Objectives:
Students will be able to

• Label evaporation on the hydrologic cycle map (student page)
  
  
• Record observations accurate to within 1°C and 2 mm
  
  
• Identify the substance that has precipitated out of the solution as it evaporated
  
  
• List some factors which influence the rate of evaporation
  
  
• Explain in essay form where the disappearing water went

Time:
One day
20 minute set up
One afternoon for observation
20 minute cleanup/discussion

Materials Needed:

• 2 shallow pans of water
• 2 Celsius thermometers
• 50 ml graduated cylinder
• Fan
• Heat lamp
• Salt
• Stopwatch or clock
• Copy of provided chart to record observations

Vocabulary:

boiling point
evaporation
solution

Included National Parks & other sites:

Death Valley National Park

Background:

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.

Instructional procedures:

1. Place two pans on a level table or countertop.
2. Fill the graduated cylinder with 150 ml of water. Pour this into one of the empty pans.
3. Fill the cylinder with another 150 ml of water and add 5 grams of salt and stir. (Placing a few drops of food coloring may make it easier to see the salt). Stir until dissolved.
4. Pour this solution into the empty pan on the table.
5. Secure a thermometer in each pan. Make sure the thermometer is always in the water and record initial temperatures of both pans of water.
6. Record observations at ½ hour intervals, including temperature and volume. (Check volume by pouring the pan of fresh water into the cylinder. Record volume. Pour back into pan. Then pour the salt water into the cylinder. Record volume. Pour back into pan. Be sure to rinse the cylinder and allow it to dry between each ½ hour volume reading.)
7. Continue this process as long as possible or until the pans are dry. If they dry at different times, record the time it took each pan to dry.

Discussion:

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?

Variations:

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?

Extension:

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.