PROPERTIES OF WATER LAB Introduction: Water’s chemical formula is H2O. As the diagram to the left shows, that is one atom of oxygen bound to two atoms of hydrogen. There are more electrons orbiting the oxygen atom compared to the hydrogen atoms, resulting in a water molecule having a slightly positive charge on the side where the hydrogen atoms are and a slightly negative charge on the side with the oxygen atom. This uneven distribution of charge is called polarity. Since opposite electrical charges attract, water molecules tend to attract each other, making water a “sticky” molecule. The INTRAmolecular bonds that hold the hydrogen and oxygen atoms together to make a water molecule are covalent bonds. Station 3 – Water molecules are not only attracted to each other but can also be attracted to substances and surfaces around them. This attraction is called adhesion. Place a strip of construction paper vertically into a cup filled with 25 mL of water. Be sure that the end of the construction paper is submerged into the water. Wait 5 minutes. What happens to the water and paper? What does this tell you about the molecules that make up the paper? Are they polar or nonpolar? Explain your answer. Station 4 – The ability of water to move along small tubes is called capillary action. This is demonstrated by placing a stalk of celery in a cup containing colored water. Let it sit for at least an hour and then make your observations. If water enters a plant through the roots, how do you think the properties of adhesion and cohesion are involved in capillary action to help the water get to the leaves? Station 5 – Fill a cup with water and try to balance a paperclip on the surface of the water. Why is the paperclip able to sit on the top of the water? The floating paperclip is very similar to how animals like the water strider can walk on water. Describe what is happening when insects like this can walk on the water, and explain why humans cannot walk on water. Fill a second cup with water and add a teaspoon of dish detergent. Try to balance a paperclip on the surface of the water in the second cup. In order to clean clothes or wash dirty dishes/hands, surface tension must be reduced so water can spread and wet surfaces. Chemicals that are able to do this effectively are called surface active agents, or surfactants. They are said to make water “wetter.” Surfactants perform other important functions in cleaning, such as loosening, emulsifying (dispersing in water), and holding soil in suspension until it can be rinsed away. Surfactants can also provide alkalinity, which is useful in removing acidic soils. The second cup contains dish detergent. Soap works because it has a polar end that has an affinity for water and a nonpolar end that has an affinity for greasy dirt. By attaching to both water and dirt at the same time, the soap can wash away the dirt from your hands or your clothes. Why was it more difficult to float the paperclip in the second beaker? What does soap do to hydrogen bonds? Station 6 – Observe ice in a cup of water. The hydrogen bonds between water molecules in ice are farther apart than when water is a liquid. How do you think this affects the amount of space water takes up when frozen as opposed to as a liquid? How would life in a lake be affected if ice sank and lakes froze from the bottom up? How do you think this property of water is responsible for potholes in roads in places like South Dakota? Station 7 – Because of hydrogen bonds between water molecules, it takes a significant amount of heat energy to make water change temperature. The amount of heat needed to make water change temperature by one degree Celsius is known as the specific heat. This number is much larger for water than it is for most other substances. It also takes a large amount of heat energy to change water from a liquid to a gas. This energy is known as the heat of vaporization. Put a drop of water and a drop of rubbing alcohol on a piece of wax paper. Which drop evaporates first? Explain your results. Make sure that you discuss the role of hydrogen bonds in your results.