Glycerol is purified by:

Options

A. steam distillation
B. vacuum distillation
C. sublimation
D. simple distillation.

Answer

To determine the best method for purifying glycerol, we need to analyze the options given based on the properties of glycerol, particularly its boiling point and behavior during distillation.

### Step-by-Step Solution:

1. **Understand the Properties of Glycerol**:
- Glycerol has a high boiling point of approximately 298 degrees Celsius. This high boiling point can lead to decomposition if subjected to high temperatures.

**Hint**: Remember that the boiling point is crucial for determining the suitable distillation method.

2. **Evaluate the Distillation Methods**:
- **Simple Distillation**: This method is generally used for liquids with lower boiling points. Since glycerol has a high boiling point, simple distillation is not suitable as it may lead to decomposition.

**Hint**: Consider the boiling point when selecting a distillation method.

3. **Consider Sublimation**:
- Sublimation involves converting a solid directly into a gas. Glycerol is a liquid at room temperature, so sublimation is not applicable for its purification.

**Hint**: Identify the physical state of the substance to rule out inappropriate methods.

4. **Analyze Steam Distillation**:
- Steam distillation is typically used for immiscible liquids and can help separate components without reaching their boiling points. However, since glycerol decomposes at its boiling point, steam distillation is not suitable.

**Hint**: Think about the decomposition of glycerol when evaluating steam distillation.

5. **Choose Vacuum Distillation**:
- Vacuum distillation reduces the pressure during the distillation process. Lower pressure decreases the boiling point of glycerol, allowing it to be distilled without decomposition. This makes vacuum distillation the best method for purifying glycerol.

**Hint**: Remember that reducing pressure can help in distilling substances with high boiling points safely.

### Conclusion:
The best method for purifying glycerol is **vacuum distillation**. This method allows glycerol to be distilled at a lower temperature, preventing decomposition.