Our World of Water
Physical Properties of Water
The chemical formula for water is H20, which means it is a molecule consisting of two atoms of hydrogen and one of oxygen. These three atoms are bonded tightly together, more so than the atoms of most other substances. This tight bond and arrangement of atoms in the water molecule results in the following five unusual properties:
The water molecule—two hydrogen atoms and one oxygen atom bonded together. |
Water is the only substance that occurs naturally on earth in three forms: solid, liquid, and gas. In liquid water, the molecules of hydrogen and oxygen are close together but are able to slip past one another, which is why it flows. Examples of this are a river, a waterfall, or water coming out of your faucet.
When the temperature drops, the water molecules slow down and become sluggish. As it becomes cold enough for the water to freeze, the molecules rearrange themselves into hollow rings. This is why water expands when it freezes, unlike most other substances which contract. This expansion in the solid phase is the reason why ice cubes float in a glass of water. The ice is actually lighter or less dense than the liquid water.
Water also occurs in the gaseous phase, such as steam rising from a boiling tea kettle. As water is heated, the molecules move about violently, colliding with one another, until some break free and form a vapor, or gas.
EXPERIMENT #1
To see how easily water can change forms, try the following experiment. First melt some ice cubes in a pan on the stove. Bring the same water to a boil and place a cover on the pan to catch the steam. The steam should condense into small droplets of water when it contacts the cover. Next, place the cover with the droplets of water into the freezer until the droplets freeze. Can you think of how humans use this ability of water to change forms? Think of how you change forms of water when you use it. Find out if it is possible for water to change from a solid form (ice) to the gaseous form (vapor) without becoming liquid.
When various materials are dissolved in water, they can change the properties of the water. To see this, take two containers of equal size and put into each the same amount of water. Pour salt into one of the containers, label the container, stir until the salt dissolves and then place both containers in the freezer and mark the time. (Note: Do not use large amounts of water because it will take a long time for this to freeze.) See how long it takes for both to freeze. Can you think of why they put salt on icy roads during the winter? Ask your parents why anti-freeze is important for their car. |
Water has a high surface tension. This is the ability of a substance to stick to itself. A drop of water falling from the rim of a faucet will stretch itself very thin before it drops off. Then it immediately forms a sphere and resists any kind of shape change. This high surface tension enables a water surface to support small objects like waterbugs, because their weight distribution will not permit them to break through.
EXPERIMENT #2
Can a needle float on water? Drop a sewing needle into a container of water and watch it sink. Now, take a strip of paper and make a loop. Carefully rest the needle in the loop and lower it slowly onto the water, being careful not to break the water surface with the needle. Keep pushing the paper down slowly, and gently pull it away after the needle has floated. This may take several tries before it is accomplished. Look very closely at the contact between the needle and the water. Notice the indentation the needle makes on the water surface.
 Have you ever held water between your fingers? Place the tips of your thumb and index finger together in water. When you pull them out of the water, slowly open up a small space between them. You should catch some water between your fingers and be able to hold it there no matter how you move your hand. See what happens when you open up your fingers. Does the water stay between your fingers? Try this with very soapy water. Can you still capture some of the soapy water between your thumb and index finger? What does the soap do to the surface tension of the water? |
Another unusual property of water is displayed when it is heated. Water has an extremely high heat capacity, which is the ability of a substance to absorb heat without becoming extremely hot itself. This is why it takes a long time for water to boil. An empty pan placed over a hot flame will become red hot and then burn black. However, if some water is placed in the pan over the same flame, the pan will become hot, but not red hot as before since most of the heat will be absorbed from the pan by the water. In like manner, your body cools when you sweat because body heat is absorbed when sweat evaporates.
The heat capacity of water enables the oceans to act as huge reservoirs of solar warmth and keeps our weather from going to great extremes of heat or cold. The moderating effect of water is noticeably absent from a desert, where days tend to be very hot and nights cold.
The most remarkable aspect of water is its ability to dissolve so many substances; that is, to act as a solvent. For example, some caves form when acidic ground water dissolves limestone bedrock. The substance that is dissolved is called the solute, and the liquid mixture is called a solution. Most water on the earth is actually a solution. Rainwater is the purest naturally occurring solution of water and contains few dissolved substances.
EXPERIMENT #3
| Collect rainwater in a clean glass or metal container and fill another container of similar shape with an equal amount of water from your faucet. Label the containers and place them in a warm place to evaporate. When all of the water has evaporated from both containers, check them for any residue. Which container has the most residue in it after the water evaporates? |
The degree to which water has a distinctive taste or odor depends on the types of substances dissolved in it. Since water is not changed chemically when it acts as a solvent, it can be recovered for reuse after undesirable dissolved substances are removed. The amount of dissolved substances in water is affected by factors such as water temperature and the nature of the material water moves through.
EXPERIMENT #4
Take two containers of equal size and fill one with cold water and the other with hot water from your faucet. Make sure each container has an equal amount of water. Measure a ¼ teaspoon of salt into each container, stir to dissolve. Keep adding salt by the same amount to each container and see which temperature of water will dissolve the most salt. Keep a record of the number of times you added a ¼ teaspoon of salt to each container. Be sure to stir the water each time you add the salt. What would happen if you dissolved as much salt as possible in hot water and let the water cool to near freezing temperatures? Try this. Do you think that instant coffee or cocoa would dissolve as rapidly in cold water as hot water? Think of some environmental problems related to water's ability to dissolve so many substances and the effects of dumping hot water containing dissolved pollutants into cooler river water.
Mix some salt with water until it has a definite salty taste. Pour this into a pot and bring to a boil. Catch some of the steam using a pan lid. When it cools, taste the water collected on the pan lid. What happened to the salt? Find out what distilled water is and how it is prepared. Why is it best to use distilled water in a steam iron? If all the water in the oceans evaporated, what would be left? Look in an encyclopedia and find out how the Bonneville salt flats were formed. |
Streams running through areas where there are few people will generally have a better quality of water than streams running through populated areas. Can you tell why? What do you think happens to the quality of polluted water when it evaporates? How would evaporation act as a natural purifier of polluted water?
Conductivity is the ability of a substance to carry an electric current. Water will conduct an electric current only if dissolved ions are present because water molecules do not act as a conductor. Measuring conductivity is a good way to determine the amount of dissolved solids in a sample of water and, thus, to determine its purity.
EXPERIMENT #5
 Construct an electric circuit using a flashlight bulb, wire, and a 6-volt dry cell battery. Wire the circuit such that two ends of the wire are submerged in a glass of water, as shown in the diagram. See if the bulb will light up when there is only water in the container. Start adding salt to the water, always stirring. Watch and see if the bulb starts to get brighter and brighter. Do you think that seawater would be a good conductor? |
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