Margaret D. Andrews
Water Wonders
If you just look at the chemical formula for water (H2O), it seems that water is a simple enough compound consisting of two parts of hydrogen and one part of oxygen. However, the structure of this small, light weight molecule gives it several unique characteristics that have contributed significantly to the patterns of life on this planet. Consider first of all, that water is one of only three naturally occurring liquids under normal atmospheric conditions and that the other two liquids are petroleum and mercury, then try to imagine an ecological system based on mercury! Other important characteristics of water include:
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-an uncommon tendency for water molecules to cling together due to a form of attraction between the hydrogen and oxygen atoms in different water molecules.
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-an ability to absorb or release large amounts of heat/energy before changing temperature.
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-a uniquely temperature dependent density pattern: water is most dense at 4 degrees Centigrade or 39 degrees Fahrenheit and becomes less dense at above and below this key temperature. transparency, or the ability to allow light to pass through the substance of water.
-the ability to dissolve many different substances or compounds.
There are many other molecules of similar size and weight we can compare to water, such as ammonia (NH3), methane (CH4), and carbon dioxide (CO2), but none of these substances exhibit any of the properties of water.
Molecular Structure
The secret of water’s unique characteristics lies in the arrangement of the atoms within the molecular structure of water. Both hydrogen atoms are arranged to one side of the oxygen atom. Hydrogen atoms carry a positive charge (like an electrical charge) and oxygen atoms carry a negative charge. Because of this arrangement of atoms, a water molecule ends up acting like a very tiny magnet, with a “positive” side and a “negative” side, and like magnets, the oppositely charged ends of different molecules are attracted to each other. A molecule with this arrangement is referred to as “polar”, because while the total charge of the molecule is zero, the asymmetrical distribution of the charges creates “magnetic poles” within the molecule.
Water absorbs heat
The attractive force between water molecules is known as a “hydrogen bond”. It is not as strong as the bonds that join the hydrogen to the oxygen within the water molecule, but breaking this bond to separate the water molecules does require energy. It is the hydrogen bonds formed between water molecules that keep water in liquid form at room temperature.
To move from a liquid to a gaseous form (evaporation), each gram of water must absorb 540 calories of energy. When the water reverts to its liquid form (condensation) all that energy is released to the air around the water molecules. The release of energy through the condensation process provides the energy that powers hurricanes and thunderstorms (this is where Lightning comes from). Energy is also released when water goes from a solid to a liquid form, and energy is required for ice to melt back to a liquid, but the quantity of energy for these events is significantly smaller than is used in the processes of evaporation and condensation. The large amounts of energy absorbed and released through the heating and cooling of water help moderate the atmospheric temperature (climate) (which explains why it always seems 1 0 degrees cooler near the water in the summer, warmer in the winter), and also helps living organisms maintain a constant body temperature.
Learning activities
Molecular Tag
Materials:
*tags or arm bands in two colors
*a large cleared area or an outside location
Divide the group into groups of three, preferably with one child being somewhat larger than the other two. Give each small group three tags, two representing hydrogen and one representing oxygen. Have the groups arrange themselves in a triangle, clasping hands in the middle with the identifying mark on their outside arm.
Mark boundaries for the “container” and tell the kids that they are liquid water and have them start moving around within the “container”, forming and breaking hydrogen bonds as they pass near other groups. Tell them the temperature is getting colder and they are moving slower and the hydrogen bonds are getting harder to break (and colder and slower and colder and slower . . . ) until you reach zero and the water freezes. At zero have everyone grab hold where their hydrogen bonds are formed and see if they can successfully create the crystalline structure of ice.
Once they’ve seen how they look, reverse the process by telling them the temperature is rising and the ice is melting, etc. As the “water temperature” rises, the groups should be moving around faster and faster until some groups are accidentally moving outside the “container” boundaries. Groups that fall out of the boundaries have broken all their bonds and evaporated. You can try getting the “water” all the way to boiling, but only if the children don’t appear to be in danger of hurting themselves. End by “cooling” down the water again.
Ask where they think the energy came from to heat up the water and where it went when the water cooled down
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Water Molecules Stick Together
Materials:
*construction paper
*markers or pencils
*scissors
*waxed paper
*toothpicks
*squeeze bottles of colored water
*student worksheet “Water Drops”
*pennies
*eyedroppers
Make models of water molecules out of construction paper, with one large circle for the oxygen and two smaller circles for the hydrogen. Put several of the molecules together, keeping the opposite charges next to each other, to show how the attractive forces between the molecules make them stick together. To illustrate, give everyone a square of waxed paper and some toothpicks, then have them sprinkle drops of different colors of water onto the waxed paper. Each person can experiment with the drops, following the ideas on the worksheet.
Pass out one penny, a container of water and an eyedropper to small groups of two or three. Ask each group to guess how many drops they think they can put on a penny before the water runs off. Then have them try to it, writing down their guess and the actual number of drops. You’ll be surprised!
Density
Water is densest at 4 degrees Centigrade and less dense at higher and lower temperatures. This unique density pattern is due to the polar configuration of water molecules. While in its liquid form, water molecules move around each other randomly and hydrogen bonds are continually forming and breaking between different molecules depending on the energy available. Decreasing water temperatures indicate a decreasing availability of energy. When the water temperatures reach 4 degrees Centigrade, there is not enough energy available to break hydrogen bonds once they have formed. As the temperature decreases further, the continual formation of hydrogen bonds force water molecules to start lining up in crystalline formation, which becomes visible to us when the temperature reaches 0 degrees Centigrade and ice forms. The rigidity of these crystalline structure forces the water molecules to take up more room than while they were moving together randomly in their liquid form, so ice, the solid form of water, is actually less dense than the liquid form. This is why ice floats and why the most dense water, at 4 degrees Centigrade, is found at the bottom of lakes and ponds. When the water gets colder than this temperature, it rises to the surface and freezes, allowing life to persist in the deeper water throughout the winter months. (You might want to consider what life would be like if water sank when it froze instead of floating.)
The density of water is also affected by whether or not it contains other dissolved materials. Salt water is denser than fresh water because of the quantities of salt and other minerals it contains. Water from the open ocean has a salinity of 35 parts per thousand (ppt) which means that 35 grams of salts have been dissolve in 1 000 grams of water. Density can be a complicated concept to explain. I generally think about a bucket of feathers and a bucket of sand: which one is heavier and which one is bigger. Since a bucket of sand is heavier than one full of feathers but takes up the same amount of room, sand is denser than feathers.
Water is transparent
Because water is transparent, light can travel deep below it surface. The availability of light enables plants of all sizes to grow under water. Phytoplankton, tiny one celled plants that float in waters of our lakes and oceans, are responsible for about one half of all the photosynthetic activity that occurs on our planet, thus providing the material for the base of many food chains. When water is clear it will appear blue in color. Other colors are caused by small particles suspended in the water. Oceanographers and limnologists (scientists who study our oceans and lakes) can often determine what sort of particles are in the water by looking at the apparent color of the water. A brown color is often caused by suspended sediments or silts or clays, although some phytoplankton can cause the water to look green or yellow or red.
Water as a universal solvent
The polar structure of water enables it to dissolve all polar compounds (like salts that are composed of one positive and one negatively charged particle) and many non polar compounds (like sugars which have no charges within their structure). Water also dissolves gases like oxygen, nitrogen and carbon dioxide. This ability to dissolve almost anything (given enough time) make water ideal for transporting substances through living bodies or around the planet. Water, in the form of blood, carries nourishment around your body and removes wastes. It carries nutrients into plants and salts from eroding rocks into our oceans.
There is a difference between materials that are dissolved in water and materials that are suspended in water. When a substance has dissolved in water (making a solution), it has generally been divided to a size where it actually fits between the water molecules. It is very difficult to remove dissolved materials from water without some energy intensive procedure like evaporation or reverse osmosis. If a substance is suspended in water, it will eventually settle out of still water with smaller particles settling at a much slower rate than larger particles. Somewhere in between dissolved substances and suspended substances comes a group of substances that when mixed with water, create a “colloidal suspension”. The materials involved are not dissolved in the water but they also will not settle out of the water. Milk is an excellent example of a colloidal suspension. One way to differentiate between a solution and a suspension is to determine whether you can see through the liquid. Solutions are generally fairly transparent while suspensions are generally opaque.
Learning Activities
Water Dissolves Many Things Materials:
*student worksheet “Water Dissolves Things”
*transparent containers
*measuring spoons
*cold water
*salt
*baking soda
*vegetable oil
*sand
Following the procedure on the student worksheet, introduce the activity. Depending on the group size and time available, you may want to divide the group into mini groups and have each group experiment with one substance, or have each group test each substance for its ability to dissolve. You should expect that one cup of cold water will dissolve five to seven teaspoons of salt. Baking soda dissolves less readily than salt, oil will form a suspension but not really dissolve, and the sand will not dissolve at all. You should explain with the sand that if the sand was in the water for a very long time it would eventually dissolve. This activity can be varied by comparing the dissolving powers of cold and hot water.
Ask what they think caused the different substances to dissolve differently
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