Question

Which instrument is generally used to measure the specific gravity of milk?

Answer 1

Hint: Specific gravity is a tool to derive the relative density of a liquid. It is expressed as the ratio of the substance’s density of which the relative density is to be calculated to the reference liquid’s density. The relative density of a substance is termed as the specific gravity when the reference liquid is water.

Complete solution:
Specific gravity can be measured with an instrument based on the concept of buoyancy. Buoyancy is a force exerted by a liquid to a partially or wholly immersed body in that liquid. Buoyancy always acts opposite to the direction of weight of the partially or wholly immersed body.
Hydrometers are the instruments used to determine the specific gravity of a liquid. They can be calibrated in many scales to measure the specific gravity of various liquids. A hydrometer has two parts: a broad, sealed bottom part and a narrow tube with measurement graduations. The bottom part is sealed with lead or mercury, and the testing liquid is poured into the narrow upper tube. Then the instrument is sunk in the testing liquid until it can float freely. The testing liquid then moves upwards due to the action of buoyancy. Finally, the marking of the scale is recorded to determine the specific gravity of that liquid. Hydrometers calibrated to determine the specific gravity of milk is known as the Lactometer.

Note: The Lactometer measures the creaminess of the milk. While measuring the specific gravity, one must be cautious about taking the same volume of the testing liquid and reference liquid. However, the value of specific gravity and the liquid’s real density is the same under room temperature.