Q. Lactometers (used to determine the purity of a sample of milk) and hydrometers (used to determine the density of liquids) are based on which principle?
Answer: Archimedes
Notes: Lactometer is a device used for finding the purity of a milk sample. It works on the principle of Archimedes' principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced. If the milk sample is pure, then the lactometer floats on it and if it is adulterated or impure, then the lactometer sinks .The hydrometer makes use of Archimedes' principle: a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid.