Name: LAB: Phase Changes of Water

Transcription

1 Name: LAB: Phase Changes of Water Introduction: Water is a substance commonly found on Earth and in the atmosphere in all three phases of matter (solid, liquid, and gas). This unique property of water is a major factor in determining weather patterns on Earth. Objective: You will observe the temperature changes of a volume of water as heat is added and the water undergoes phase changes of solid to liquid and liquid to gas. You will compare the relative amounts of heat energy needed for each step. Hypothesis: Make a prediction about which phase change requires the most energy. Vocabulary: Water Vapor: Latent Heat: Melting: Vaporization: Procedure: YOU MUST WEAR SAFETY GOGGLES WHEN HEATING MATERIALS! 1. Fill a 400 ml beaker with ice and water. 2. Turn on your heat source. Do not put the beaker on the heat source yet. The source of energy must remain constant throughout the experiment. 3. Insert a thermometer into the beaker and use it as a stirring rod - be sure to hold the thermometer so that it does not touch the sides or the bottom of the beaker. 4. Stir the solution gently throughout the experiment. 5. When the thermometer reaches its lowest reading, record this under Time 0 on the Report Sheet. 6. Quickly place the beaker on the heat source. 7. Read and record the temperature every 30 seconds, continuing for at least 10 minutes after the water reaches a full, rolling boil. Remember to continue stirring throughout the experiment. 8. Record the time in your data: a. When the ice begins to melt b. When the ice is entirely melted c. When the water begins to boil 9. Graph your data on the graph provided placing Time on the horizontal axis.

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3 Discussion Questions: 1. According to your graph, did the temperature of the water/ice mixture increase while the ice was melting? 2. According to your graph, what happened to the temperature of the water between the time the ice melted and the water boiled? 3. According to you graph, what change occurred in temperature after the water began to boil? 4. What can you tell about the rate of temperature change between the time the ice melted and the water boiled?

4 5. From the rate of temperature change (question #4), what can you infer about the rate of energy input during each minute? 6. Before the temperature began its steady rise, for what was the added heat energy being used? 7. During the time of steadily increasing temperature, what change in energy occurred because of the added heat? 8. During the last ten minutes, what changes occurred because of the added heat? 9. What is another term for the potential heat energy that is stored during a phase change? Conclusion: According to the graph, which phase change required the most added heat energy, and how did this compare to your hypothesis? Reading Comprehension Read the portion of the article on the different Phases of Water below and answer the following questions based on the reading. Use complete sentences. Water's Three Phases are a Key to the Weather 05/20/ Updated 11:05 AM ET By Jack Williams, USATODAY.com Water is so common that most of us, most of the time pay little attention to it even though without it life as we know it would not exist. Everyone is aware of water when it falls from the sky as rain, snow, or ice. Its importance to weather, however, goes far beyond the various forms that fall on us. Water, as it turns out, is one of the main sources of the energy needed to run the Earth's weather machine. To see why, let's look at some basic chemistry and physics.

5 All substances, including water of course, can exist in three phases: Solid, liquid, and gas. Without getting into too much detail, the average speed of molecular motion determines a substance's phase. Temperature, in turn, is a measure of the average speed of molecular motion. But, difference substances change from one phase to another at vastly different temperatures. Nitrogen, for instance, becomes solid (freezes) at -346 degrees F. Since the coldest temperature ever recorded on Earth is -129 degrees F (at the Russian Vostok Station in Antarctica), nitrogen never naturally freezes on Earth. Metals, on the other hand, remain sold at temperatures above 300 degrees F, much hotter than any life yet found can survive. Water is special because it's the only substance that can exist in all three phases at Earth's ordinary temperatures, and it's common to have all three phases together at the same time. In other words, water's gas form - water vapor, the liquid form (the water we buy in expensive plastic bottles to drink), and the solid form, ice are often found together in clouds. The water molecules in ice are locked into a hexagonal crystal and do not move freely. Liquid water molecules are moving fast enough to break free from the crystal structure, but they are still attached to each other. This allows liquid water to assume the shape of its container. When water is a gas - vapor - the molecules are free to move about. To understand the weather, you need to understand what happens when water changes its state. These changes are: Evaporation: From liquid to gas (water vapor). Condensation: From gas (water vapor) to liquid. Freezing: From liquid to solid (ice). Melting: From solid to liquid. Sublimation: From gas directly to solid without becoming liquid. Sublimation: From solid directly to gas. (Note: In chemistry a change from vapor to solid is called "deposition," and meteorologists in other English-speaking countries use this term for the change of water vapor directly to ice. But American meteorologists generally use "sublimation" for both the change to ice and directly back to vapor.) Water's phase changes help drive the weather because each change either releases or takes up energy in the form of latent heat. 1. What determines a substance s phase? 2. Why is water special? 3. When water ice melts, what happens to its density? 4. Dry ice is a substance that does no melt and has no liquid phase. Which phase change does dry ice go through when it is warmed?