The specific gravity of a substance is measured using:

The specific gravity of a substance is measured using:

• A. Thermometer
• B. Barometer
• C. Refractometer
• D. Hydrometer

Answer: (D)

Measuring specific gravity relies on how dense a liquid is compared to water, using buoyancy. A hydrometer is designed for this: it’s a weighted float that sinks into the liquid until the buoyant force equals its weight. The depth it sinks depends on the liquid’s density—the denser the liquid, the higher it floats; the lighter the liquid, the deeper it sinks. Because the instrument is marked with a scale calibrated to show relative density, you can read the specific gravity directly from the scale. Temperature matters because density changes with temperature, so readings are usually taken at a standard temperature or with temperature compensation. Thermometer only measures temperature, barometer measures atmospheric pressure, and a refractometer measures refractive index. None of these provide a direct readout of specific gravity like a hydrometer does.

Measuring specific gravity relies on how dense a liquid is compared to water, using buoyancy. A hydrometer is designed for this: it’s a weighted float that sinks into the liquid until the buoyant force equals its weight. The depth it sinks depends on the liquid’s density—the denser the liquid, the higher it floats; the lighter the liquid, the deeper it sinks. Because the instrument is marked with a scale calibrated to show relative density, you can read the specific gravity directly from the scale. Temperature matters because density changes with temperature, so readings are usually taken at a standard temperature or with temperature compensation.

Thermometer only measures temperature, barometer measures atmospheric pressure, and a refractometer measures refractive index. None of these provide a direct readout of specific gravity like a hydrometer does.