VI. Unique properties of water

A. Thermal properties (A good way to organize, understand, and learn the thermal properties is to use the change of state diagram for H₂0).

1. Melting and boiling points (or temperatures)– H₂0 changes state from solid to liquid at the melting temperature and from liquid to gas at the boiling temperature. Water melts and boils at an amazingly high temperature for a compound made up of such light elements. The graph shows you the melting and boiling points of water compared to those of some similar compounds that form between hydrogen and the other elements of group VI-A in the periodic table (hydrogen sulfide, hydrogen selenide and hydrogen telluride). [It is a common procedure in science to use the known properties of substances to predict the property of a similar substance that has not yet been determined.] Notice how the temperatures of water are way out of line with that which would be predicted by the behavior of the other similar compounds.

a. The reason for the high melting and boiling temperatures is the hydrogen bonding between water molecules that causes them to stick together and to resist being pulled apart which is what happens when ice melts and water boils to become a gas.

b. Importance = Without this 'stickiness' water would not be a liquid over much of the surface of the Earth where temperatures are relatively high and we would not have an ocean.

2. Specific Heat or Heat Capacity - The specific heat capacity of a compound is a measure of how difficult it is to get the molecules of the compound to vibrate. Because after all temperature is just a measure of molecular motion – the more the molecules of a compound vibrate - the higher the temperature of the compound. Because of the hydrogen bonding of water molecules it is very difficult to get them to vibrate compared to the molecules of any other common substance. Water is therefore said to have a high specific heat capacity or specific heat.

a. Strict definition of heat capacity = the amount of heat required (measured in calories) to raise the temperature of 1 gram of a substance by 1^oC.

b. Remember that heat and temperature are not the same thing. Heat is a form of energy, whereas, changes in temperature are a response to changes in the amount of heat energy available. The degree of molecular motion in a substance is measured by its temperature – the more th molecules vibrate – the higher the temperature of the substance. As a result, heat energy can be added to a system without causing an increase in temperature. Heat input only increases the temperature if it causes an increased vibration of the molecules. In the case of liquid water, because the hydrogen bonds suppress vibration, the temperature does not increase as much as it would without the presence of strong hydrogen bonds.

c. Importance = Since water can absorb a lot of heat with only a small increase in temperature, the temperatures of large standing bodies of water remain relatively constant. This thermal buffering protects life on Earth from otherwise possibly lethal temperature fluctuations.

3. Changes of State of H₂O

a. The states of matter are solid, liquid and gas. Because of the hydrogen bonding it is very difficult to separate the water molecules from the surface of a liquid to form a vapor. It is also difficult to separate the molecules from the surface of ice to form liquid water.

b. It requires much more heat (measured in calories) than expected to change H₂O from ice to water and from water to gas.

4. Latent heats of fusion and vaporization – measures how much heat you must add to a substance to melt or vaporize it once you have increased the temperature to its melting and boiling points. The excessive energy needed to melt and boil H₂O is due to the H-bonds.

a. Importance = Liquid water can absorb a lot of heat at one place on the Earth (such as in the Tropics) when evaporation occurs and then transport this heat somewhere else where the water cools, condenses and releases the stored heat. This means that evaporation dissipates much of the Sun's energy thus moderating and stabilizing Earth's surface temperature without appreciable change in the ocean temperature. This heat storage and transportation also has significant consequences for climates and storms such as hurricanes.

5. Density - because of the unique bonding and structure of the water molecule solid H₂O (ice) is less dense than liquid water. Therefore, when water freezes the ice floats on top of the denser liquid water instead of sinking to the bottom. This is different from most compounds which are denser in the solid state than in the liquid state.

a. Importance - Freshwater lakes in mid-latitudes don't freeze solid. Instead ice floats on top and like a blanket, insulates the rest of the lake from the freezing temperatures. This profoundly influences the cycles of organisms living in these lakes.

1) The same effect is not seen in seawater because of the salt In seawater the salt content is as important or more important to density than temperature.

B. Solvent Properties - dipolar liquids like water are excellent solvents for ionic substances such as NaCl. Water is probably the best solvent in nature. That is, it is good at dissolving solids into ions in solution. The dipolar water molecules attach their charged ends to the oppositely charged atoms of solid substances immersed in them and pull components of the solid into the solution as dissolved ions. Ionic substances are most susceptible to this because they consist of a framework of positively and negatively charged particles.

1. Saturated-when water has dissolved all of a given solid that it can hold

2. Dissolved salts raise the boiling point and depress the freezing point of water

3. When evaporation and freezing occur the dissolved materials stay behind and mainly pure water goes to gaseous or solid form.

C. Light transmission

1. Seawater transmits the visible wavelengths of sunlight thus allowing plants to live in seawater. Not all wavelengths of visible light are transmitted equally.

a. Red wavelengths are absorbed in about the first meter.

b. Yellow wavelengths are absorbed in about the first ten meters.

c. All that’s left are the blue wavelengths and because color perception is due to the reflection back to our eyes of wavelengths of a particular color, the ocean usually appears blue-green. These are the wavelengths being absorbed least readily.

D. Sound transmission

1. The fact that water transmits sound is important to a few life forms.

a. Whales, dolphins, fish, etc. use their “sonar” to track prey and/or escape predators.

b. Humans have used this property as a research tool and sub-finder.