Hydrometer Use: Determining Liquid Properties

A hydrometer is a scientific instrument designed specifically to measure the specific gravity of liquids. Specific gravity is the ratio of the density of a liquid to the density of a reference substance, usually water.

Principle of Operation

The operation of a hydrometer is based on Archimedes' principle, which states that a body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the body.

• A hydrometer consists of a sealed glass float with a weighted stem.
• When placed in a liquid, it sinks until it displaces a volume of liquid whose weight equals the hydrometer's total weight.
• In less dense liquids, the hydrometer sinks lower, displacing more liquid to achieve the required buoyant force.
• In denser liquids, it floats higher because less volume needs to be displaced to equal its weight.
• The stem is calibrated with markings corresponding to specific gravity values, allowing for direct reading.

Evaluating the Options

• Relative humidity: This measures the amount of water vapor in the air and is measured using a hygrometer.
• Buoyant force: While buoyancy is the principle behind the hydrometer's function, the hydrometer itself doesn't measure the buoyant force directly; it uses the effect of buoyancy to measure specific gravity.
• Specific gravity of liquids: This is the primary measurement a hydrometer is designed for. The calibration on the stem directly indicates the specific gravity.
• Viscosity of liquids: This measures a liquid's resistance to flow and requires instruments like viscometers.

Therefore, the correct use of a hydrometer is to determine the specific gravity of liquids.