The instrument to measure the relative density of liquid is

Analysis

The question asks for the specific scientific instrument used to determine the relative density (or specific gravity) of a liquid.

Concept Explanation

Relative density is the ratio of the density of a substance to the density of a standard substance (usually water). To measure this efficiently in a laboratory setting, we use the principle of flotation (Archimedes' Principle). An object floats in a liquid when the weight of the liquid it displaces is equal to its own weight.

• Hydrometer: This is a glass instrument designed to float in a liquid. It is calibrated with a scale that allows you to read the relative density directly based on how deep the instrument sinks into the liquid. The denser the liquid, the higher the hydrometer floats.

Step-by-Step Evaluation

• A. Thermometer: Used to measure temperature, not density.
• B. Hydrometer: Specifically designed to measure the relative density of liquids. This is the correct tool.
• C. Spring balance: Used to measure weight or force. While it can be used in an experiment to calculate density (by weighing an object in air and then in liquid), it is not an instrument designed for direct relative density measurement.
• D. Beaker: A simple glass container used for holding or mixing liquids; it has no measurement scale for density.

Conclusion

The hydrometer is the standard instrument used for this purpose. It is commonly used in battery maintenance (to check electrolyte density) and in the dairy industry (to check the quality of milk).

Exam Tip: Remember the "Hydro-" prefix usually relates to water or liquids. A Hydrometer measures liquid properties, while a Hygrometer (often confused with it) measures humidity in the air. Don't mix them up!