# Seawater Density Calculator

Photo by Silas Baisch on Unsplash

This online seawater density calculator is based on the UNESCO formula which calculates the density of water as a function of water temperature (T), salinity (S) and pressure (p).

## Water Temperature and Density

Within the functional temperature range of this calculator (i.e. 0°C < T < 40°C) liquid water density is inversely proportional to temperature. At a molecular level, as the temperature of the water rises, or in other words, as the kinetic energy of the molecules increase, the increase in molecular movement and collisions means that on average, molecules are positioned further apart, resulting in an increase in volume per unit mass. This means that an increase in temperature results in a slight decrease in density.

## Water Pressure and Density

In many cases, water can be assumed to be incompressible. This is particularly useful in simplifying calculations or analyses related, but not “sensitive”, to water density. On the other hand, oceanographers require density measurements to be accurate to the fifth decimal place [1]. The reality is that water is compressible, albeit a very small amount, and only noticeably so at high pressures. In the context of ocean water density, water pressure increases with depth at a rate of 1 atmosphere for every 10.06 metres of descent [2]. This means that at a depth of 1 km, the water pressure is approximately 100 atmospheres or 101.3 bar. Therefore, as the pressure increases (at deeper depths of the ocean), the water is slightly compressed and the water density increases.

## Salt and Water Density

When salt (sodium chloride or NaCl) dissolves in water, there is a significant increase in mass of the solution due to the relatively higher molecular mass of the dissolved ions Na (22 g/mol) and Cl (35.5g/mol) when compared to water or H2O (20 g/mol).

Interestingly, the dissolved salt does not increase the volume of the water by the volume of the added salt, and this is due to the charge of the Na and Cl ions and the H2O molecules. Because of the geometry of water molecules, they are essentially dipoles with “positive and a negative ends”. The positive Hydrogen ends of the water molecules are attracted to the negatively charged Cl ions, and the positive oxygen ends are attracted to the positively charged Na ions. In this more ordered arrangement, the ions effectively fill the voids between the water molecules, and the volume of the water only increases slightly.

See this excerpt from a study on the volumetric effects due to ion-solvent interaction in aqueous electrolyte solutions:

The interaction of the electrostatic field of an ion with water tends to align the dipolar water molecules in the direction of the field. In this way the field tends to disrupt hydrogen bonded structures in liquid water, and to compress the water molecules surrounding an ion. These electrostatic effect give rise to a shrinkage of the water.

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