reflect Water is found in many places on Earth. In fact, about 70% of Earth is covered in water. Think about places where you have seen water. Oceans, lakes, and rivers hold much of Earth s water. Some water is trapped in glaciers and icebergs. Even the air holds water! If you have ever been outside on a foggy day, you have seen water in the air. Fog is an example of water that forms from a gas called water vapor. Even on clear, sunny days, there is water vapor in the air. Water is found in different physical forms, or states, and it can change between these states. Water can change from a solid to a liquid. It can change from a liquid to a gas. Water can even change from a gas to a liquid. But how, exactly, does water change between its different states? states: classifications of matter as a solid, liquid or gas. What are some properties of water? Can these properties change? How? Pure water is an amazing substance. It can exist naturally as a solid, liquid, or gas. When enough heat is added to or taken away from water, it will change its physical state. For example, water sometimes collects in puddles during a rainstorm. With enough heat from the Sun, the water will eventually change from a liquid (in the puddle) to a gas (water vapor in the air). This is an example of evaporation. Then the gas, or water vapor, can change again. If it cools enough in the air, water vapor will change into a liquid and fall back to Earth as rain. Changing from a gas to a liquid is called condensation. look out! When a substance warms up, it gains heat. But what happens when a substance cools? A cooling substance loses heat it does not gain cold. Think about an ice cube tray. You can fill it with water and put it in the freezer to make ice. As the water changes from a liquid to a solid, it loses heat. what do you think? Look at the photographs below. The picture on the left shows water changing from a liquid to a gas. The picture on the right shows water that has changed from a liquid to a solid. What caused the physical state of each water sample to change? 7
Discover Science: Water on Mars? Is there water on other planets? As far as we know, Earth is the only planet in our solar system containing liquid water. But scientists wondered for many years whether other planets had moving water at one time. Recently, they found exciting evidence to suggest water once flowed on Mars. When water flows over rocks for a long time, the rocks break down and become smooth. On Mars, some rocks look like they were smoothed by water long ago. Scientists are not sure what happened to the water. But, because matter cannot disappear, the water most likely evaporated or froze. (Large ice caps are visible at each pole on Mars.) Scientists also think water may be buried beneath the planet s surface. This image compares rocks found on Mars (left) with rocks on Earth (right). Both sets of rocks look like they were smoothed by flowing water. Which tool can we use to measure different temperatures? Losing or gaining heat causes the temperature of a substance to change. If water loses heat, the water s temperature decreases. Similarly, if water gains heat, the water s temperature increases. Temperature is measured in units called degrees. The most common tool for measuring temperature is a thermometer. Sometimes temperature is measured on the Fahrenheit scale. When you hear a weather report, you might hear the temperature will be 65 degrees Fahrenheit, or 65 F. However, scientists use the Celsius scale to measure and record the temperatures of different substances. A temperature of 65 F equals about 18 degrees on the Celsius scale (18 C). temperature: a measure of the amount of heat in a substance. Fahrenheit: temperature scale commonly used in the United States. Celsius: temperature scale used by scientists. On this scale, water freezes at 0 and boils at 100. try now Take a little time to explore how to measure temperature on the Celsius scale. Remember that many thermometers are made with glass, so handle yours carefully. 1. To complete this activity, you will need the following materials: Celsius thermometer A container of room temperature water, about one cup 8
Four or five ice cubes A spoon 2. Place the thermometer in the container of water. Wait for about 1 minute. 3. With the thermometer in the water, note where the liquid on the thermometer stops along the number line. This is the water temperature. Record it in degrees Celsius, or C. 4. Take the thermometer out of the water. Add the ice cubes and stir with the spoon. 5. Repeat Steps 2 and 3. 6. Think about how the temperature would change if you added heat. Would it increase or decrease? At what temperatures on the Celsius scale will water freeze or boil? At what temperature will ice melt? Like other substances, water needs to reach a certain temperature in order for its physical state to change. The temperature at which a substance changes from a liquid to a gas is called its boiling point. The temperature at which a substance changes from a liquid to a solid is called its freezing point. As you might have guessed, the temperature at which a substance changes from a solid to a liquid is called its melting point. All substances have specific boiling, melting, and freezing points. For example, the boiling point of pure water is 100 C. You can place water in a pot and heat it on the stove. Once the water temperature reaches 100 C, it will begin to boil. This boiling point does not change whether you have a large pot of water or a small droplet of water. Boiling, melting, and freezing points are constant, meaning they do not change unless the substance itself is changed. If you add salt to water, you are changing pure water into a different substance. This new substance will have a different boiling point. Melting and freezing are reverse processes. Because of this, the melting point and freezing point of water are the same. They are both 0 C! 9
What do you know? Water has constant boiling, melting, and freezing points that cause its physical state to change. Color each thermometer in the chart below to show how the water s temperature changes in each example. Then, read the descriptions of changes in physical state. Decide which change matches each example. Write your answers in the final row of the chart. For each example, color the thermometer on the left to show the water s starting temperature. Color the thermometer on the right to show the water s ending temperature. Water starts at 60 C and is heated until it boils. Water starts at 40 C and is cooled until it freezes. Water starts at 10 C and is heated until it melts. The water changes from: The water changes from: The water changes from: Changes in Physical State solid to liquid liquid to solid liquid to gas 10
connecting with your child Properties of Water at Home To help students learn more about the properties of water, set up an experiment to compare the time it takes to boil different quantities of pure water. You will need the following materials to get started: Eight cups of water set to room temperature Measuring cup Hot plate or access to stove burner Small pot Timer or stopwatch Heat-resistant Celsius thermometer that measures at least 120 C (optional) Remind students to use extreme caution when using the hot plate or stove burner. In order to bring the water to room temperature, let it sit in a room for about an hour before the experiment. Set up the experiment by adding two cups of room temperature water to the pot. If possible, have students measure and record the water s temperature. Next, carefully place the pot on the hot plate or stove and start the timer. As soon as the water begins to boil, record the elapsed time. If possible, have students carefully measure and record the water s temperature. (Measurements should confirm that water boils at 100 C.) Turn off the burner and pour out the water. Rinse the pot with cold water and drain. Now, add one cup of water to the pot. Repeat the experiment by measuring the temperature of the water, placing the pot on the burner (be sure to use the same setting on the burner), and recording the amount of time it takes for the water to boil. Repeat these steps for the following quantities of water: one-half cup and four cups. Compare the results from each trial. If time allows, try a similar experiment with salt water. Simply add a quarter-cup of salt to a cup of water and stir until the salt is dissolved. Record the amount of time it takes the salt water to boil, and compare this to the pure water results. Here are some questions to discuss with students: Did the amount of water affect the amount of time it took for the water to boil? Did the amount of water affect the boiling point? If you were not able to obtain boiling point measurements, think about what you observed during the experiment. Suppose you added salt to the water. How do you think this would affect the boiling point? Do you think it would affect the boiling time? 11