The unit of heat called the gram calorie is defined as the amount of heat required to raise the temperature of one gram of water 1 °C. The kilocalorie, or food calorie, is the amount of heat required to raise one kilogram of water 1 °C. Heat capacity is the amount of heat required to raise one gram of material 1 °C under constant pressure. In the International System of Units (SI), the heat capacity of water is one kilocalorie per kilogram per degree Celsius. Water has the highest heat capacity of all common Earth materials; therefore, water on Earth acts as a thermal buffer, resisting temperature change as it gains or loses heat energy.

The heat capacity of any material can be divided by the heat capacity of water to give a ratio known as the specific heat of the material. Specific heat is numerically equal to heat capacity but has no units. In other words, it is a ratio without units. When salt is present, the heat capacity of water decreases slightly. Seawater of 35 psu has a specific heat of 0.932 compared with 1.000 for pure water.

Pure water freezes at 0 °C and boils at 100 °C (212 °F) under normal pressure conditions. When salt is added, the freezing point is lowered and the boiling point is raised. The addition of salt also lowers the temperature of maximum density below that of pure water (4 °C [39.2 °F]). The temperature of maximum density decreases faster than the freezing point as salt is added.

At 24.70 psu salinity, the freezing point and the temperature of maximum density coincide at −1.332 °C (29.6 °F). At salinities typical of the open oceans, which are greater than 24.7 psu, the freezing point is always the temperature of maximum density.

When water changes its state, hydrogen bonds between molecules are either formed or broken. Energy is required to break the hydrogen bonds, which allows water to pass from a solid to a liquid state or from a liquid to a gaseous state. When hydrogen bonds are formed, permitting water to change from a liquid to a solid or from a gas to a liquid, energy is liberated. The heat energy input required to change water from a solid at 0 °C to a liquid at 0 °C is the latent heat of fusion and is 80 calories per gram of ice. Water’s latent heat of fusion is the highest of all common materials. Because of this, heat is released when ice forms and is absorbed during melting, which tends to buffer air temperatures as land and sea ice form and melt seasonally.

When water converts from a liquid to a gas, a quantity of heat energy known as the latent heat of vaporization is required to break the hydrogen bonds. At 100 °C, 540 calories per gram of water are needed to convert one gram of liquid water to one gram of water vapour under normal pressure. Water can evaporate at temperatures below the boiling point, and ice can evaporate into a gas without first melting, in a process called sublimation. Evaporation below 100 °C and sublimation require more energy per gram than 540 calories. At 20 °C (68 °F) about 585 calories are required to vaporize one gram of water. When water vapour condenses back to liquid water, the latent heat of vaporization is liberated. The evaporation of water from Earth’s surface and its condensation in the atmosphere constitute the single most important way that heat from Earth’s surface is transferred to the atmosphere. This process is the source of the power that drives hurricanes and a principal mechanism for cooling the surface of the oceans. The latent heat of vaporization of water is the highest of all common substances.