Question

Instrument used for measuring the density or relative density of liquids is known as.

Answer 1

Relative density is the ratio of the density of a substance to the density of a given reference material. Instrument used for measuring the density or relative density of liquids is known as hydrometer. This instrument was invented by William Nicholson in 1790. It is a simple device which largely resembles the thermometer.

Complete step by step solution:
Hydrometer is also called an aerometer that consists of a weighted, sealed, long-necked glass bulb which is immersed in the liquid being measured; the depth of flotation gives an indication of liquid density, and the neck can be measured to read the density and specific gravity.
This instrument is used for measuring the relative density of liquids based on the buoyancy concept. Density is tough to measure directly and accurately, as it depends on two separate properties such as mass and volume. However, density comes into play with fluids and floatation. For example, a denser object will always sink into a less dense fluid, and float on a fluid of greater density. Based on density, a hydrometer will float at a different level within a liquid.
Hydrometers have different uses, such as a lactometer for measuring the density of milk, a saccharometer used to measure the sugar density in a liquid, or an alcohol meter for measuring higher levels of alcohol in spirits.

Note:
Hydrometer is not only used to determine the density of liquids but also to measure the volume in a graduated cylinder and then needing to weigh the liquid to determine its mass. Different hydrometers use different scales for measuring density. For measuring the liquid density, the unit of hydrometer is gram per centimeter cubic.