Contents
Physical properties, such as density, boiling point, and solubility, are important aspects of Chemistry Topics as they describe the characteristics of substances.
What is an Irreversible Change? Is it possible that a substance can undergo both reversible and irreversible changes?
A change which cannot be reversed to form the original substance (or substances) is called an irreversible change. This will become clear from the following example. If we burn a piece of paper, it changes into ash and smoke (see Figure). Now, we cannot combine the ash and smoke to form the original
piece of paper. So, the burning of paper is a change which cannot be reversed. In other words, the burning of paper is an irreversible change.
There are a very large number of irreversible changes around us. Some of the examples of irreversible changes (or changes which cannot be reversed) are : Burning of paper ; Burning of fuels (like Wood, Coal and LPG); Formation of curd from milk ; 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 biogas ; Sawing (cutting) of a log of wood ; Breaking of a toy; Cutting of paper ; Bursting a balloon ; Burning of wax (in the form of candle); Burning of incense stick (agarbatti); Setting of Plaster of Paris (POP) on mixing water ; Setting of cement on mixing water ; Growth of nails on fingers ; Making painting on a drawing sheet; and Baking a clay pot in an oven. We will discuss some of these changes in somewhat detail.
Curd is made from milk. This is done as follows : 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. This curd, however, cannot be changed back into milk by any means. So, the formation of curd from milk is an irreversible change (which cannot be reversed).
When wheat is ground, it is converted into flour. The flour cannot be converted back into wheat grains. So, the grinding of wheat to form flour is an 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 materials. So, the cooking of food is an irreversible change (which cannot be reversed). 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 (which cannot be reversed).
If we drop a toy accidently, it breaks into a number of pieces. We cannot get back original toy again from the broken pieces. This means that the change which takes place during the breaking of a toy, cannot be reversed. In other words, the breaking of a toy is an irreversible change. When we draw and paint a picture on a drawing sheet, there is a change in the drawing sheet. We cannot convert the painted drawing sheet back into the original, blank drawing sheet. So, the making of painting on a drawing sheet is an irreversible change (which cannot be reversed).
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 joined properly. The change which occurs in POP during setting cannot be reversed. 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. If a bag of cement lying in the open gets wet due to rain during the night, some changes occur in cement due to which it gradually sets into a hard mass. Even if we keep the wet cement in bright sunshine the next day, the changes which have taken place in cement on getting wet, cannot be reversed. Thus, the setting of cement on mixing with water is 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 the 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 change.
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) (see Figure). 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 (see Figure). 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 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. 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 change. 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 [see Figure (a)]. 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) [see Figure (b)]. 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. 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 [see Figure (a)]. 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 [see Figure (b)]. 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 of 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 question now.
Example Problem.
Some changes are listed below. For each change write whether the change can be reversed or not (Yes/No) :
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
Answer:
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