Properties of Water: 3. Specific Heat of Water is High

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3. The specific heat of water as a liquid is high. The temperature of water increases as it absorbs heat and decreases as it releases heat. However, the temperature of liquid water rises and falls more slowly than those of most other liquids. Stated another way, water absorbs heat without immediate rise in temperature. It also retains its temperature much longer than other substances.

This high specific heat of water prevents abrupt changes in temperature and helps the body maintain temperatures that are relatively constant. Otherwise plants and other living organisms, which contain large amounts of water, will easily become susceptible to overheating or underheating. This is the reason why places adjacent to bodies of water are less prone to severe changes in temperature. Likewise, moist soil slowly gets hot but retains high temperature much longer. It is the reverse with dry soil.

Heat is different from temperature although they are interrelated. As defined in Cambridge Dictionary of Science and Technology (Walker 1988), in physics “heat is energy in the process of transfer between a system and its sorroundings as a result of temperature differences”  while “temperature is a measure of whether two systems are relatively hot or cold with respect to one another.”

Raising the temperature of 1 gram of liquid water by 1°C at 1 atmospheric pressure requires an expenditure of 1 calorie of heat energy. This (1 cal/g.deg) is the specific heat of water as a liquid, or specific heat capacity of liquid water. One calorie= 4.184 joules; 1 joule= 1 kg(m)2(s)-2 = 0.239005736 calorie.

In contrast, other substances need only about one-half of this amount of heat or lower. For example, here are the specific heats of water in other forms and of some common substances in cal/g.deg (from Bettelheim and March 1998): ice- 0.48, steam- 0.48, lead- 0.038, iron- 0.11, rock (typical)- 0.20, aluminum- 0.22, wood (typical)- 0.42, and ethyl alcohol- 0.59. This means that compared to liquid water, these substances and materials easily get hot, or cool off.

The increase in temperature results from the increase in the kinetic energy of the molecules of a substance as heat is absorbed. It consequently causes the temperature of the substance to rise which means an increase in the vibrational movement of the molecules.

The high specific heat of water is attributed to the many hydrogen bonds that join the multitude of water molecules. In order to increase the temperature of water, the molecules have to vibrate. Because there are so many hydrogen bonds, a high amount of heat energy is needed to cause the water molecules to vibrate and, eventually, to break them.

Likewise, heated water slowly cools down. As heat is dissipated, temperature decreases and the vibrational movement of water molecules slows down. The heat that is given off counteracts the cooling effect of the loss of heat from the liquid water.

Comparing the same mass or weight of liquid water with other substances with lower specific heats at the same temperature, for example 90°C, water will cool down more slowly. This is because to reach the temperature, water had to absorb more heat. Consequently, at the same rate of heat loss, it will take more time to dissipate the higher amount of heat absorbed by water. Similarly, a greater volume of water will be packed with more heat energy and will therefore retain high temperature much longer than a smaller amount of water.

(Ben G. Bareja March 2013)

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