Water Density

The density of water (SEA WATER or ocean) is one of the most important water parameters, as all water circulation and currents depend on it.

In simple terms, and explained according to lifelong physics/chemistry, the density is the weight contained in a volume. In other words density=mass/volumeand expressed in grams/litre. The value of 1000 grams/L (1Kg/L) corresponds to distilled water.which has no salt content.

Water density varies mainly according to water salinity and temperature:

• Water components o Water SalinityThe more dissolved components the water has, or the higher the salinity, denser will be water.
• Water temperature. The higher the temperature, the faster the molecules are moving, the more they are separated from each other and the lower their density.
• Atmospheric pressure. Density also varies according to atmospheric pressure. In the liquid state, as water is an incompressible solid, it will be the factor that has the least effect, but not in the case of ice or water vapour.

HOW MUCH IS THE DENSITY OF WATER

The base density of the distilled liquid water at a pressure of 1 atm and a temperature of 4 °C is 1000 kg/m³, i.e. 1 kg/dm³, 1Kg/L or 1 g/cm3. If we consider 1atm and 20ºC, the density of water would be  0,998Kg/L

The value of 1 is therefore for distilled water (pure H2O, which is not what we find in the tap, or in the ocean or sea). It is difficult to give a density value for water as it varies greatly depending on temperature and salinity (the content of salts and components).

To avoid the variation due to the temperature factor in the water, the value of the salinity. Salinity is the concentration of compounds (grams) in 1L of water, and is expressed in g/L or per thousand.

 

TYPE OF WATER/SEA	SALINITY	DENSITY 1 atm 4ºC	DENSITY 1 atm 20ºC
Distilled water	0 g/L	1 Kg/L	0,998 Kg/L
Tap water	3 g/L	1.002 g/cm3	0,9982 Kg/L
Ocean Water	33 - 37 g/L	1,020 to 1,029 Kg/L	1,020 to 1,029 Kg/L

MAIN SEA DENSITY

Based on the salinity of the water, one type of water or another is defined, starting with distilled water (pure water - whose salinity is 0). Then there is drinking water or fresh water, whose salinity is less than 0.05%. Then there is brackish water, which is water that is neither as low in salinity as drinking water nor as seawater. Then we have seawater, whose salinity is between 30 and 50 g/L . Finally, we have brine, whose salinity is higher than 50 g/L and prevents the existence of living beings, as in the case of the Dead Sea.

You can complete the information on density in seawater in the section on seawater salinity.

SEA	SALINITY	DENSITY 25º 1atm
Distilled water	0 g/L	998g/cm3
Tap water	3 g/L	1000 g/cm3
Baltic Sea	6 - 18 g/L	1006-1008 g/cm3
North Sea	32 g/L	1026 g/cm3
Oceans	33 - 37 g/L	1027 g/cm3
Mediterranean Sea	38 g/L	1028 g/cm3
Red Sea / Persian Gulf	38 - 43 g/L	1033 g/cm3
Dead Sea	230 g/L	230 g/cm3

Salinity and density are closely related. However, density is also very dependent on temperature. In this graph you can see the density for different temperatures and densities.

 

The Dead Sea is not technically a sea, it is a lake where the Jordan River flows into the sea. Its salt composition is therefore different from other seas or oceans.

MEASURING THE DENSITY OF WATER

There are several ways to measure the density of water.  Here you can see a device and here how to measure the density of water.

THE DENSITY OF WATER IN THE BEHAVIOUR OF THE OCEAN

DENSITY DISTRIBUTION BASED ON DEPTH

The density of water, or any fluid in which you immerse yourself, is directly proportional to the upward force that this fluid will exert on you. In other words, the denser the water, the more you will be able to float on it. This is the basis of the Archimedes principlewith its famous Eureka.

In relation to the buoyancy of bodies, it does not only depend on the density of the water, or the fluid. Buoyancy also depends on the weight/volume of the body being immersed. In this case we are going to focus on the density of the water/fluid.

In the oceans or seas, because of the density or pressure they exert, the densest waters are those that will be at the bottom. The less dense waters will be the surface waters. According to what we have seen, the hot water is less dense than cold water and will therefore be above cold water. This effect can be felt in the sea itself without the need to swim very deep. You will be able to see how in some areas there is a superficial layer of warm water and below it areas of colder water.

Saltier water is also denser. Therefore, in the same section of the ocean, surface water will be less dense than denser water.

In this graph you can see how density, temperature and salinity vary with depth.

HOW BODIES FLOAT BASED ON THE DENSITY OF WATER

The densest waters, which as we have seen will be the coldest and the saltiest, are the ones that will give you the greatest buoyancy. Besides intuitively, the explanation was given by Archimedes with the principle of buoyancy of bodies.

For example, in the Mediterranean Sea, where the salinity is much higher than in the ocean (due to the evaporation of the water), you will have an easier time floating than in the ocean. You may also have seen a photo in the Dead Sea, where due to its very high salinity (there are no living creatures because of the salinity), you float much more than in any other water.

This image, reading effortlessly, floating in the Dead Sea is now a classic for anyone who visits the area.

CLIMATE REGULATION BASED ON OCEAN CURRENTS

As you can see in this article, on the gulf streamThe density plays a very important role in the circulation of the ocean current, and therefore in the regulation of the climate. Since cold water is denser than warm water, as soon as the water cools at the North Pole, it sinks. This will act as a motor in the circulation and allow the warm current to rise to the surface and cool down. Read more about the Gulf Stream.

 

DENSITY AS A FUNCTION OF TEMPERATURE AND SALINITY

If you are more interested in the mathematical terms pressure, density, temperature here we will look at them in more detail.

The density formula for solid bodies would be as follows:

ρ= m / V

This is the basic formula for the calculation of any density. It depends on the mass (weight colloquially) and the volume it occupies. So if we have a steel ball of the same size as a wooden ball, and the steel ball weighs more, we can conclude that steel is denser than wood. Knowing the weight and the volume it occupies, you can get its density value. For common materials there are also tables that give you the density values without the need to calculate it.

For gases, which are bodies whose volume varies with temperature, density is inversely proportional to temperature. If we put a balloon in the sun, we will see that it swells, just as the wheels of a car swell as they heat up. Mathematically, the expression that defines the relationship between density, pressure and temperature is known as the Law of Ideal Gases:

ρ= p / T R*

p is the pressure of the gas, T corresponds to the temperature and R*= R / M is the ideal gas constant divided by the molecular mass of the gas.

In oceanic waters, the relationship is more complex, since in the ocean we will have water with salts on the one hand, but we will also have dissolved materials and gases. The density in the ocean will therefore also depend on the amount of dissolved materials we have. This property is called salinity (S) and measures the grams of salts in 1 kg of ocean water.

The relationship of density in ocean water will be a relationship to pressure, salinity and pressure. Calculations are much more complex than in the case of solid bodies or ideal gases. Therefore, in many cases, rather than working with calculations, we work with empirical tables.

ρ=ρ(S, T, p)

In this graph, we will see how, by maintaining salinity, density varies with temperature:

In this graph we will see how, maintaining the temperature, the density varies, based on salinity.

MORE ON DENSITY

If you want more information about the density of alcohol and other liquids, you can read about it here: density.