Relative density, also known as specific gravity, describes how dense a substance is compared to a reference substance, usually water. It's a dimensionless quantity, meaning it has no units.
Properties of Relative Density
Here's a breakdown of the key properties of relative density:
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Relationship to Mass and Volume: Relative density directly reflects the relationship between a substance's mass and its volume. A higher relative density indicates that a substance has more mass packed into the same volume compared to the reference.
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Comparison to a Reference: It's always expressed relative to something else, almost always water at a specified temperature (usually 4°C, its densest state). The formula is:
Relative Density = (Density of substance) / (Density of reference substance)
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Intensive Property: Relative density is an intensive property, meaning it doesn't depend on the amount of the substance present. A small sample of gold will have the same relative density as a large gold bar. This contrasts with extensive properties like mass or volume, which do depend on the amount of the substance.
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Dimensionless Quantity: Since relative density is a ratio of two densities (or weights), the units cancel out, making it a dimensionless number. For example, if a substance has a relative density of 2, it's twice as dense as water.
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Determines Floating/Sinking: A substance with a relative density less than 1 will float in water. If it's greater than 1, it will sink. If it equals 1, it will be neutrally buoyant.
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Temperature Dependence: The density of most substances, including the reference (usually water), changes with temperature. Therefore, relative density is also temperature-dependent.
Examples
- Gold: Gold has a relative density of approximately 19.3. This means it's 19.3 times denser than water and will sink.
- Ice: Ice has a relative density of approximately 0.92. This is why ice floats in water.
- Ethanol: Ethanol has a relative density of approximately 0.79. Therefore, ethanol will float on water.
In summary, relative density is a useful property for comparing the densities of different substances to a standard, predicting whether they will float or sink in water, and characterizing materials based on their mass-to-volume ratio.
