Properties of Water

Without water, there would be no oceans, no lakes, no rivers, no rain, no snow, no hail, no clouds, no polar ice caps, no Jolt cola, nothing to drink whatsoever, and probably no you, no me, no nothing! Water is everywhere; it defines our planet; it is intricately involved in just about every process on this planet in one way or another. Water rules!

Yet how does this simple molecule, composed merely of two hydrogen atoms and one oxygen atom (hence the chemical designation H2O) do all the amazing things that it does? That's what we are about to find out.

Here are ten properties of water that are familiar to us all:

  1. It's colorless;
  2. It's tasteless;
  3. It's odorless;
  4. It feels wet;
  5. It's distinctive in sound when dripping from a faucet or crashing as a wave;
  6. It dissolves nearly everything;
  7. It exists in three forms: liquid, solid, gas;
  8. It can absorb a large amount of heat;
  9. It sticks together into beads or drops;
  10. . It's part of every living organism on the planet.

Let's look at some of the chemical properties of water that give it these characteristics.

Water is a polar molecule

Water's unique properties are largely a result of its simple composition and structure. As mentioned above, water is composed of two hydrogen atoms bound to one oxygen atom. As shown in your book, the two hydrogen atoms are smaller (the smallest atom there is, in fact) and they rest on both sides of the larger oxygen atom at an angle of 105. When the hydrogen atoms combine with oxygen, they each give away their single electron and form what is known as 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 creates what is known as a polar molecule, meaning a molecule that has an electrical charge on its surface somewhere. Although the water molecule as a whole has no charge, the parts of it, the hydrogen wings and the oxygen body, do exhibit individual charges.

Water has a shape

The polarity of water allows it to "hook up" with other molecules, including itself. As shown in the figure, water molecules can form hydrogen bonds, which give shape to water as a whole. One single water molecule can form bonds with four other water molecules, and while these bonds aren't particularly strong, the fact that water can form so many of them gives water its "sticky" nature. These hydrogen bonds also give water many other unique properties as we shall see in a minute.

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. Supercooled water (water below the freezing point) also has water molecules that are structured in a certain way. Snowflakes have yet another shape. If you were to take some V-shaped paper clips and arrange them in any three dimensional pattern you can think of, you would probably have some idea of the variety of forms in which water can exist.

Water dissolves almost anything

More substances dissolve in water than in any other liquid. For this reason, water is often called the "Universal Solvent." The reason for water's excellent dissolving capability relates to its polarity; water offers positive and negative charges to which other atoms of molecules can attach.

Note how water molecules can surround the positive sodium ion or the negative chloride ion, the common components of table salt. Water surrounds positive atoms (or the positive end of a polar molecule) with the negative charge of the oxygen atom. Around negatively charged atoms or molecules, water places the positive hydrogen atoms first.

Look at the difference in the spacing between water and positive or negative atoms. Around the sodium atom, the positive hydrogen atoms are still free to bind with other atoms. However, in the case of chlorine, the packing of the atoms is tighter. The arrangement of water molecules around any other atom or molecule leads to differences in water's ability to dissolve a substance. Hence, some things are easier to dissolve in water than others.

Water exists in three forms

Water can exist on our planet in three physical states (i.e. state of matter) under them ambient conditions that normally occur. Water can be a liquid (water), a gas vapor (clouds), or a solid (ice). Think about this. If the ambient conditions of Earth were much cooler (or at higher pressure), we would all be frozen. Alternatively, if the Earth was hotter than Hades, we would be bathed in perpetual clouds (like Venus).

As most of you know, water turns into a vapor (i.e. boils) at 100C and turns into ice (i.e. freezes) at 0C at standard atmospheric pressure (i.e. at sea level). However, what you might not know is that these changes of state require energy input or removal in addition to the energy input/removal required to change the temperature.

Water has a neutral pH

pH is a measure of the acidity or alkalinity of a substance. Formally, pH is defined as the negative logarithm of the concentration of hydrogen ions in an aqueous solution. For our purposes, you just need to know that some liquids are acidic (having more hydrogen ions) and some are basic (having more hydroxyls, or OH ions).

The pH scale ranges from 0 to 14. The lower the pH the greater the acidity. Conversely, the higher the pH, the more alkaline a substance.

Pure water has a pH of 7, which is neutral. Seawater with its dissolved minerals tends to be slightly basis, around 8 or so. The pH scale lists some common substances and their pH values. Take a few moments to familiarize yourself with the pH values of everyday items.

Water has the highest latent heat of vaporization

To increase the temperature of water, energy in the form of heat must be added. This heat-energy is measured in calories. One calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1 degree. Thus, to raise the temperature of 1 gram of water from 0 degrees to 100 degrees would require 100 calories of heat. However, to change 1 gram of water from a liquid to a gas requires 540 calories. No change in temperature occurs; there is only a change in the physical state of the water as it turns from a liquid to a gas. The heat needed to change water from a liquid to a gas is called the latent heat of vaporization. Water's exceptionally high latent heat of vaporization is what makes water so hard to boil.

As we all know, vaporization of water (known as evaporation) occurs at other temperatures as well. This process has to do with random motion of molecules at liquid-gas interfaces and physical laws governing vapor pressure. Simply put, not all water molecules in a glass of water have the same energy. Those that have lots of energy leave first, i.e. they vaporize. As they leave, they take their heat energy with them. That is why the surface of water cools first and why you feel cool when water evaporates from your body either through sweating or by getting wet.

However water turns into vapor, the rules of the latent heat of vaporization still apply. On average, 540 calories of heat are required to evaporate 1 gram of water, although this number changes slightly as temperature changes.

Water has a high latent heat of fusion

At the other end of the temperature spectrum, energy removal is required to change water from a liquid state to a solid state, i.e. ice. As with other state changes, energy input/removal is required; the temperature doesn't change but the physical state does. The heat removal required to change water into ice is called the latent heat of fusion. For 1 gram of water, 80 calories of heat must be removed. This is what a lake doesn't freeze immediately even though the temperature is 0 degrees C. Water has the highest latent heat of fusion, except for ammonia.

Water has a high heat capacity

Water has the highest heat capacity of any liquid or solid, except ammonia. The heat capacity of a substance is defined as the amount of heat that is required to raise the temperature of 1 gram of a substance by 1 degree. As we learned above, this amount of heat for water is defined as 1 calorie. This means that the heat capacity of all other substances is lower, i.e. that it takes less than 1 calorie to raise the temperature of 1 gram of the substance by 1 degree.

The main point here is that water can absorb a tremendous amount of heat. For this reason, the oceans of the world tend to vary in temperature much less than land. The average range of temperatures in the ocean is from -2 degrees to 35 degrees C. On land, temperatures may vary anywhere from -70 degrees to 57 degrees C. Compare also the moon, which has no water. Temperatures here range from -155 degrees to 135 degrees C.

Thus, water acts like a heat buffer for the globe. Its ability to absorb heat at one location and transport it to another locations is extremely important in moderating the climate of our globe. On a smaller scale, anyone who has lived near the beach has probably noticed that days are not as hot during summer and nights are not as cold during winter. This is because of the high heat capacity of water and its ability to absorb to release tremendous amounts of heat without changing temperature.

Water has a high surface tension

The ability of water molecules to quickly break and re-form hydrogen bonds gives it a property called cohesion. Water's high amount of cohesion makes it "sticky" such that across the air-water interface, a kind of "water barrier" is set up that allows things to float easier on the surface and causes water to form beads. Many of you have probably seen insects skate across the surface of the water. They can do this because water has a high surface tension. Try placing a toothpick lengthwise across a glass of water, or try filling a glass of water above the rim. The fact that you can do this is testimony to the high surface tension of water.

Water is essential to all living organisms

So far as we know, every living organism on the planet, from the simplest virus to the largest whale, vitally depends on water. All the plants, all the animals, all the bacteria contain water. It is the blood of life and it connects all of us in one common thread. In fact, as Grandfather says: "...through water, [we can] communicate with all the waters of the world, even the waters that pulse through the veins of man, animal, plant, and even the rains in the sky. Water then is not just a spiritual entity, but a living, breathing, and thinking being."

Question: How do we use water every day? What properties of water make our daily life easier?

The Paradox of Ice

Have you ever wondered why ice floats? Have you ever thought how different the world would be if ice didn't float? Well, start thinking!

Ice floats because it is less dense than water. This is true about any two substances: if one is less dense than the other, then it will float. Good examples here are oil on water, floating logs, and ducks and witches (a reference to Monty Python's Holy Grail; for those not fortunate enough to have seen their marvelous treatise on the density of objects, rent this movie and watch it!).

The density of a substance (liquid, solid, or gas) is defined as the mass of that substance per unit volume. It is an expression of the amount of molecules packed into a particular volume of space. Think about a cereal box that is packed by weight not volume. When the cereal is loaded into the box, the box is full and has a certain density, for example, one pound of cereal per box. As the box is shipped and the cereal settles, the box is no longer full. The one pound of cereal now occupies a smaller volume (i.e. less than a full box) and thus, the density of the cereal has increased. The cereal grains are packed closer to each other.

The density of liquids and gases can change depending on the temperature. Increases in temperature usually decrease the density of substances, i.e. the space between the molecules in the substance expand. Decreases in temperature typically cause the density to increase, that is, the molecules in the substance get closer together, i.e. they contract.

Variations in density also occur as a function of pressure. As pressure on a substance increases, its density increases. Where decreases in pressure occur, substances expand and become less dense. Obviously, the effects of pressure are greater on gases than liquids or solids; nonetheless, pressure affects all substances.

We should all be comfortable with the concept of density and try to understand how changes in temperature and pressure cause changes in the density of substances, particularly water. Density differences between different masses of seawater are one of the major driving forces of deep-sea circulation and may have a major influence on the climate.

Let's take a moment to look at the effects of temperature on the density of seawater. As the temperature of water decreases, water becomes more dense, as expected. However, at temperatures below 4 degrees C, a very unusual thing happens to water -- it begins to expand. In other words, the density of water reaches a maximum at 4 degrees C; below and above this temperature, the density of water decreases.

This unusual property of water is what allows ice to float. Because water freezes below 4 degrees C, i.e. at 0 degrees C, ice is less dense than water. The reason for this apparent anomaly is that at 4 degrees C, water molecules are packed as tight as they will go. Any attempt to push them closer together, such as by lowering the temperature, only makes the water molecules push back harder, i.e. they repel each other. Water molecules at the freezing point form a crystal lattice structure, like ice and snow, that is significantly less dense that liquid water. Like Ivory soap, ice floats.

Imagine a world where ice didn't float. If ice sank, it is very likely that ice skating would never have been invented; lakes would take forever to freeze from the bottom up. The polar ice caps would not be as large as they are; all the sea ice would be at the bottom of the ocean. I would venture to say that polar bears would not exist and penguins would be packed onto a much smaller Antarctic continent. On the other hand, if there were no icebergs, the Titanic never would have sunk.

Question: Can you think of any examples where organisms would be severely affected if ice didn't float?

Highly Suggested Reading:

True-false test on water
http://ga.water.usgs.gov/edu/sc3.html

Please also read this excellent brief summary about sea ice.

Sea Ice Formation
http://www.antarctic.com.au/encyclopaedia/physical/Ice.html

Useful Links:

Michael Pidwirny's Introduction to Hydrology Page
Great summary of water chemistry with beautiful illustrations
http://www.geog.ouc.bc.ca/physgeog/contents/8a.html