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Now that we've looked at the relationship between water's structure and chemical properties, let's focus on its physical properties. These include cohesion, adhesion, surface tension, and specific heat. Since the first three are closely related, we'll discuss them together.
Cohesion is the attractive force between similar molecules in the same phase. Water's hydrogen bonds are largely responsible for its cohesion. Cohesion can be thought of as the sum of all the hydrogen bonds in a collection of water molecules. Cohesion has important biological consequences. Cohesion helps hold a column of water together against the force of gravity. This lets the water from a tree's roots be transported all the way to its leaves.
A second physical property related to water's structure is adhesion. Adhesion is the attractive force between molecules in one phase, such as liquid, and different molecules in another phase, such as solid. Adhesion between water molecules and the molecules in tube-like structures in the tree trunk is also important in transporting water from a tree's roots to its leaves.
We've already talked a bit about a third physical property of water: surface tension. It's the energy needed to increase the surface area of a liquid by a given amount.
Now that we know something about the structure of water, let's take another look at the molecular basis of surface tension. Our knowledge of hydrogen bonding will help us understand why surface tension is defined in terms of energy.
The molecules on the surface of water are held together by hydrogen bonds. These bonds are strong enough to withstand the forces applied by the water bug's feet. If the frog tries to increase the water's surface area by pressing down on the surface with greater force, some of the hydrogen bonds will break from the applied force. The network of hydrogen bonds is no longer strong enough to support the frog's weight, so it breaks through the surface.
Specific heat is the last physical property of water we'll consider. Specific heat is the amount of heat needed to raise the temperature of a 1-gram sample of a substance by one degree Celsius. The specific heat of water is 4.184 joules per gram per degree Celsius. A joule is a unit of energy in the metric system, just like a calorie is a unit of energy in the British system. We can also express specific heat in calorie units. The specific heat of water is 1 calorie per gram per degree Celsius. What does specific heat mean in practical terms? Let's compare the specific heats of a few substances.
Water has a specific heat of 4.184 joules per gram per degree Celsius, glass has a specific heat of 0.84, copper has a specific heat of 0.387, and steel has a specific heat of 0.45. The lower the number, the greater the temperature change when a certain amount of heat energy is added. That's why a lot of cookware has copper on the bottom. Copper has a lower specific heat than stainless steel, so it takes less heat energy to raise its temperature high enough to boil the water.
Water has a high specific heat because its polar structure lets it form a network of hydrogen bonds. When heat energy is added, much of the energy is used to disrupt the hydrogen bonds, rather than to heat the molecules.
Water's high specific heat has some important biological consequences. Water is the most abundant compound in our body. Since water can absorb or lose a significant amount of heat before its temperature changes, we're more able to withstand a wide range of environmental temperatures.
Another consequence of water's high specific heat is that, like our body, the Earth is also relatively resistant to temperature changes. The oceans can store large amounts of heat energy with only modest temperature changes. Aquatic animals thrive when the temperature is relatively stable. Also, as the warm water cools when the sun goes down, the released heat energy warms the atmosphere in the immediate vicinity. This helps maintain mild climates on land as well as in the water.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education