Relative mass density, often referred to simply as relative density or specific gravity, is a comparative measure. It describes how dense a substance is compared to a reference material. The core concept revolves around ratios, not absolute values.
Understanding Relative Mass Density
According to our reference, density is fundamentally the ratio of a body's mass to its volume. Relative density, in contrast, is defined as:
The ratio between the density of a body and the density of some other reference body.
This means we are not interested in the actual density of an object, but rather its density relative to something else, typically a standard material.
Key Aspects of Relative Mass Density
- Ratio: Relative density is a dimensionless quantity expressed as a ratio. It does not have units because the units of density cancel out in the calculation.
- Reference Material: Water is most often used as the reference substance, particularly for liquids and solids. For gases, air is usually the reference.
- Calculation: It's calculated by dividing the density of the substance by the density of the reference material.
$$
Relative \ Density = \frac{Density \ of \ Substance}{Density \ of \ Reference \ Material}
$$
Examples of Relative Mass Density
Here's a simple table demonstrating relative densities of a few common substances, using water as the reference (density ≈ 1000 kg/m³ or 1 g/cm³):
| Substance | Approximate Density (kg/m³) | Relative Density (approximate) |
|---|---|---|
| Water | 1000 | 1.0 |
| Aluminum | 2700 | 2.7 |
| Lead | 11340 | 11.34 |
| Ethanol | 789 | 0.79 |
| Air (at STP) | 1.225 | 0.001225 |
- Floating and Sinking: A relative density greater than 1 means the substance is denser than water and will sink, while a value less than 1 implies the substance is less dense than water and will float. For example, aluminum (2.7) will sink, and ethanol (0.79) will float on water.
Applications of Relative Mass Density
Relative density is very useful for:
- Quality control: Checking the purity or composition of materials, for example, in the food or chemical industries.
- Engineering: Material selection for specific applications, such as ship building where a material's density relative to water is crucial.
- Geology: Identifying minerals and rocks by comparing their relative densities.
Importance
In essence, relative mass density gives us a quick and easy way to understand the comparative denseness of different materials without having to know their exact densities. It allows us to directly compare how heavy a substance is for a given volume with a well-known standard.
