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 a tube light

Explanation

To determine which of the following is a chemical change, we first need to understand what a chemical change is. A chemical change is a process where new substances are formed through a chemical reaction, and this change is generally irreversible.

### Step-by-Step Solution:

1. **Identify the Characteristics of Chemical Changes:**
- A chemical change results in the formation of one or more new substances.
- It is usually irreversible under normal conditions.
- The molecular composition of the substances involved changes.

2. **Analyze the Options Provided:**
- Look at each option and determine whether a new substance is formed.
- Consider if the change can be reversed easily.

3. **Example of a Chemical Change:**
- **Formation of Rust:** When iron (Fe) is exposed to oxygen and moisture, it reacts to form rust (iron oxide, Fe2O3·xH2O). This is a chemical change because:
- A new substance (rust) is formed.
- The process is irreversible under normal conditions.

4. **Examples of Physical Changes:**
- **Formation of Clouds:** This involves water changing states from liquid to gas and back to liquid. No new substance is formed, so this is a physical change.
- **Manufacture of Milk Powder:** This process involves changing the state of milk but does not create a new substance, thus it is a physical change.
- **Black Coating on Tube Light:** This is a change in appearance, but no new substances are formed, making it a physical change.

5. **Conclusion:**
- Among the options, the formation of rust is the only example of a chemical change.

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

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