Physical Properties of Water

Links between Chemical Structure and Physical Properties

Water (H2O) is a very unusual substance with many strange and unique properties that are so important to Life on Planet Water - Life which is based on Water and adapted to its unique and anomalous properties.

How does this simple molecule, composed of two hydrogen atoms and one oxygen atom, behave the way it does and how does it support life?

Some familiar properties of water are: Many of water's unique properties are largely a result of its chemical structure.

The two hydrogen atoms bound to one oxygen atom to form a 'V' shape with the hydrogen atoms at an angle of 105°. When the hydrogen atoms combine with oxygen, they each give away their single electron and form a covalent bond. Because electrons are more attracted to the positively charged oxygen atom, the two hydrogens become slightly positively charged (they give away their negative charge) and the oxygen atom becomes negatively charged.

This separation between negative and positive charges produces a polar molecule, that is a molecule that has an electrical charge on its surface. The hydrogen lobes have positive charges, and the oxygen atom on the opposite side has two negative charges (associated with two lobes. The net interaction between the covalent bond and the attracting and repulsion between the positive and negative charges repelling charges produces the 'V' shape of the molecule.

The polarity of water allows it to bind with other molecules, including itself. The water molecules form hydrogen bonds, giving shape to water as a liquid. Each single water molecule can form bonds with four other water molecules in a tetrahedral arrangement. Although these bonds are weak they lead to many other unique properties.

The V-shape of the water molecule is also important because it allows for other configurations of water to be formed.

Summary Table of Physical Properties



Molar mass


Molar Volume

55.5 moles/liter

Boiling Point (BP)

100 degrees C at 1 atm

Freezing point (FP)

0 degrees C at 1 atm

Triple point

273.16 K at 4.6 torr

Surface Tension

73 dynes/cm at 20°C

Vapor pressure

0.0212 atm at 20°C

Heat of vaporization

40.63 kJ/mol

Heat of Fusion

6.013 kJ/mol

Heat Capacity (cp)

4.22 kJ/kg.K

Dielectric Constant

78.54 at 25°C


1.002 centipoise at 20°C


1 g/cc

Density maxima


Specific heat

4180 J kg-1 K-1 ( T=293 C 373 K)

Heat conductivity

0.60 W m-1 K-1 (T=293 K)

Melting heat

3.34 x 105 J/kg

Evaporation heat

22.6 x 105 J/kg

Critical Temperature

647 K

Critical Pressure

22.1 x 106 Pa

Speed of sound

1480 m/s (T=293 K)

Relative permittivity

80 (T=298 K)

Index of refraction (relative to air)

1.31 (ice; 589 nm; T=273 K; p=p0)

1.34 (water; 430-490 nm; T=293 K; p=p0)

1.33 (water; 590-690 nm; T=293 K; p=p0)

Physical Properties of Water and Links to Life

Life on Planet Water depends on the unique and unusual properties of water. Water is 'mother' and 'matrix' for life.

Hydrophylic ('Water Loving') and Hydrophobic ('Water Hating') Molecules

Hydrophylic Molecules

Substances that dissolve readily in water are termed 'hydrophilic'. They are composed of ions or polar molecules that attract water molecules through electrical charge effects. Water molecules surround each ion or polar molecule on the surface of a solid substance and carry it into solution. Ionic substances such as sodium chloride dissolve because water molecules are attracted to the positive (Na+) or negative (Cl-) charge of each ion. Polar substances such as urea dissolve because their molecules form hydrogen bonds with the surrounding water molecules.

Hydrophobic Molecules

Molecules that contain mostly nonpolar bonds are usually insoluble in water and are termed 'hydrophobic'. This is true, especially, of hydrocarbons, which contain many C-H bonds. Water molecules are less attracted to such molecules than they are to other water molecules and so have little tendency to surround them and carry them into solution.

But the so-called 'Hydrophobic Effect' does not mean that nonpolar molecules are not attracted to water! When a highly polar substance, such as water, is mixed with a nonpolar or weakly polar substance, such as most oils, the substances will separate into two phases. This phenomenon is usually rationalized in introductory chemistry text books by saying that oil is hydrophobic.

Most people wrongly believe that this means that individual water and oil molecules repel each other, or at least attract each other very weakly. However, this is clearly wrong and misleading!

In fact an individual oil molecule is attracted to a water molecule by a force that is much greater than the attraction of two oil molecules to each other. This can be demonstrated when a drop of oil is placed onto a clean surface of water. Originally the oil will be in the shape of a spherical droplet, because the oil molecules are attracted to one another and a spherical shape minimizes the number of oil molecules that are not surrounded by other molecules. When the oil droplet hits the surface of the water, it spreads out to form a thin layer. This happens because the oil and water bonds formed by the oil forming a layer on the surface of the water are stronger than the oil-oil attraction in the oil droplet. If a sufficiently small drop of oil is put on the surface, it will spread to form a single molecular layer of oil.

Given these strong interactions, why doesn't each oil molecule dive into the water solution? and become completely surrounded with water molecules? The reason is that the water-water bonds are much stronger! Displacing the water molecules would cost more energy. Consequently most of the oil molecules stay out of the water, though as many as will fit will hang on to the surface water molecules that do not have a full complement of partners. A similar explanation applies for the meniscus, that is the curved surface of a liquid in a graduated cylinder or any other small diameter glassware. Water adheres to the sides of any container creating a "cup" of surface tension.

The induced structure produced through the interaction with water molecules is very important as it is related to the structure and function of membranes which are very characteristic of life as we know it. Membranes in bacteria are composed of phospholipids and proteins. Phospholipids contain a charged or polar group (often phosphate, hence the name) attached to a 3 carbon glycerol back bone. There are also two fatty acid chains dangling from the other carbons of glycerol. The phosphate end of the molecule is hydrophilic and is attracted to water. The fatty acids are hydrophobic and are driven away from water.

Because phospholipids have hydrophobic and hydrophilic portions, they do remarkable things. When placed in an aqueous environment, the hydrophobic portions stick together, as do the hydrophilic bits. A very stable form of this arrangement is the lipid bilayer. This way the hydrophobic parts of the molecule form one layer, as do the hydrophilic. Lipid bilayers form spontaneously if phospholipids are placed in an aqueous environment. The cytoplasmic membrane is stabilized by hydrophobic interactions (i.e. water induced) between neighboring lipids and by hydrogen bonds between neighboring lipids. Hydrogen bonds can also form between membrane proteins and lipids. These are known as membrane vesicles and are used to study membrane properties experimentally. There is some evidence that these structures may form abiotically and may occur on particles that rain down on earth from space.

Extract air to make oil and water mix

One of the great truths of life, that oil and water do not mix, has been turned on its head. The secret to making them mix without chemicals, according to Ric Pashley, a chemist at Canberra's Australian National University, is extracting all the dissolved air from the water. "It makes an emulsion, not quite as cloudy as milk," the chemist said. The discovery, which could lead to everything from new medicines to paints and perfumes, has delighted scientists around the world.

In 1982, Professor Pashley discovered something called long-range hydrophobic force, now accepted as the reason oil and water do not normally mix. He explained that oil droplets can attract each other over a distance as large as their own radius. As a result, oil droplets merge rather than disperse in water.

A typical litre of water, he noted, contains about two millilitres of dissolved air. Suspecting that was the problem, he extracted 99.999 per cent of the dissolved air from some water. To his joy, it mixed with oil, forming an emulsion that did not separate.

Water as a Solvent - Acids & Bases - pH - Hydration

Water as a Solvent

Many substances, such as salt and sugar, dissolve in water. That is, their molecules separate from each other, each becoming surrounded by water molecules. When a substance dissolves in a liquid, the mixture is termed a solution. The dissolved substance (in this case salt or sugar) is the solute, and the liquid that does the dissolving (in this case water) is the solvent. Water is an excellent solvent for many substances because of its polar bonds.


Substances that release hydrogen ions into solution are called acids. Many of the acids important in the cell are only partially dissociated, and they are therefore weak acids-for example, the carboxyl group (-COOH), which dissociates to give a hydrogen ion in solution. This is a reversible reaction.


Substances that reduce the number of hydrogen ions in solution are called bases. Some bases, such as ammonia, combine directly with hydrogen ions. Other bases, such as sodium hydroxide, reduce the number of H+ ions indirectly, by making OH- ions that then combine directly with H+ ions to make H2O. Many bases found in cells are partially dissociated and are termed weak bases. This is true of compounds that contain an amino group (-NH2), which has a weak tendency to reversibly accept an H+ ion from water, increasing the quantity of free OH- ions.

Hydrogen Ion exchange

Positively charged hydrogen ions (H+) can spontaneously move from one water molecule to another, thereby creating two ionic species. Since the process is rapidly reversible, hydrogen ions are continually shuttling between water molecules. Pure water contains a steady state concentration of hydrogen ions and hydroxyl ions (both 10-7 M). pH The acidity of a solution is defined by the concentration of H+ ions it possesses. For convenience we use the pH scale, where pH = _log10[H+]. For pure water [H+] = 10_7 moles/liter

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