. Again, let us take a balloon and blow air into it so that it gets inflated to its full size. Tie the mouth of the balloon tightly with a thread. Now, press the balloon between the palms of your hands. The balloon bursts. This burst balloon cannot be changed back to its original shape and size. So, the change which occurs during the bursting of a balloon cannot be reversed. In other words, the bursting of balloon is an irreversible change.

5. Melting of wax and burning of wax

Wax is a solid substance. Let us take some wax in a porcelain dish and heat it over a burner. On heating, solid wax melts to form a liquid which is called molten wax (or melted wax). We now remove the burner and allow the molten wax to cool down. On cooling, the molten wax (or liquid wax) becomes solid again. thus, solid wax melts on heating, and melted wax becomes solid on cooling. So, melting of wax is a reversible change. In other words, melting of wax is a change which can be reversed.

A candle is made of wax. Wax take a small wax candle and measure its length with a scale. Now, fix this candle on a tile kept on a table and light it with a burning matchstick. Allow the candle to burn for some time (say 15 to 20 minutes). Now, blow out the burning candle and measure its length again. We will find that the length of this remaining candle is much less than its original length. This happens as follows: When we light the candle, then the wax present in it starts burning to produce heat and light. The burning of wax also produces carbon dioxide gas, water vapour and soot (unburnt carbon particles), which all go into air. We cannot combine all the products o burning of wax to get back the original candle. So, the burning of wax is an irreversible change. In other words, burning of wax is a change which cannot be reversed. Let us answer one questions now.

Sample Problem: Some changes are listed below. For each change write whether the change cab be reversed or not (Yes / N):

            1. Sawing of a piece of wood              2. Melting of ice candy

            3. Dissolving sugar in water                4. Cooking of food

            5. Ripening of a mango                       6. Souring of milk

Ans.    

 

Change

Can be reversed (Yes / No)

1.

Sawing of a piece of wood

No

2.

Melting of ice candy

Yes

3.

Dissolving sugar in water

Yes

4.

Cooking of food

No

5.

Ripening of a mango

No

6.

Souring of milk

No

            We are now in a position to answer the following questions:

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