Question 1 What are irreversible changes?
Question 2 Give few examples of irreversible changes?
Question 3 Milk can be changed into curd but curd cannot be changed back into milk. What is the name of the change?
Question 4 Burning of paper is an irreversible change. Explain ?
Question 5 When water is mixed with POP and allowed to dry, it sets into a hard mass. Explain that this is an irreversible change?
Question 6 Give few examples to show that the same material can undergo reversible and irreversible change?
Question 7 Explain why, the burning of an incense stick is an irreversible change?
A change which cannot be reversed to form the original substance (or substances) is called an irreversible change.
If we burn a piece of paper, it changes into ash and smoke. Now we cannot combine the ash and smoke to form the original piece of paper. So, the burning of paper is an irreversible change which cannot be reversed.
Some examples of irreversible changes are burning of paper, Burning of fuels (like Wood, Coal and LPG), Cooking of food, Rusting of iron , Grinding of wheat grains into flour, Baking of chapatti (roti), Growth of a plant, Formation of flower from bud, Falling of leaves from a tree, Ripening of fruits, Ageing of man and animals, Death and decay of plants and animals, Weathering of rocks (Breaking down of rocks); Printing of paper; Souring of milk ; Boiling of an egg (Raw egg to boiled egg), Making cheese (paneer) from milk ; Cow-dung to bio gas ; Sawing (cutting) of a log of wood, Bursting a balloon ,Burning of wax (in the form of candle); Burning (agarbatti), Setting of Plaster of Paris (POP) on mixing water, Setting of cement on mixing on fingers Making painting on a drawing sheet and Baking a clay pot in an oven.
A very small quantity of previously made curd is added to warm milk. The milk is then stirred and kept aside for a few hours at a warm place. During this time milk changes into curd, cannot be changed back into milk by any means. So, the formation of curd from milk is irreversible change (which cannot be reversed).
When we heat raw food materials, we get cooked food. The cooked food cannot be converted back into raw food. So, the cooking of food is an irreversible change. A young man ages and ultimately becomes an old man. But we cannot change an old man back into a young man. So, the ageing of man is an irreversible change.
Plaster of Paris (POP) is a white, powdery substance which immediately sets to a hard mass on adding water to it. In hospitals, a thick coat of the paste of plaster of Paris is applied over the bandage on the fractured bone of a person. POP sets and becomes hard on drying and keeps the fractured bone in place to get the joined properly. Thus, the setting of plaster of Paris on mixing water is an irreversible change.
When water is added to cement, it sets into a hard mass after sometime, the setting of cement on mixing with water an irreversible change.
If we light an incense stick (agarbatti) with a burning matchstick we find that after some time the whole incense stick burns away. During burning of incense stick, some pleasant smelling gases are produced (which go into the air) and ash is left behind. We cannot recombine the gases and the ash to get back the original incense stick. So, this is a change which cannot be reversed. Thus, the burning of an incense stick is an irreversible change.
Reversible and Irreversible Changes Involving Same Materials
Even the same material can undergo reversible change or irreversible change under different set of conditions.
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. 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 pieces of paper to get back exactly the original sheet of paper So, the cutting of paper is an irreversible change.
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 stove. We will get 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
Potter 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 a 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, an irreversible change.
4 Inflating a Balloon and Bursting a Balloon
We take a balloon and blow air into it. The shape and size of balloon change. The balloon gets inflated. Let us now allow the air filled in the in the balloon to escape. We will find that when all the air escapes, balloon comes back to its original shape and size. This means that inflating a balloon is a change which can be reversed. 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 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 (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. We 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 candle to burn for some time. 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 of burning of wax to get back to the original candle. So, the burning of wax is an irreversible change.
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