Water is the most abundant, most accessible and one of the most studied chemical compounds. The fact that it is located throughout the planet, its crucial importance to man's survival and its ability to exist in solid, liquid and vapor phases and transform between them readily has made this liquid uppermost in the thinking of man throughout time.

Water plays an important role in many religions and in 500 BC was considered by many secular philosophers as the sole fundamental principle in nature. Thales of Miletus said,"It is water that, in taking different forms, constitutes the earth, atmosphere, sky, mountains, gods and men, beasts and birds, grass and trees, and animals down to worms, flies and ants. All these are different forms of water. Meditate on water." Although this sounds strange and perhaps farfetched, even to the ears of freshman chemists, we should remember that many marine invertebrates are 96 - 97 % water and the human embryo, during its first month, is 93 % water. (As Steve Wright, the deadpan comedian, would say we are this far {show two finger barely separated from each other} from drowning.) Aristotle consider water as one of the four elements along with, earth, fire and air.

Water is distributed very unevenly around the globe with 97.33 % as salt water and 2.67 % as freshwater. One can understand with so little fresh water why purification and recycling is so important. Lakes and streams account for only 0.01% of all the water on our planet. Groundwater is the largest accessible reserve but still accounts for only 0.6% of the total water and only about half of this is within reach of even the deepest wells. Water makes up about 0.023 % of the total mass of the earth, with about 1.38 x 1021 kg distributed, on, and below the earth's surface. The volume of this amount of water is 1.4 billion cubic kilometers. Over 80 % of the surface of the earth is covered by water (ice caps, lakes and streams, and oceans). Water is essential to most forms of life and accounts for more than 60 % of the mass of the human body.

There are 328 million cubic miles, or 3.612 x 1020 gallons (361,200,000,000,000,000,000 gallons) of sea water in the ocean. If all that water were piled on top of the United States the land would be submerged under 88.2 miles of water. If the same amount of sea water was to be piled up onto all of the available land above sea level on this planet, the land would be 5.7 miles below the water's surface.

Water is really pretty interesting.

Most solids expand up to 10 % of their volume when they melt. Water expands by the same amount when it freezes!

Most solids are more dense than the liquid phase. However, ice has a density of 0.917 g/cm3 while liquid water has a density of 0.998 g/cm3. Lakes freeze from the top down, insulating the liquid water below. If the density of the solid were greater than that of water, lakes would freeze from the bottom up and all the animal and plant life in the aquatic environment would die.

Water has a melting point which is 100 degrees C higher than expected for its group of hydides.

Water has a boiling point 200 degrees C higher than expected for its group of hydides.

Water is a liquid at room temperature. The hydrogen compounds around oxygen are all toxic gases, CH4, NH3, H2S, HF.

Water has the highest surface tension of any liquid except mercury. High surface tension helps plants move water from the earth surface up into the tree limbs and leaves.

Water is an excellent solvent, dissolving many ionic compounds which are insoluble in other compounds.

Water has the highest heat of vaporization of all known substances. The heat needed to vaporize 1 g of water at 100 degrees C is 20,000 joules. The result is perspiration when it evaporates from our body cools because the heat from our body is absorbed by the water as it changes phase from liquid to vapor.

Water has a high heat capacity. That means it can absorb large amounts of heat without large changes of temperature. Its heat capacity is 10 times copper or iron. This property accounts for the moderating influences on the climate around cities near large bodies of water. Without water the temperature changes between day and night would be much greater than we experience. Large bodies of water release heat absorbed from the sun during the day, at night, or during the winter. Most of the water is contained in the oceans and the high heat capacity of this large volume of water (1.35 million cubic kilometers) buffers the Earth surface from large temperature changes such as those observed on the moon.

All of these seemingly anomolous properties can be understood in terms of the intermolecular attractive forces which exist between water molecules. The particular intermolecular attractive force is called hydrogen-bonding.

Lets look at a simulation which gives us an idea of what is meant by the term attractive forces.

What follows is a narrative of what I might say during the attractive forces animation. You might print these notes out so you could read along as the simulation ran.

Let's begin by considering a sample of a gas. We will view this sample of gas in terms of the kinetic-molecular model. That is a gas consists of a collection of widely spaced particles in constant, chaotic motion. The kinetic energy of the gas particles, which is proportional to the temperature of the gas, is much greater than any attractive force that exists between the particles. So all collisions occuring between particles are elastic.

As we cool a gas, lower its temperature, the molecule's velocity drop and they will begin to 'stick' together. This is not a chemical bond being formed but an attraction that molecules feel for each other which is not observed at high temperature because of the high kinetic energy of the molecules. In a gas when the particles are widely spaced there are no attractive forces between the particles. However, as the temperature drops the velocity of the particles drops and they come closer together.

The intermolecular attractive forces in a liquid are sufficient to hold the particles in close proximity, yet there is still chaotic motion. So liquids can flow. In liquids the particles are closer together, and the density of liquids are greater than gases. There is very little free space between particles in the liquid phase, so liquids are not very compressible. Liquids have definite volumes, but do take the shape of the container which they occupy.

In solids the intermolecular forces are sufficiently strong relative to the kinetic energies that molecules are virtually locked in place. Each particle occupies a certain position relative to its neighbors often in a regular pattern that extends throughout the solid. Solids, like liquids are not very compressible because of the lack of space. Because particles are not free to undergo long range movement solids are rigid they do not flow. Translational motion is restricted, but they can vibrate in position.

The 'stickyness' exhibited by particles at lower temperature, which result in the formation of liquids and eventually solids is due to intermolecular attractive forces. Intermolecular means between molecules. Intramolecular means between atoms. Intramolecular forces are what we call covalent bonds and are very strong (100 - 1000 kJ/mol). Intermolecular forces are between molecules and are weak (.1 - 40 kJ/mol). It is intermolecular forces which explain the formation of liquids and solids in covalent compounds. The intermolecular attractive forces are electrostatic in nature.

Water is an interesting compound! Lets consider its structure..its geometry. What is the shape of water? We can answer this question by drawing its Lewis structure. Water has a central oxygen atom with two lone-pairs of electrons and two bonding pairs of electrons. The geometry about water is bent.

The oxygen has two unpaired electrons occuping orbitals around the oxygen. Those electrons are not balanced by the electrons bonding to the hydrogen atoms. Oxygen is sharing two pairs of electrons with the hydrogen. But because oxygen has so many more protons than hydrogen it tends to hold the electrons closer to it than do the hydrogens. The result is oxygen has some partial negative charge on it, causing the hydrogens to have a partial positive charge. This makes water like a small magnet.

Water is called a polar compound. Anytime a central atom has a lone pair of electrons the compound is most likely polar. Polar compounds act like little magnets and are attracted to each other. The unique properties of water are a result of the shape of the water molecule and its polarity.


The boiling points of the Noble Gas elements and the hydrides of Groups IV - VI show some interesting patterns. Here is an interactive movie of these boiling points. Notice the boiling point of the Noble Gases increases with increasing molar mass. This is as expected. Proceeding down the group each succeeding Noble Gas has more electrons. It is larger, more massive and more polarizable. Since the Noble Gases are monoatomic the only intermolecular attractive force which occurs between adjacent atoms are of London Dispersion type. The more electrons, the more polarizable, the stronger the instantaneous-induced dipole interaction the higher the boiling point. The trend for the nonpolar Group IV hydrides is similar, as is the explanation of the observed trend. However, in the Group V, VI and VII hydrides an unusual observation is made. The lightest member of each series has boiling point which is significantly higher than would be predicted. (Note: Predicting the boiling point of the lightest member would be accomplished by extrapolation from the data of three heavier members of a series.) In the case of water it is 200 degree higher than expected. Clearly the intermolecular attractive forces occurring in H2O, NH3 and HF are significantly stronger compared to the other members of a series.

A site at New York University has an introduction to hydrogen-bonding interactions in water and ice. This site also has several very nice animations depicting the hydrogen-bonding interaction between water molecules. Another page at this site provides VRML and PDB views of different numbers of water molecules. It should be noted that Mac users may have trouble viewing the VRML versions of the water structures at this site.

Here is an mpeg movie which shows two water molecules interacting in the vapor phase at 400 K. This movie was produced by Dr. Lars Ojamäe is an assistant professor at the Department of Physical Chemistry, Stockholm University, Stockholm, Sweden.

Here are images of water in the liquid phase

and water in the solid phase.