The relative density of dilute tetraoxosulphate (vi) acid in a battery is measured with a

Step 1: Analyze the question The question asks which instrument measures the relative density (specific gravity) of a liquid, here dilute tetraoxosulphate(VI) acid (H₂SO₄) in a battery.

Step 2: Explain the concept Each option is an instrument with its own purpose:

• Barometer: measures atmospheric pressure.
• Hydrometer: measures the relative density of liquids. It floats in the liquid, and the depth to which it sinks shows the density.
• Manometer: measures gas or fluid pressure.
• Hypsometer: determines the boiling point of water, used to calibrate thermometers (the upper fixed point) or to estimate altitude.

Step 3: Apply to the battery A hydrometer floats higher in a denser liquid and lower in a less dense one. A fully charged lead-acid battery has a higher acid density, about 1.25 to 1.28. As the battery discharges, the acid becomes more dilute and the reading drops. Mechanics therefore use a hydrometer (often called a battery tester) to check the state of charge.

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

Exam tip: Remember that hydrometer goes with density of liquids. Link the other names too: baro = air pressure, mano = pressure of gas, hypso = height or boiling point.