The properties of water (part 1)
How would you describe water to someone who had never seen it before? You might say that pure water has no color, no taste, and no odor. You might even say that water is a rather plain, ordinary substance. But if you asked a chemist to describe water, the response would be different. The chemists would say that water is very unusual. Its properties differ from those of most other familiar substances.
Are you and the chemist talking about the same substance? To understand the chemist's description of water, you need to know something about the chemical structure of water.
Water's Unique Structure
Like all matter, water is made up of atoms. Just as the 26 letters of the alphabet combine in different ways to form all the words in the English language, about 100 types of atoms combine in different ways to form all types of matter. Atoms attach together, or bond, to form molecules. Two hydrogen atoms bonded to an oxygen atom form a water molecule. A short way of writing this is to use the chemical formula for water, H2O.
Have you ever played with bar magnets? If so, you know that the opposite poles of two magnets attract each other. The same is true with polar molecules, except that an electric force rather than a magnetic force causes attraction. The positive hydrogen ends of one water molecule attract the negative oxygen ends of nearby water molecules. As a result, the water molecules tend to stick together. Many of the water's unusual properties occur because of this attraction among the polar water molecules.
Surface Tension
Have you ever watched a water strider? Insects can skate across the surface of a pond without sinking. They are supported by the surface tension of the water, Surface tension is the tightness across the surface of water that is caused by the polar molecules pulling on each other. The molecules at the surface are being pulled by the molecules next to them and below them. The pulling forces the surface of the water into a curved shape. Surface tension also causes raindrops to form round beads when they fall into a car windshield.
Capillary Action
The next time you have a drink with a straw in it, look closely at the level of the liquid outside and inside the straw. You will see that the liquid rises higher inside the straw. Similarly, water will climb up into the pores of a brick or piece of wood. How does water move up against the force of gravity? Just as water molecules stick to each other, they also stick to the sides of a tube. As water molecules are attached to the tube, they pull other water molecules up with them. The combined force of attraction among water molecules and with the molecules of surrounding materials is called capillary action. Capillary action allows water to move through materials with pores or narrow spaces inside.
Capillary action causes water molecules to cling to the fibers of materials like paper and cloth. You may have seen outdoor or athletic clothing that claims to "wick moisture away from the skin." The capillary action that occurs along the cloth's fibers pulls water away from your skin. By pulling the water away from your skin, the fibers keep you dry.
Water, the universal solvent
What happens when you make lemonade from a powdered mix? As you stir the powder into a pitcher of water, the powder seems to disappear. When you make lemonade, you are making a solution. A solution is a mixture that forms when one substance dissolves another. The substance that does the dissolving is called the solvent. In this example, the water is the solvent.
One reason that water is able to dissolve many substances is that it is polar. The charged ends of the water molecule attract the molecules of other polar substances. Sugar is a familiar polar substance. When you add a sugar cube to a cup of hot tea, the polar water molecules in the tea pull on the polar sugar molecules on the surfaces of the cube. As those sugar molecules dissolve, other sugar molecules are exposed to the water. Eventually, the sugar cube dissolves into many individual molecules too small to see. The result is a solution of sweetened tea.
Water dissolves so many substances that it is often called the "universal solvent." It can dissolve solids, such as salt and soap, and liquids, such as bleach and rubbing alcohol. Water also dissolves many gases, including oxygen and carbon dioxide. These dissolved gases are important for organisms that live in the water.
However, some substances, such as oils and wax, do not dissolve in water. You have observed this if you have ever seen the oil separate from the vinegar and water in salad dressing. The molecules of oil are nonpolar molecules-they have no charged regions. Nonpolar molecules do not dissolve well in water.
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