Heat versus Temperature

The energy relationships of the water molecule are very important to understanding why water is unique. However, the layman defines temperatures as warm or cool. To the layman, if you add heat, a substance will warm up. This is not always true.

The chemist or physicist makes a distinction between the two. Temperature is a measure of the motion of the molecules. Higher temperatures mean more rapid molecular motion. Heat, on the other hand, is defined as the energy added or subtracted from a system.

We feel temperature changes and can measure them with a thermometer. It is more difficult to measure the changes in energy. Energy can often be added to a system without a change in temperature. Even on a hot sunny day, a snowpack can absorb a great deal of solar energy without experiencing a temperature change above freezing. On the same sunny day, our bodies emerging from a swimming pool will be cooled off as the water evaporates from our swim suit. The temperature of a boiling pot of water does not change even when we add more energy from the stove burner.

A common measure of heat energy is a gram-calorie. It is defined as the amount of heat needed to raise a gram of pure water from 14.5-degrees C. to 15.5-degrees C. at sea level air pressure. The amount of heat necessary to raise the temperature of a substance one degree C. under a set of standard conditions. Hence, the specific heat of water is one. All values of specfic heat are related to the value defined by water.

Water exists in three phases: solid (ice), liquid (water), and gas (vapor).

Ice has a rigid molecular structure held together by strong bonds between molecules by the "hydrogen bond". When heat is added to the ice it warms up until it reaches the freezing temperature of 32-degrees F. (0-degrees C.). At freezing temperature, as you add heat -- the temperature doesn't change because the molecules motion stays the same until the hydrogen bonds holding the molecules together is broken. This takes about 79.7 gram-calories per gram of ice. This amount of energy is called the latent heat of fusion/melting. Latent heat must be removed before 32-degree water freezes or 32-degree ice melts. Compared to most substances, the latent heat of fusion is remarkably high for water.

At the other extreme, as we add energy the temperature of a gram of liquid water increases at the rate of one degree C. for each gram-calorie until the boiling temperature is reached at 212 degree F. (100 degrees C.). When water boils, the liquid water evaporates but it takes 539.4 gram calories to evaporate (convert the liquid water to gas) each gram of water. Again. this is a remarkable amount of energy to effect a phase change. This is called the latent heat of evaporation/condensation. It takes that much energy to break the liquid hydrogen bond and free the water molecules as indiviual gaseous molecules.

The extremely high latent heat values are due to the hydrogen bonds being hard to form or to break. These unusual properties account for much of our climate and environmental survival.