What instrument is used to determine the density of a liquid relative to water?

What instrument is used to determine the density of a liquid relative to water?

• A. Barometer
• B. Pycnometer
• C. Hydrometer
• D. Densitometer

Answer: (C)

Understanding whether a liquid is denser or lighter than water relies on buoyancy. A hydrometer floats in the liquid and is calibrated so its depth of immersion translates directly into a density reading relative to water. Since water has a density of about 1 g/mL at standard conditions, the hydrometer essentially reads the liquid’s specific gravity: a denser liquid makes the device float higher, while a less dense liquid makes it sink deeper. This direct, quick reading is why the hydrometer is the go-to instrument for comparing density to water in laboratory and field tests. Other instruments either measure something different (a barometer relates to atmospheric pressure) or require more involved procedures to determine an absolute density (a pycnometer measures density by a known volume, yielding an exact value, not a direct relative reading), or provide density values that aren’t specifically expressed as a ratio to water (a densitometer/density meter).

Understanding whether a liquid is denser or lighter than water relies on buoyancy. A hydrometer floats in the liquid and is calibrated so its depth of immersion translates directly into a density reading relative to water. Since water has a density of about 1 g/mL at standard conditions, the hydrometer essentially reads the liquid’s specific gravity: a denser liquid makes the device float higher, while a less dense liquid makes it sink deeper. This direct, quick reading is why the hydrometer is the go-to instrument for comparing density to water in laboratory and field tests. Other instruments either measure something different (a barometer relates to atmospheric pressure) or require more involved procedures to determine an absolute density (a pycnometer measures density by a known volume, yielding an exact value, not a direct relative reading), or provide density values that aren’t specifically expressed as a ratio to water (a densitometer/density meter).