Activity 3: Evaporation
WHERE DID IT GO? EVAPORATION?
Evaporation turns water into water vapor, water in its purest form. Everything else that was dissolved or suspended in the evaporating body of water is left behind. The activity demonstrates the difference in evaporation of fresh and salt water.
Instructional Method: Experiment
Goal: To show students what evaporation is.
Objectives: Students will be able to:
Set up: 20 min.
2 shallow pans of water
Evaporation will be the starting point for our exploration of the Hydrologic Cycle. Our Sun provides the energy required for evaporation. Oceans and other bodies of water rarely boil in nature, so there must be a way for water to be changed into vapor without reaching its boiling point. Evaporation is the act of water changing from a liquid state into a vapor. Evaporation separates water into individual water molecules. Millions of times smaller than raindrops, these tiny bits of water are so light weight that they float upward into the sky. Although water can only freeze at temperatures at or below 32° F (0° C), evaporation can occur at almost any temperature, as long as the surrounding atmosphere is dry enough.
Evaporation occurs almost everywhere; however, evaporation is most extreme in the deserts. Death Valley National Park has the highest rate of evaporation in North America. This deep valley collects the rain and melting snow from a huge area of California and Nevada, on rare occasions turning much of the valley into a shallow lake. Yet even after a big rain all the water quickly evaporates, leaving only a new layer of sand, silt and salt behind.
How did all this new sediment come to Death Valley? It was captured by the water that came streaming off the surrounding mountains and highlands. The heavy pieces of sand are released from the water when they reach the calm shallow lake at the valley floor. Silt, like downy feathers, will also eventually sink to the bottom, but the salt remains firmly held by the water.
It is the 120° F (47° C) heat of Death Valley and the dryness of the desert air that applies the energy needed to free the salt from the water. Under these extreme conditions, evaporation happens so quickly that the billions of water molecules can actually be seen rising through the air. This phenomenon is what causes distant objects to become blurry or to shimmer when we look at them during a hot and dry day. By comparison, salt crystals are way too big and heavy to fly. As water escapes everything else is left behind. This is why the lowest point of Death Valley is crusted in a thick layer of salt.
Which pan dried faster? Where did the water go? What is the stuff left behind? How does this evaporation process affect natural lakes and oceans? If the oceans are salty, why isn't the rain? What would have made the water evaporate faster?
Repeat experiment with fresh water in both pans, one with a fan blowing on it, the other with a heat lamp shining on it. Record the times and temperatures. This will simulate wind and direct sun. Both pans should have rates of evaporation much higher than the first round of experiments. How do wind and sun affect evaporation rates? Why does the water in these pans evaporate faster than that in the first pans? If water can evaporate out of this pan, can water evaporate out of the ocean? Out of the ground? From a plant? From where does the water on the tips of plant leaves come?
Discuss with the students the purification effects of evaporation. Encourage them to design a device that would make ocean water drinkable. Once evaporated the water molecules have to be recaptured, cooled, and condensed to make drinkable freshwater. This leads the class into condensation activity.
Alternatively discuss the polluting effects of evaporation. As evaporation continues, the water that is left behind becomes increasingly salty and is no longer useable for drinking or irrigation. Encourage the class to design a system that would minimize the amount of evaporation lost in a farmer's irrigation system.
Included National Parks and other sites:
Utah Science Core:
1st Grade Standard 1 Objective 1