Question

How can you change burnt paper back into its original form?
A.By cooling it
B.By heating it
C.By leaving it at room temperature
D.It can’t be changed

Answer 1

Hint: A reversible process can be reversed in order to obtain the original form of a system. And an irreversible process cannot be reversed in order to obtain the original state of the system. The irreversible process may result in formation of new compounds.

Complete answer:
To answer the above question let us glance at some basic definitions. The above question is an example of reversible and irreversible changes. A reversible change is a process that can be reversed in order to obtain the initial state of a system. And an irreversible change is the process that cannot be reversed in order to obtain the original state of the system.
Now using the above concept we try to solve the question. When a paper is burnt, the molecules of paper undergo changes and formation of new substances like smoke and water vapor takes place. The burnt paper completely turns into an ash and we know that by any means we cannot change those ashes back into their original paper form. Thus, the change taking place here is irreversible. Hence, a burnt piece of paper cannot be brought back to its original form.

Therefore the correct answer is D. It can't be changed.

Note:
The process of burning a paper is a chemical change as the ash is formed. This ash is a new compound and oxides of carbon are also released during the process. By definition of the chemical change, we come to know that during chemical change a new substance must form. These changes are irreversible in nature.