Which of the following is a chemical change ?

Options

A. Formation of clouds
B. Formation of rust
C. Manufacture of milk powder from milk
D. Formation of black coating on the walls of tube light

Explanation

To determine which of the following is a chemical change, we need to understand what a chemical change is. A chemical change is a process where substances transform into different substances with different properties, and this change is usually irreversible.

### Step-by-Step Solution:

1. **Identify the Options**: We need to look at the options provided to us. The options are:
- Formation of clouds
- Formation of rust
- Manufacture of milk powder from milk
- Formation of black coating on the tubes of a light bulb

2. **Analyze Each Option**:
- **Formation of Clouds**: This is a physical change. Clouds can form and dissipate, and the water vapor can return to its liquid state (rain). Thus, this is reversible.
- **Formation of Rust**: This is a chemical change. When iron (Fe) reacts with water (H2O) and oxygen (O2), it forms rust (Fe2O3·xH2O). This process is irreversible because once rust is formed, it cannot be converted back to iron without a chemical reaction.
- **Manufacture of Milk Powder from Milk**: This is a physical change. The process of evaporating water from milk to make milk powder can be reversed by adding water back to the powder.
- **Formation of Black Coating on Tubes of Light Bulbs**: This is also a physical change. The black coating is due to the evaporation of the electrodes, which can be cleaned off, making it reversible.

3. **Conclusion**: Among the options analyzed, the only irreversible change is the formation of rust. Therefore, the answer to the question is the formation of rust.

### Final Answer:
The chemical change is the **formation of rust**.

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