______ is added to prevent the evaporation of water.

Explanation

To solve the question, "______ is added to prevent the evaporation of water," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Evaporation**:
- Evaporation is the process where water molecules at the surface gain enough energy to escape into the air as vapor. This occurs when water is exposed to air.

**Hint**: Think about what happens to water when it is left out in the open.

2. **Identifying the Need to Prevent Evaporation**:
- In certain situations, we may want to prevent water from evaporating. This could be for preserving water in a container or maintaining humidity in a specific environment.

**Hint**: Consider why we might want to keep water from disappearing.

3. **Monomolecular Layer Concept**:
- To prevent evaporation, one effective method is to create a barrier on the surface of the water. This can be achieved by adding a substance that forms a thin layer over the water.

**Hint**: Think about what kind of substances can create a layer on the water's surface.

4. **Identifying the Substance**:
- The substance that is commonly used to create this barrier is **cetyl alcohol**. When added to water, cetyl alcohol forms a monomolecular layer, which hinders the escape of water molecules into the air.

**Hint**: Recall the name of a fatty alcohol that can be used in this way.

5. **Final Answer**:
- Therefore, the answer to the question is **cetyl alcohol**.

### Final Answer:
**Cetyl alcohol** is added to prevent the evaporation of water.

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