What is the unit of relative density?

Answer

To find the unit of relative density, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Relative Density**:
- Relative density is defined as the ratio of the density of a substance to the density of water at 4 degrees Celsius.

2. **Write the Formula**:
- The formula for relative density (RD) can be expressed as:
\[ RD = \frac{\text{Density of the substance}}{\text{Density of water}} \]

3. **Identify the Units**:
- The unit of density is typically expressed in kilograms per cubic meter (kg/m³).

4. **Substitute the Units into the Formula**:
- When we substitute the units into the relative density formula, we have:
\[ RD = \frac{\text{kg/m}^3}{\text{kg/m}^3} \]

5. **Simplify the Expression**:
- Since both the numerator and denominator have the same unit (kg/m³), they cancel each other out:
\[ RD = 1 \quad (\text{unitless}) \]

6. **Conclusion**:
- Therefore, relative density is a unitless quantity.

### Final Answer:
The unit of relative density is **unitless**.