When CaO is heated in an oxy-hydrogen flame,

Options

A. it decomposes to give oxygen
B. it melts to give fused `CaO`
C. it becomes incandescent
D. no observable change takes place.

Explanation

To solve the question "When CaO is heated in an oxy-hydrogen flame, what happens?", we can follow these steps:

### Step 1: Understand the Components
- **Calcium Oxide (CaO)**: This is a chemical compound commonly known as quicklime.
- **Oxy-Hydrogen Flame**: This flame is produced by burning a mixture of hydrogen (H2) and oxygen (O2) gases. It burns at a very high temperature, approximately 2572 degrees Celsius.

### Step 2: Heating CaO
- When CaO is subjected to the high temperatures of the oxy-hydrogen flame, it undergoes a physical change rather than a chemical decomposition.

### Step 3: Observing the Effect
- As CaO is heated, it becomes incandescent. Incandescence is the emission of light from a hot body. The light emitted is known as "limelight."

### Step 4: Analyzing the Options
- **Option A**: It decomposes to give oxygen. (Incorrect)
- **Option B**: It melts to give CO. (Incorrect)
- **Option C**: It becomes incandescent. (Correct)
- **Option D**: No observable changes take place. (Incorrect)

### Conclusion
- The correct answer is **Option C**: It becomes incandescent.