Chemistry

On the basis of whether new substances are formed or not we can classify all the changes into two groups.
Physical change
A change in which no new substances are formed but physical form of the substance changes is known as physical change.
The product formed in such changes is chemically identical to the starting substance.
Example. When ice is heated, it changes into liquid water, on further heating it changes into steam. But water in the solid form (ice) or liquid form or in gaseous form (steam) is chemically the same substance.
Thus, this transformation represents a physical change. Physical changes can be reversed easily.
Example. Steam on colling forms liquid water, which on further cooling changes into ice.
Chemical change
A change in which one or more substances change into new substances is known as chemical change.
Such a change cannot be reversed easily. Chemical changes are also known as chemical reactions.
Example. When electricity is passed through water. it decomposes into two new substances, hydrogen and oxygen. Thus, it represents a chemical change. Similarly, burning of candle, rusting or iron and calcination of lime-stone are also examples of chemical changes.
Differences between physical and chemical changes

Physical Change	Chemical Change
1. No new substance is formed in a physical change	1. A new substance is formed in a chemical change
2. A physical change is a temporary change.	2. A chemical change is a permanent change.
3. A physical change is easily reversible.	3. A chemical change is usually irreversible.
4. Very little heat (or light) energy is usually absorbed or given out in a physical change	4. A lot of heat (or light) energy is absorbed or given out in a chemical change
5. The mass of a substance does not alter in a physical change.	5. The mass of a substance does alter in a chemical change.