What Is the Water Cycle?
Water is one of the few substances that can be found in all three states—solid, liquid and gas—at any given time somewhere on Earth. For example, snow and ice always are present at the poles, as well as on the tops of high mountains. Liquid water is abundant in many places on Earth, including lakes, rivers, oceans and underground. Water vapor, the gas phase of water, usually makes up a small component of the air around us (up to 5%), and can be observed as steam when liquid water is heated.
When talking about this important resource, we usually think of liquid water. However, if water were not continuously cycling among its three states, the world’s stores of freshwater quickly would become depleted or too polluted to use. Fortunately, our supply of freshwater continually is collected, purified and redistributed as part of the water cycle. Also known as the hydrologic cycle, this continuous process replenishes our water sources through precipitation (rain, mist, snow and sleet, for example). Some water from precipitation soaks into the ground. The rest runs off into streams, lakes and the oceans. Heat from the sun causes water to evaporate from the land and from bodies of water. Water vapor collects in the atmosphere until there is too much for the air to hold in clouds, leading once again to rain or snow.
This activity allows students to explore properties of water that are important to the water cycle.
Session 1: Making the model
- Have each group line the inside of its box by pressing a single sheet of aluminum foil along the bottom and up the sides of the box.
- Direct groups to take turns measuring out two cups of sand and placing it in a pile at one end of their boxes.
- Have each group smooth the sand to create a hill at one end of the box, gradually sloping it toward the other end. This will form the “land” in the model.
- Have each group place 20 ice cubes on top of the “land” in the box. The ice cubes will represent “snow” and “ice” in the model.
- Help the groups cover each box with a sheet of clear plastic wrap and secure it with a large rubber band. (If using plastic storage boxes, cover them securely.)
- Discuss the models with the class. Ask, Which part of the box and its contents could represent land? Which part could represent snow on the tops of mountains or ice in the winter? Do you think a lake could form? If so, where would it be?
- Ask students, What do you think will happen if we put the boxes in the sun? Have each student fold a sheet of drawing paper in half. Then direct students to use one-half of the sheet to draw a “side view” of what they think the box will look like at the end of the day. Place the boxes in a sunny window or a under a lamp with an incandescent (not fluorescent) light bulb. If possible, have the students observe their boxes at intervals throughout the day. Otherwise, have them observe the boxes within the next day or so.
Keywords: condensation | evaporation | ground water | ground water | hydrologic cycle | lake | ocean | physical science | polarity | precipitation | properties of water | river | three states of water | transpiration | water | water cycle | water molecule | water source | water supply | lesson
- Illustration © Baylor College of Medicine\M.S. Young.
- Moreno, N., Tharp, B., and Dresden, J. (2011) The Science of Water Teacher’s Guide. Baylor College of Medicine: Houston.
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Funded by the following grant(s)
My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932
Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education