Education: Classroom Activities - The Water Cycle

• Evaporation occurs when a liquid is changed into a gas.
• Evaporation occurs when the temperature of a liquid is increased.
• Condensation occurs when a gas is changed into a liquid.
• Condensation occurs when the temperature of the vapor decreases.
• When the sun heats up water in rivers, lakes or the ocean, it turns into vapor or steam. The water vapor or steam leaves the body of water and goes into the air.

Essential Questions

• How does water change?
• How does water move?
• How does life depend on water?
• Where does the water that forms on the outside of your drinking glass come from?
• Where does the water go when it is evaporated?

Knowledge and Skills

• Observe the relationship between temperature and condensation and temperature and evaporation.
• Water vapor in the air gets cold and changes back into liquid water.
• When water is heated it changes into a gas.
• Use a model to simulate the water cycle.

Prior Knowledge

• Basic familiarity with the water cycle as a diagram and introduction to the terms used to describe the various stages water goes through in a cycle.
• Some events in nature have a repeating pattern - such as daily weather patterns or changes in temperature and the appearance of rain and snow at different times of the year.
• Water can be a liquid, solid, or gas and can go back and forth from one form to another.
• Water left in an open container disappears, but water in a closed container does not disappear. (Might be helpful to complete Evaporation Investigation before this activity.)

Common Preconceptions

• The water in a drinking glass seeps through the wall of the glass, or evaporates from the inside and condenses on the outside.
• Students have a difficult time accepting the idea of invisible particles of water in the air.
• Students understand the concept of boiling and freezing before understanding evaporation and condensation.
• The water cycle involves freezing and melting of water.
• Condensation occurs because the "coldness changed the water," or "cold caused oxygen and hydrogen to change to water."
• When water evaporates, it just disappears and ceases to exist.
• When water evaporates, it immediately goes up to the clouds or into the sun.

Concept Map
A concept map can be a helpful evaluation tool for an activity such as this, see Assessment Activities for further ideas. The map provided here relates to the branch "Hydrologic Cycle " from the comprehensive Aquarius Concept Map - Water and its patterns on Earth's surface.

Background
From the US Geological Survey - The Water Cycle

The water cycle has no starting point. But we'll begin in the oceans, since that is where most of the Earth's water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures causes it to condese into clouds. Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff, and groundwater seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration.

Evaporation drives the water cycle. Evaporation from the oceans is the primary mechanism supporting the surface-to-atmosphere portion of the water cycle. After all, the large surface area of the oceans (over 70% of the Earth's surface is covered by the oceans) provides the opportunity for such large-scale evporation to occur. On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation. Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10% of the water evaporated from the oceans is transported over land and falls as precipitation. Once evaporated, a water molecule spends about 10 days in the air. The process of evaporation is so great that without precipitation, runoff, and discharge from aquifers, oceans would become nearly empty over time.

Condensation is the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation. You don't have to look at something as far away as a cloud to notice condensation. Condensation is responsible for ground-level fog, for your glasses fogging up when you go from outside on a cold winter day and to inside a warm room, for the water that drips off the outside of your glass of iced tea, and for the water on the inside of the windows in your home on a cold day.