Density of Water

Properties of Water

Water is a tasteless, odorless transparent substance, which can be found in three states: liquid (water), solid (ice), and gaseous (vapor). One molecule of water (H2O) is composed of two hydrogen atoms and one oxygen atom, bound by two covalent bonds. The density of water i.e. mass per unit volume of water has some interesting features, different from other liquids. Unlike ordinary liquids, the density of water decreases from 40C to 00C. Above 40C, the density of water decreases with increasing temperature. This behavior is called “density anomaly”. Due to this property, the density of ice is less than the density of water at 40C (the maximum density).

What is Density?

The density of a substance is defined as the mass present in a unit volume of that substance. Another useful notion is number density, mostly used in the study of gas theory. It is given by the number of molecules present in unit volume. Density is an intrinsic property of matter having unit kg/m3 in SI and g/cm3 in CGS methods.

The Density of Water At Room Temperature

The density of water is defined similarly to other substances. At room temperature ( ~ 200C), its value is 998.2 kg/m3. The density of water at 25 degrees Celsius is 997 kg/m3. At room temperature, water remains in a liquid state. The density of distilled water is the same. Seawater has salt and minerals, which make its density higher than normal water. At the sea surface, the density is about 1027 kg/m3.

Effect of Pressure on Density

Density increases when pressure increases and decreases when pressure decreases. As pressure increases, the molecules of a substance come closer resulting in a higher density. On the other hand, when pressure decreases, the molecules become distant. Due to this, the density reduces. This occurs with water also. 

Effect of Temperature on the Density of Liquid

Generally, a liquid expands as temperature increases. As a result, its density decreases. The expansion of the unit volume of liquid for one unit increase in temperature is defined as the coefficient of expansion (\[\gamma\]) of liquid. If the volume and density at temperature \[T_{1}\] are \[V_{1}\] and \[\rho_{1}\] respectively, the volume \[V_{2}\] and density \[\rho_{2}\] at temperature \[T_{2}\] can be obtained as,

\[V_{2}\] = \[V_{1}\] [ 1 + \[\gamma\](\[T_{2}\] - \[T_{1}\]) ]

\[\rho_{2}\]  = \[\rho_{1}\] [ 1 - \[\gamma\](\[T_{2}\] - \[T_{1}\]) ]

For most liquids, is positive i.e. density increases with decreasing temperature. But water does not behave in this conventional way.

Anomalous Expansion of Water

The density of water increases from 00C  to 40C, unlike usual liquids. The density of water is maximum at 40C and the volume reaches a minimum. Beyond this temperature, water behaves like a usual liquid i.e. its density decreases. The maximum density of water is 1000 kg/m3. The density and volume graphs with temperature are shown below,

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Volume and density plots

This behavior is also called the density anomaly.

The density of pure water for temperatures in the range 00-1000C  is listed below:

The Density of Water at Different Temperatures

Cause of the Density Anomaly

• Behavior Above 40C : When water is cooled from high temperature, the thermal energy of the water molecules reduces. The molecules come closer and the density increases. 

• Behavior at Below 40C : In this temperature range as the water molecules come closer, they become able to make stable Hydrogen bonds. Due to lack of thermal motion, more and more hydrogen bonds are formed. This prevents the molecules from coming closer and the density is less.

• Behavior at 40C : Around this temperature, the thermal agitation and the hydrogen bond formation tend to balance each other to give a stationary behavior of density. So, the density of water at 4 degrees Celsius is the maximum and the specific volume is minimum.

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Hydrogen bonds in water and ice

Examples

• Floating Icebergs: Density of ice is one ninth of the density of water due to the density anomaly of water. So, icebergs float on oceans with 90% of their entire volume being underwater. 

• Effect on Marine Life: In cold regions, the temperature of the atmosphere gradually decreases to 00C and further below. The water on the surfaces of lakes and oceans cool down and become heavy. The cold water comes below and the hot water rises. This convection process goes on until the water of lower surfaces become around 40C . Since this water density value is maximum, the water from upper surfaces cannot come down anymore and the convection process stops. The upper layers cool down even more and become ice. The ice then floats on the surface of water. The lower layers however stay at 40C , which is sufficiently warm for aquatic animals to survive.

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Water areas in cold region

Did You Know?

• The density of water at 40C  is maximum and it is in liquid state at this temperature. Ice has less density than water.

• Icebergs float on water but 90% volume of an iceberg remains underwater. It was the reason behind the sinking of the Titanic.

• The “competition” between hydrogen bond formation and thermal motion of water molecules is the reason behind the maximum density of water at 40C.

• The density vs temperature graph is flat around 40C i.e. density is stationary. The density of water at 40C is used to define the unit of density. It was also used to define the unit of mass.