Specific gravity is the ratio of a substance’s density to the density of water. It can determine purity and concentration, and is often measured at 39°F/4°C. Archimedes discovered it by comparing the density of gold to that of water. Other substances can be used as a reference point.
Specific gravity usually refers to the density of a material relative to the density of water. This number is given as a ratio, meaning there are no units when discussing it. Using a known specific gravity for a particular substance, it is possible to determine which sample of the substance is pure or how concentrated it is. In some industries, the density of two substances may be compared to each other rather than water when determining this measurement.
In most cases, specific gravity is given as the ratio of the density of a certain substance to the density of the same amount of fresh water at 39 degrees Fahrenheit (4 degrees Celsius). At this temperature, fresh water is at its maximum density, which is 1 gram per milliliter. A substance with a specific gravity lower than that of water will float above water, while an object with a higher one will sink. Temperature and atmospheric pressure alter the specific gravity of a substance, so standard specific gravity values are determined at this temperature and one atmosphere of pressure, which is the air pressure at sea level.
In addition to the standard measurements, it is also possible to compare the density of two substances with each other instead of comparing them with the density of water. Many different industries use their own sets of density comparisons which help them control concentration. When testing the density of gases, air is often used as the standard, and liquids and solids are often tested against water, although other liquids can be used. It is also possible to measure a value known as the bulk specific gravity of a substance by creating a ratio of the weights of equal volumes of the test substance to a reference substance, such as water.
The concept of specific gravity was discovered by the Greek scientist and inventor Archimedes. When he was tasked with determining whether the new king’s crown was made of solid gold, Archimedes discovered that he could take the crown and an equal mass of solid gold and see if the two displaced the same amount of water from a tub. Soon after this discovery, the densities of a number of different substances were calculated so that it was easy to determine their purity. It is also possible to take an unknown substance and experimentally determine its density relative to water to narrow down the possible materials it could be composed of.
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