Water has a high heat capacity

Recall that the three states of matter include solid, liquid, and gas. When you think of water, you probably think of water in its liquid state. When water is boiled, the liquid is heated into a gas called water vapour. The water molecules move very fast in water vapour as compared to liquid water. When water is frozen, the liquid is cooled into ice. The water molecules in ice move much more slowly than in liquid water.

Recall that energy is the capacity to do work. Heat and temperature are two different measures of energy. Heat is the total kinetic energy within a system. Temperature is the average kinetic energy of a system. You may find it easier to understand the difference between these two measures of energy using the following example.

Water requires a large investment of heat energy before a small change in water temperature can occur. In other words, water has a high heat capacity. Imagine that you are boiling water to make tea. The amount of heat required to boil water is the total amount of energy you need to put in to raise the temperature of water to ~100°C. Water seemingly takes a long time to boil because energy must be invested to break the hydrogen bonds that hold water molecules together before those water molecules can move freely and form water vapour. Once the hydrogen bonds have been broken, individual water molecules move faster (gain kinetic energy), resulting in a change in temperature.

Similarly, boiled water takes a seemingly long time to cool. Once you have brewed your tea, you must wait for the tea to reach a drinkable temperature. As water cools, the molecules slow their movement and energy is released as the hydrogen bonds between water molecules re-form. That energy keeps the temperature of water relatively constant, even though you have stopped heating the water.

The high heat capacity of water makes Earth habitable. Heat from the atmosphere is absorbed by water in oceans, ensuring that the temperature on Earth’s surface does not exceed a livable temperature. Water can release heat to cooler atmospheric air, again, keeping the temperature on Earth’s surface relatively constant. Life on Earth is possible because of the ability of water molecules to form hydrogen bonds!