The change in which the shape , size, appearance or state of a subtance may alter but its chemical composition remains the same is called a physical change . In a physical change, no new substance is formed. Any change that involves the formation of a new substance and leads to a transformation of chemical identity is called a chemical change.
Chemical changes are usually accompanied with heat exchanges. Chemical changes are permanent changes which are usually irreversible.
An example of a chemical change is

Options

A. formation of clouds
B. glowing of an electric light
C. dropping sodium into water
D. dissolving of salt in water.

Explanation

To answer the question regarding an example of a chemical change, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Chemical Changes**:
- Chemical changes involve a transformation that results in the formation of new substances. This occurs through a chemical reaction where the molecular composition of the reactants changes.

2. **Identifying Characteristics of Chemical Changes**:
- Chemical changes are typically irreversible and permanent. Once a chemical change occurs, it is often not possible to revert to the original substances without undergoing another chemical reaction.

3. **Examples of Chemical Changes**:
- A classic example of a chemical change is the reaction of sodium with water. When sodium is dropped into water, it reacts vigorously, producing sodium hydroxide (NaOH) and hydrogen gas (H2). This reaction is exothermic, meaning it releases heat.

4. **Observing the Reaction**:
- During this reaction, the heat produced can ignite the hydrogen gas, demonstrating that new substances are formed (sodium hydroxide and hydrogen) and that the original sodium and water have changed chemically.

5. **Conclusion**:
- Therefore, the reaction of sodium with water is a clear example of a chemical change due to the formation of new products and the change in molecular composition.

### Final Answer:
An example of a chemical change is the reaction of sodium with water, which produces sodium hydroxide and hydrogen gas.

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