Several terms are used to express the mass (weight) of equal volumes of different substances - absolute density, apparent density, relative density, and specific gravity.

**Absolute density** is the ratio of the mass of an object, determined in or referred to a vacuum, at a specified temperature, to the volume of the object at the same temperature. This relationship is expressed mathematically as:

Absolute Density= Mass in grams (in a vacuum)/ Volume in millimeters

**Apparent density** differs from absolute density only in that the mass of the object is determined in air; the mass is influenced by the difference in the buoyant effect of air on the object being weighed, and on the standard masses (weights) used for comparison.

If the object and masses are made of the same material, or have the same density, there will be no difference, and the apparent density will be identical with the absolute density.

**Relative density** is an expression sometimes employed to indicate the mass of 1 mL (not cc, which is very slightly different) of a standard substance, such as water, at a specified temperature, relative to water at 4°C taken as unity. Thus, at 4°C the relative density of water is 1.0000, whereas its absolute density at the same temperature is 0.999973. Water attains its maximum absolute density of 0.999973 at 3.98°C. To convert a relative density of water to absolute density, the former should be multiplied by 0.999973.

**Specific gravity** may be defined as the ratio of the mass of a substance to the mass of an equal volume of another substance taken as the standard. For gases, the standard may be hydrogen or air; for liquids and solids, it is water.

#### Calculations

**Specific gravity**

The principle underlying the determination of the specific gravity of either a liquid or a solid is the same to find the ratio of the mass (weight) of the substance to that of an equal volume of water. This may be expressed by a simple relationship:

Specific gravity= Ws/Ww

Where, Ws is the weight of the substance, and Ww the weight of an equal volume of water.

**Density**

Density is defined as the mass of a substance per unit volume. The equations for calculating density weight, and volume are:

Density= Weight/Volume

Given any two variables, the third one can be calculated.

**Examples**:

1. A pharmacist weighs out 2 kg of glycerin (density, 1.25 g/mL). What is the volume of the glycerin?

Volume = 2000 g ÷ 1.25 g/ml = 1600 mL.

2. What is the weight of 60 mL of oil whose density is 0.9624 g/mL?

Weight = 60 mL × 0.9624 g/mL = 57.7 g

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