Relative density and actual density differ primarily in that actual density measures mass per unit volume, while relative density is a ratio comparing a substance's density to that of a reference substance (usually water), making it dimensionless.
Here's a breakdown of the key differences:
Density (Actual Density)
- Definition: Density is defined as mass per unit volume. It measures how much "stuff" is packed into a given space.
- Formula: Density = Mass / Volume (ρ = m/V)
- Units: Commonly expressed in kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³).
- Nature: It is an absolute measure of a substance's compactness.
- Example: The density of aluminum is approximately 2700 kg/m³. This means that one cubic meter of aluminum has a mass of 2700 kilograms.
Relative Density (Specific Gravity)
- Definition: Relative density (also known as specific gravity) is the ratio of the density of a substance to the density of a reference substance, typically water at 4°C (whose density is approximately 1000 kg/m³ or 1 g/cm³).
- Formula: Relative Density = Density of substance / Density of reference substance
- Units: It is a dimensionless quantity (i.e., it has no units) because it's a ratio of two densities.
- Nature: It's a relative measure, indicating how much denser or less dense a substance is compared to the reference substance.
- Example: If a substance has a relative density of 2, it means it's twice as dense as water.
Table Summarizing the Differences
| Feature | Density (Actual Density) | Relative Density (Specific Gravity) |
|---|---|---|
| Definition | Mass per unit volume | Ratio of a substance's density to a reference substance's density |
| Formula | ρ = m/V | Density of substance / Density of reference substance |
| Units | kg/m³ or g/cm³ | Dimensionless |
| Nature | Absolute measure | Relative measure |
| Reference Needed | No | Yes (usually water) |
In essence, density tells you how heavy something is for its size, while relative density tells you how heavy something is compared to water. Relative density simplifies comparison, especially when assessing whether a substance will float or sink in water (a relative density less than 1 indicates the substance will float).
