Which instrument is used to determine the specific gravity of solids, minerals, and liquids?

Which instrument is used to determine the specific gravity of solids, minerals, and liquids?

• A. Westphal balance
• B. Hydrometer
• C. Pycnometer
• D. Refractometer

Answer: (A)

Specific gravity is determined by comparing density to that of water, which for solids and minerals is most reliably found from buoyancy. The Westphal balance is built to weigh a sample in air and then in water on the same instrument, so the change in weight equals the buoyant force from the displaced water. Using Archimedes’ principle, this directly yields the sample’s volume and, together with its air weight, its specific gravity relative to water. This makes it especially practical for irregular solid forms like minerals, where measuring volume directly is difficult. It can also be used for liquids by the same buoyancy principle, but other tools serve different primary purposes: a hydrometer is for liquids only; a pycnometer requires precise volume control and is more involved for solids; a refractometer measures refractive index, not density.

Specific gravity is determined by comparing density to that of water, which for solids and minerals is most reliably found from buoyancy. The Westphal balance is built to weigh a sample in air and then in water on the same instrument, so the change in weight equals the buoyant force from the displaced water. Using Archimedes’ principle, this directly yields the sample’s volume and, together with its air weight, its specific gravity relative to water. This makes it especially practical for irregular solid forms like minerals, where measuring volume directly is difficult. It can also be used for liquids by the same buoyancy principle, but other tools serve different primary purposes: a hydrometer is for liquids only; a pycnometer requires precise volume control and is more involved for solids; a refractometer measures refractive index, not density.