The specific gravity of water, which is defined as 1, is a convention chosen for ease of comparison and calculation. Specific gravity is the ratio of the density of a substance to the density of a reference substance, typically water at a standard temperature and pressure.
The specific gravity scale was initially developed based on the observation that different substances have different densities. Density is a measure of how much mass is packed into a given volume. Water was chosen as the reference substance because it is a common and readily available liquid with a well-known density.
The specific gravity of water being defined as 1 means that the density of water is also 1 gram per cubic centimeter (g/cm³) or 1 kilogram per liter (kg/L) at the standard temperature and pressure, which is typically around 25 degrees Celsius (77 degrees Fahrenheit) and atmospheric pressure.
By convention, if a substance has a specific gravity less than 1, it is less dense than water, and if it has a specific gravity greater than 1, it is denser than water. For example, if a substance has a specific gravity of 0.8, it is 0.8 times less dense than water, while a substance with a specific gravity of 2 is twice as dense as water.
It is important to note that the specific gravity of water can vary slightly with temperature and impurities present in the water. However, for most practical purposes, it is considered to be 1.
In summary, the specific gravity of water being defined as 1 is a convention chosen for comparative purposes, with water at a standard temperature and pressure serving as the reference substance.