Relative density, also known as specific gravity, is a dimensionless ratio comparing a substance's density to a reference substance's density. Water is usually the reference for liquids. Calculating it is straightforward:
Calculating Relative Density
To calculate the relative density of a liquid, follow these steps:
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Measure the density of the liquid: This involves determining both the mass and volume of a sample of the liquid. The density (ρ) is calculated as mass (m) divided by volume (V): ρ = m/V. You can use various instruments for this measurement such as a graduated cylinder and a balance.
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Divide the liquid's density by the reference substance's density: The reference substance is typically water at a specific temperature (usually 4°C, where its density is 1 g/cm³ or 1000 kg/m³). Therefore, relative density (RD) is: RD = ρliquid / ρwater.
Example Calculation
Let's say you measure 50 ml of an unknown liquid. Its mass is found to be 56.5 grams.
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Step 1: Density of the liquid: ρliquid = 56.5 g / 50 ml = 1.13 g/ml
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Step 2: Relative density (assuming the density of water is 1 g/ml): RD = 1.13 g/ml / 1 g/ml = 1.13
Therefore, the relative density of the unknown liquid is 1.13. This means the liquid is 1.13 times denser than water.
Alternative Methods
While the direct density measurement is the most common, other methods exist, including using:
- A hydrometer: This instrument floats in the liquid, and its submerged depth indicates the relative density. The hydrometer scale is calibrated to directly read relative density.
- Archimedes' principle: This principle can be used to determine the relative density by measuring the apparent weight loss of an object when submerged in the liquid and comparing it to the weight loss in water.
Understanding the Results
The relative density provides a valuable comparison of liquid densities without needing to specify units. This is particularly useful in various applications, from industrial processes to laboratory experiments.
