Which of the following devices is used to determine the relative density of an acid?

Step 1: Analyze the question We need the instrument used to measure the relative density (specific gravity) of a liquid, in this case an acid. This is the kind of thing you'd check in a car battery (lead-acid) to see its charge level.

Step 2: Explain the concept Relative density is the ratio of the density of a substance to the density of water. A hydrometer is a sealed, weighted glass tube that floats in a liquid. It floats lower in less dense liquids and higher in denser ones. The level at which the liquid surface meets the scale gives the relative density directly.

Step 3: Check each option

• A. Manometer: measures fluid pressure (e.g., gas pressure) using a U-tube of liquid. It does not measure relative density.
• B. Hydrometer: measures the relative density of liquids such as acids, milk, and alcohol. ✔
• C. Hypsometer: measures the boiling point of water, used to calibrate thermometers' upper fixed point or to estimate altitude. It is not for density.
• D. Hygrometer: measures humidity (moisture in the air).

Step 4: Answer The correct option is B. hydrometer.

Exam tip: Match the Greek root to the meaning:

• Hydro = liquid/water → hydrometer (density of liquids)
• Hygro = moisture → hygrometer (humidity)
• Mano = pressure → manometer
• Hypso = height → hypsometer (boiling point/altitude)