A hydrometer is an instrument used in measuring?

Analyze the question: We are asked what a hydrometer measures.

Explain the concept: A hydrometer is a sealed, weighted glass tube with a graduated stem. It floats upright in a liquid. The denser the liquid, the higher the hydrometer floats, because less of it needs to be submerged to displace a weight of liquid equal to its own weight (the principle of flotation). The depth to which it sinks is read off the scale on the stem.

The scale gives the relative density (specific gravity) of the liquid, which is the ratio of the density of the liquid to the density of water. Examples include:

• A lactometer, which tests milk.
• A battery hydrometer, which tests the acid in a car battery.

Check each option:

• A. Vapour pressure of a fluid: Wrong. Vapour pressure is measured with a manometer or similar device.
• B. Density of a liquid: Close, but a hydrometer is conventionally calibrated to read relative density (a ratio with no units), not density in kg/m³. This is the trap option.
• C. Relative humidity of a liquid: Wrong. Relative humidity applies to air and is measured with a hygrometer. Note the similar-sounding names.
• D. Relative density of a liquid: Correct.

Answer: D.

Tip: Remember "Hydrometer = Density (relative)" and "Hygrometer = humidity". The hydrometer's "d" in "hydro" can remind you of "density". Also, when both "density" and "relative density" appear as options, JAMB usually wants "relative density" for the hydrometer.