Which of the following changes is slow and periodic?

Options

A. Rotation of the earth
B. Rusting of iron
C. Curdling of milk
D. Evaporation of petrol

Answer

To determine which of the following changes is slow and periodic, we will analyze each option based on the definitions of slow change and periodic change.

### Step-by-Step Solution:

1. **Understanding Slow Change**:
- A slow change is defined as a change that takes a long duration of time to complete.

2. **Understanding Periodic Change**:
- A periodic change is one that occurs at regular intervals or in a predictable manner over time.

3. **Analyzing the Options**:
- **Option 1: Rotation of Earth**
- The Earth takes 24 hours to complete one rotation and 365 days to complete one revolution.
- This is a periodic change because it occurs at regular intervals (every 24 hours and 365 days).
- It is also a slow change since both the rotation and revolution take a long time to complete.
- **Conclusion**: This option is correct.

- **Option 2: Rusting of Iron**
- Rusting occurs when iron reacts with oxygen and moisture, forming rust (iron oxide).
- This change is slow but it is not periodic because it does not occur at regular intervals.
- **Conclusion**: This option is incorrect.

- **Option 3: Curdling of Milk**
- Curdling is a process where milk is converted into curd, typically due to the action of bacteria.
- This change is slow but it is also not periodic as it does not happen at regular intervals.
- **Conclusion**: This option is incorrect.

- **Option 4: Evaporation of Petrol**
- Evaporation is the process where liquid petrol changes into vapor.
- This change is not periodic and occurs quickly, especially at room temperature.
- **Conclusion**: This option is incorrect.

4. **Final Conclusion**:
- The only option that is both slow and periodic is **Option 1: Rotation of Earth**.

### Final Answer:
**The correct answer is Option 1: Rotation of Earth.**