Question

What is the SI unit of density?

• A. $\text{kg\ dm}^3$
• B. $\text{kg\ m}^{-3}$
• C. $\text{kg\ m}^3$
• D. $\text{kg\ dm}^{-3}$

Hint:

Be very careful with negative exponents when reading options! $\text{kg\ m}^3$ implies mass multiplied by volume, whereas density requires division by volume, which is written with a negative index as $\text{m}^{-3}$.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the standard International System of Units (SI unit) used to measure mass density.

Step 2: Key Formula or Approach:
Density ($\rho$) is fundamentally defined as the mass of a substance per unit volume:
$$\rho = \frac{\text{Mass}}{\text{Volume}}$$ To find the SI unit of density, we substitute the standard SI units for both mass and volume into this formula.

Step 3: Detailed Explanation:
In the International System of Units (SI):
The standard unit of mass is the kilogram (kg).
The standard unit of volume is derived from length (meter), which gives cubic meters ($\text{m}^3$).
Substituting these into our base definition:
$$\text{SI Unit of Density} = \frac{\text{kg}}{\text{m}^3} = \text{kg\ m}^{-3}$$ This matches perfectly with option (B). Note that while units like $\text{g\ cm}^{-3}$ or $\text{kg\ dm}^{-3}$ are frequently encountered in laboratory chemistry contexts, they are not the foundational SI standard unit.

Step 4: Final Answer:
The SI unit of density is $\text{kg\ m}^{-3}$, which corresponds to option (B).