Boiling Point

Liquids will not boil until their vapor pressures have increased to the prevailing atmospheric pressure. Pressure cookers take advantage of this fact. The closed pressure cooker causes the pressure to increase substantially within the vessel. Vapor pressure is directly related to the temperature of the liquid. This pressurized water in the pressure cooker must be raised to a higher temperature before it begins to boil, hence the food is being subjected to a higher temperature and will cook faster than in an open container.

Every camper who loves the mountains and prefers a Òthree minuteÓ egg in the morning has faced the problem of cooking under reduced atmospheric pressure. Since the atmospheric pressure is lower at higher altitudes, water will boil under a lower pressure at the mountainside campsite. An egg boiled for three minutes under these conditions will be ÒrunnierÓ than the camper is accustomed to at home.

Cake bakers in the mountains also realize that an extra quantity of water must be placed in the cake mix to make proper batter. Since the water boils at a lower temperature under these lower atmospheric pressure conditions, more water boils away during the baking process. If the normal recipe is followed, the cake will not have the desired moisture level.

Latent Heat

When one gram of water freezes, 334 joules of heat are released with no change in temperature. This heat is not apparent because it is released with no temperature change; it is called latent heat. Latent means hidden. Latent heat of fusion is actually potential energy released as the liquid water molecules bond together to form solid ice.

Fruit and vegetable growers use this reservoir of latent heat to protect their crops when freezing weather threatens. As water sprayed on the crops cools and freezes, the latent heat released in the cooling and freezing process is released to the atmosphere around the crops.

This supply of heat keeps the ambient temperature higher than it would have been if water were not sprayed on the crops. For every gram of water which is evaporated from the surface of the earth by solar heating and converted into water drops in a rain cloud, a minimum of nearly 2500 joules of kinetic and potential energy are released into the atmosphere. This tremendous amount of energy produces such damaging phenomena as high winds, lightning, hail, tornadoes, and hurricanes.

Heat Capacity

Water has an extremely high heat capacity (highest of any substance). Much energy is required to speed up water molecules and in turn, these molecules release much energy as they slow down and cool. This is why a hot water bottle is such a comfort on a cold winterÕs night.

Because of the high heat capacity of water (warms slowlyÑcools slowly) areas near oceans or seas have moderate climatesÑno really hot summers or frigid winters. A case in point is the southern coast of Ireland which is bathed by the Gulf Stream. This high heat capacity water carries heat all the way from tropical areas to Ireland. Southern Ireland, although it is at a latitude above that of the U.S/Canadian border, has year-round temperatures high enough to support subtropical growth including palm trees.

The exaggerated temperatures experienced in desert areas of the earth are due in great part to the lack of moisture (water) in the soil. With little water in the sand, rather than being stored in the soil, heat from the sun radiates rapidly back into the atmosphere to heat it. At nightfall, the soil has stored little heat to radiate to the atmosphere to moderate nighttime temperatures.

The heat capacity of aluminum is approximately 1/5 that of water (aluminum warms faster and cools faster than water). Consider that although the aluminum pan containing a TV dinner may be cool enough to the touch to be carried to your easy chair, the first bite of the enclosed food items has a tendency to burn your tongue. Blame it on the high water content. There is a lot of heat released as these hot water molecules cool in your mouth.

Continued

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