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:
- It's colourless;
- It's tasteless;
- It's odourless;
- It feels wet;
- It dissolves nearly everything;
- It exists in three forms: liquid, solid, gas, and is cycled though the water cycle;
- It can absorb a large amount of heat;
- It sticks together into beads or drops;
- It flows and erodes the surface of the earth; it moves sediments to form beaches,
river banks and bars;
- It shapes lipid and protein molecules and give them their 3-dimensional form which
is critical to their function;
- It's part of every living organism on the planet.
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.
- Ice, for instance, has a very ordered lattice structure.
- Super cooled water (water below the freezing point) also has water molecules that are
structured in a certain way.
- Snowflakes have yet another shape.
Summary Table of Physical Properties
Property |
Value |
Molar mass |
18.015 |
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 |
Viscosity |
1.002 centipoise at 20°C |
Density |
1 g/cc |
Density maxima |
4°C |
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.
- The large heat capacity and high water content in organisms contribute to
thermal regulation and prevent local temperature fluctuations.
- The high latent heat of evaporation gives resistance to dehydration and considerable
evaporative cooling.
- Water is an excellent solvent due to its polarity, high dielectric constant and small
size, particularly for polar and ionic compounds and salts.
- It has unique hydration properties towards biological molecules (particularly lipids,
proteins and nucleic acids) that determine their three-dimensional structures, and hence
their functions, in solution.
This hydration forms gels that can reversibly undergo the gel-sol phase transitions that
underlie many cellular mechanisms.
- Water ionizes and allows easy proton exchange between molecules, so contributing to
the richness of the ionic interactions in biology.
- The density maximum at 4°C and low ice density means that all of a body of water
(not just its surface) is close to 0°C before any freezing can occur.
Also the freezing of rivers, lakes and oceans is from the top down, so insulating the
water from further freezing and allowing rapid thawing, and density driven thermal
convection causing seasonal mixing in deeper temperate waters.
- The large heat capacity of the oceans and seas allows them to act as heat reservoirs
such that sea temperatures vary only a third as much as land temperatures and so
moderate our climate.
- The compressibility of water reduces the sea level by about 40 m giving us 5% more land.
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.
Acids
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.
Bases
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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