Virtual Vacationland: "Hands On" Activity


PROPERTIES OF FRESH WATER AND SEA WATER
OVERVIEW The students will set up three demonstrations to observe the properties of water. They will explore the boiling point of water, the freezing point of water, and the ability of water to store heat. These activities can be done individually or as a set. About 71% of the Earth¹s surface is covered with salt water. Life on Earth is possible because of the unique properties of water.
CONCEPTS Water has unique physical properties. It is one of very few substances that occurs as all three states of matter--a solid, a liquid, and a gas--within the normal temperature range at Earth's surface. The abundance of liquid water makes life on Earth possible. Water has a high heat capacity. Ocean water contains salts and minerals that make it different from fresh water. Graphing information helps us to analyze and understand it. Water moderates Earth's climate
	MATERIALS FOR STATION #1: THE BOILING POINT OF WATER Distilled water Seawater (if seawater is not available, check the pet store for "Instant Ocean" mix) Isopropyl alcohol (optional) Hot plate 3 Flasks, each with a rubber stopper that holds a thermometer 3 Thermometers that can measure from -10 degrees to 110 degrees C Graph paper
	PREPARATION FOR STATION #1 This activity works best as a small group activity, but can be done as a demonstration. As a demonstration, use an overhead projector to record the data for the class. If sea water is unavailable, you can find salt water aquarium mix at any pet store that sells salt water fish. Alcohol may be used as a liquid in the experiment for comparison. Make sure that the alcohol is in a closed container and do not let it splash. Caution: alcohol is flammable. Fill one flask with the distilled water sample, one flask with sea water and the third flask with alcohol. Insert the thermometers through the stoppers and cap the flasks. Make sure the thermometers are sus-pended in the liquids. Set all three samples aside for half an hour so that they are all at room temperature. PROCEDURE FOR STATION #1
		Engagement In this procedure, you will explore the boiling point of water, including the differences between salt water and fresh water. Based on your intuition, which do you think will boil first: salt water or fresh water? Why?
		Activity Record the temperature of the distilled water, seawater and alcohol in the flasks. Turn on the hot plate. Begin with the distilled water. Check and record the temperature every 30 seconds. When the water begins bubbling and the temperature levels off, the water is boiling. Keep recording the temperature for 3 minutes after you see bubbles. Plot and graph your data. What is the boiling point of distilled water? How long did it take the distilled water to reach the boiling point? Repeat the experiment with seawater. Record the thermometer reading every 30 seconds. Plot and graph your data. What is the boiling point of sea water? How long did it take the sea water to reach the boiling point? Optional: Repeat the experiment with alcohol. Record the thermometer reading every 30 seconds. Plot and graph your data. What is the boiling point of alcohol? How long did it take the alcohol to reach the boiling point? Compare the results of the three experiments. Use your graphs. Are there any differences in the boiling points? How do you explain these differences?
		Explanation The boiling point of a liquid is the temperature at which it turns to gas. Water, when heated, evaporates and boils slowly compared to other liquids. This means that the heat of vaporization is high‹the highest of all common liquids. Because of the high heat of vaporization, water evaporates slowly and absorbs a lot of heat. Water's high heat of vaporization gives it a high boiling point (100 degrees C). This is why much of Earth's water is in liquid form.
	STATION #1 EXTENSION: Have students think about evaporation from the ocean surface in comparison to evaporation from the surface of a fresh water lake. How would these processes differ? Which evaporates more readily? Then have the students look at a map of surface salinity of the oceans. Is the pattern they see consistent with how much sunlight reaches different parts of the earth? Does the pattern of salinity coincide with that of sea surface temperature? What other factors might affect sea surface salinity? Think about the surface temperatures of other planets and moons in our solar system. Are any others able to host liquid water? How does their surface temperature affect each planet or moon's ability to support life as we know it?
	MATERIALS FOR STATION #2: THE FREEZING POINT OF WATER Distilled water Sea water (if sea water is not available, check the pet store for "Instant Ocean" mix) Isopropyl alcohol 3 Thermometers that can measure from -10 degrees C to 110 degrees C 3 Large test tubes each with a one hole fitted stopper 3 Pyrex beakers Dry ice chunks Gloves Graph paper
	PREPARATION FOR STATION #2 This activity works best as a small group activity, but can be done as a demonstration. As a demonstra-tion, use an overhead projector to record the data for the class. If seawater is unavailable, you can find salt water aquarium mix at any pet store that sells salt water fish. Isopropyl alcohol works nicely because it contains water. When the water in the alcohol freezes, it should sink. There are numerous stores that sell dry ice as either chunks or cubes. Ask the salespeople at the store for the best way to handle the dry ice. If you cannot find or do not wish to use dry ice, you can use a salt-ice mixture. PROCEDURE FOR STATION #2
		Engagement In this procedure, you will explore the freezing point of water, including the differences between salt water and fresh water. For pure water, the freezing point is defined as 0 degrees C, but have you ever measured it? How can we measure it? Can we put the thermometer in a solid chunk of ice or in chopped ice? What is the temperature of ice? Which will freeze more slowly, salt or fresh water? Why?
		Activity Fill one test tube with distilled water, the second with sea water, and the third with alcohol. Insert the thermometer through each rubber stopper and cap the test tubes. Make sure that the thermometer is suspended in the water. Record the temperature of each test tube. Using tongs or heavy gloves, fill the bottom of three Pyrex beakers with chunks of dry ice. DO NOT TOUCH THE DRY ICE WITH YOUR BARE HANDS! Place each test tube in a beaker of dry ice. Record the temperatures every 30 seconds until they level off. Observe the test tube of alcohol. What happens to the water that is in the alcohol? Compare it to the freezing point of the salt water and of the fresh water. Does the ice float or sink? Plot and graph your data. Compare the information on the three graphs. What is the freezing point of fresh water? Seawater?
		Explanation The temperature at which a liquid becomes a solid is called the freezing point. The solid becomes a liquid at its melting point. The freezing point and melting point of water (or any other liquid) are the same. Water also has a high latent heat of fusion. Latent heat of fusion refers to the amount of heat gained or lost when a substance changes from a solid to a liquid, or a liquid to a solid. When ice is formed, large quantities of heat are given off. Most liquids become more dense as they cool. If cooled until they become solid, the solid phase is more dense than the liquid phase. However, this is not true of water. Pure water becomes more dense as it cools until it reaches 4 degrees C and further cooling decreases the density. Thus, water ice (0 degrees C) is lighter than liquid water and floats on it.
	STATION #2 EXTENSION: The fact that water ice is lighter than liquid water has key implications for Earth. Can your students think of why this property is so important? Have them think about the implications this has for ocean life and life in our lakes and rivers. Another interesting property to consider is that when ice is formed it gives off heat. How might this affect our oceans, lakes, and rivers? Can they think of other ways this helps humankind? For example, how might farmers use this knowledge when protecting their crops from freezing air temperatures?
	MATERIALS FOR STATION #3: WATER'S ABILITY TO STORE HEAT Hot plate 2 Flasks (same size) 2 Thermometers Bucket of ice water Stop watch
	PREPARATION FOR STATION #3 This activity works best as a small group activity, but can be completed as a demonstration. Use an overhead projector to record data for the class. PROCEDURE FOR STATION #3
		Engagement In this procedure, you will examine water's ability to store heat. Water has a higher heat capacity than almost any other liquid. This means that it takes a lot of heat to change water's temperature significantly. We can measure and compare the heat capacities of water and air. Based on your experience, which will heat and cool more slowly: water or air? Why?
		Activity Fill one flask with water and leave one flask empty. This flask is filled with air. Insert thermometers through rubber stoppers and cap the flasks. Record the temperature in each flask at room temperature. Place both flasks on top of the hot plate and start the stop watch. Record the time it takes for the water to reach 33 degrees C. Also record the time when the water reaches 33 degrees C. Also record the temperature of the empty flask at that instant. Is the temperature in the flask of air higher or lower than the temperature of the flask of water? Remove both flasks from the heat and place them in ice water. Record the time it takes for each flask to reach its original room temperature. Which flask took longer to reach its original room temperature? Record your observations.
		Explanation Water, when heated, evaporates slowly in comparison to other liquids. This means that the heat of vaporization is high--the highest of all common liquids. Water also has a high latent heat of fusion. Latent heat of fusion refers to the amount of heat gained or lost when a substance changes from a solid to a liquid, or a liquid to a solid. When ice is formed, large quantities of heat are given off. Liquid water also has an extremely high heat capacity, the amount of heat required to raise its temperature (between the freezing and boiling points). The high values of the heat capacity, heat of vaporization, and latent heat of fusion mean that it takes more heat to cause a change in temperature in water than in most other substances. This makes water a strong buffer against both rising and falling temperatures.
	SECTION #3 EXTENSION Water covers about 71% of Earth's surface. So its ability to store heat strongly affects our climate. Have your students look at an image of night-to-day mean surface temperature difference. About how much does the ocean change temperature from day to night? Do land areas experience greater or lesser differences in temperature from day to night? How does this affect the climate of coastal regions? What would happen if our oceans only covered 25% of Earth's surface? Would the day to night temperature difference on land masses be more or less extreme? How does the lack of liquid water oceans affect day to night temperature differences on some of the other planets and moons in our solar system?
	VOCABULARY evaporate: the physical process of converting a liquid to a gas. Commonly considered to occur at a temperature below the boiling point of the liquid. Opposite of condensation. heat capacity: amount of thermal energy required to raise the temperature of a substance by a given amount. heat of vaporization: the thermal energy absorbed by a liquid at its boiling point as it changes to a gas. latent heat of fusion: the amount of thermal energy associated with freezing of a liquid or melting of a solid. states of matter: a classification of substance according to its structural characteristics. Four states of matter are generally recognized: solid, liquid, gas and plasma.
	SOURCE "Visit to an Ocean Planet" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory Adapted from Orange County Marine Institute Curriculum Series. Adapted from Phleger, Charles F. and Wallace, William J. Field Guide and Laboratory Manual for Oceanography: An Introduction. p. 30 - 31.