When electricity is passed through water, then a change V occurs which leads to the formation of two gases W and X. On the other hand, when water is heated strongly, then a change Y takes place leading to the formation of gas Z.
What type of change is Y ?

Answer

To determine the type of change Y that occurs when water is heated strongly, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Changes**: The question describes two different processes involving water. The first process (change V) occurs when electricity is passed through water, leading to the formation of gases W (hydrogen) and X (oxygen). The second process (change Y) occurs when water is heated strongly, resulting in the formation of gas Z.

2. **Identifying Change Y**: When water (H2O) is heated, it undergoes a transformation from a liquid state to a gas state. This process is known as vaporization, where water turns into water vapor (gas Z).

3. **Classifying Change Y**: Vaporization is a physical change because it involves a change in the state of matter (from liquid to gas) without altering the chemical composition of the water. The water molecules remain H2O whether they are in liquid or gas form.

4. **Conclusion**: Therefore, the type of change Y, which is the vaporization of water, is classified as a **physical change**.

### Final Answer:
Change Y is a **physical change**.