A hydrometer is an instrument for measuring the

Analysis of the Question

The question asks for the specific function of a hydrometer. To answer this, we need to understand the principle of flotation and how it relates to the density of liquids.

Concept Explanation

A hydrometer is a glass instrument used to measure the relative density (or specific gravity) of liquids.

1. Principle of Flotation: The hydrometer works based on Archimedes' principle. When placed in a liquid, it sinks until the weight of the liquid it displaces is equal to its own weight.
2. How it works: • In a denser liquid, the hydrometer will float higher because it needs to displace less volume of the liquid to equal its own weight. • In a less dense liquid, the hydrometer will sink deeper.
3. Calibration: The stem of the hydrometer is graduated with a scale. By observing the level at which the liquid surface touches the stem, you can directly read the relative density of the liquid.

Step-by-Step Evaluation of Options

• A (Depth of water): This is incorrect. Instruments like a sounding line or a pressure gauge are used for depth.
• B (Relative density by flotation): This is correct. As explained above, the hydrometer relies on the principle of flotation to determine how dense a liquid is relative to water.
• C (Relative density by apparent loss in weight): This is incorrect. This method describes the use of a density bottle or a Nicholson’s hydrometer (which is different from the standard glass hydrometer) using a balance to measure weight loss.
• D (Relative humidity): This is incorrect. Relative humidity is measured using a hygrometer.

Conclusion

The hydrometer is specifically designed to measure the relative density of liquids by observing how high or low it floats in the liquid.

Exam Tip: Always distinguish between a hydrometer (for liquid density) and a hygrometer (for atmospheric humidity). They sound similar, but they measure completely different physical properties!