Water Cycle - Atmosphere (3) Post Lab
 OBJECTIVES: Exploring the main ingredients of air. Contrasting the atmospheric gases. VOCABULARY: argon carbon dioxide helium hydrogen nitrogen oxygen MATERIALS:  worksheet Students use a worksheet to compare atmospheric gases.
 BACKGROUND: Air consists of gases in different proportions depending on how high you are in the atmosphere. It was a very difficult process to determine what air is composed of.  Joseph Priestly in the 1770's showed that air contained something that living organisms needed to survive.  He called it “phlogistons.”  In the 1780's French chemist Antoine Lavoisier determined that air contained a gas, which he called oxygen.  He also found out that air contained two other gases - nitrogen and carbon dioxide. Later, air was chemically found to be about 21% oxygen, 78% nitrogen, and 1% carbon dioxide and other gases.  However, it wasn’t until the 19th century that researchers discovered that the atmosphere changed with height.  James Glaisher and Robert Cowell risked their lives in balloons to find out about the atmosphere.  By 1902, unmanned balloons charted and defined the extent of the troposphere. PROCEDURE: This exercise shows two different areas in the atmosphere and the students must decide where they belong.  You may want to give them a clue, that one of the elements is essential to humans, so it must be closer to the earth.       Also ask them, if they were traveling in an airplane would there be more oxygen up there or down on the ground.  Would they be able to breathe outside the airplane?  You may have to discuss with students that there is oxygen in the airplane that is brought from the ground.  If they are climbing a tall mountain, would they have trouble breathing?  Yes, because the air has different proportions of the different gases, and oxygen becomes depleted.    Give students the worksheet.  Go through the activities on the worksheet with students.  ANSWERS:     Nitrogen = N;  Oxygen = O;  Argon = Ar;  Carbon Dioxide = CO2;   Hydrogen = H;  Helium = He. What composition of air do you think is found below 72 km?  A What composition of air do you think is found above 800 km? B  Why?  B has lighter gases; organisms need oxygen so it should be on the bottom.