A class period
U.S. National Science Standards
Advance Preparation Time
It is of obvious concern to space walking astronauts to have enough oxygen to breathe while they are conducting mission. They need enough oxygen to complete their assigned tasks and additional oxygen in case of unforeseen problems and emergencies. How much oxygen they carry with them is determined by their oxygen use rate and the time length of their mission. Physically difficult tasks cause astronauts to use oxygen more rapidly than do physically simple tasks.
To provide enough oxygen for space walk missions, NASA has had to determine oxygen rates for different levels of physical activity. Oxygen use is measured rather than air use because it was determined early in the space program that using air for space walks would be inefficient because air would require very large holding tanks. Air is approximately 80 percent nitrogen and 20 percent oxygen. By eliminating the nitrogen and providing pure oxygen, much smaller tanks can be used. In space settlements a similar strategy could be used. The amount of nitrogen could be reduced to as much as half of air at sea level, while keeping the oxygen level same. However pure oxygen is out of consideration in space settlements due to fire hazards which makes buffer gases like nitrogen necessary.
In this activity, students will determine the amount of air an average student breaths during rest and during heavy physical activity. They will calculate how much air would be needed for a six-hour space walk. If no one thinks of it, suggest they consider using pure oxygen instead of air. Have your students calculate the volume of pure oxygen that would satisfy the needs of the average student for the six-hour mission and compare this to the quantity of air that would be required for the same mission. Then extend this activity into space settlements by adding nitrogen to create air with half the pressure (0.5 atmosphere) of air at sea level (1 atmosphere).
Calibrate the glass jug in units of liters. Pour 1 liter of water into the jug and mark the water level on the side of the jug. Add a second liter and again mark the level. Repeat twice more.
Completely fill the jug with water and invert it into a basin of water so that air pressure causes the water to remain in the jug. Insert one end of the tube into the jug.
Invite several student volunteers, one at a time, to exhale through the tube into the jug. Water will be expelled from the jug. Students should breathe normally when doing this. Count how many breaths it takes to empty the water from the bottle. Also, determine the number of breaths each student takes during one minute. Record the two measurements on a chart under the headings "Breathing Volume" and "Breaths per minute."
Caution: Be sure to disinfect the end of the tube between student participants.
Calculate the averages for each of the four columns on the data chart. Use the first set of measurements to determine what volume of air an average student will consume per minute during normal activity. Next, calculate how much air is needed for one hour by that average student under normal activity and under heavy work.
Ask the students to calculate how much air would be needed by an average student-astronaut on a six-hour space walk. Typically, space walks involve both light and heavy exertion.
Prepare limewater by adding 2 g of calcium hydroxide to 1 liter of water. Stir thoroughly, and set solution aside for at least one day. The clear limewater can then be carefully decanted into a bottle. Keep the bottle stoppered; otherwise, carbon dioxide in the air will slowly react with the limewater and produce a thin scum of calcium carbonate on the surface.
Fill half of a test tube with limewater. Have a student volunteer exhale slowly through a drinking straw into the limewater, steadily, while causing bubbles. The solution will turn cloudy indicating the presence of Carbon Dioxide.
Illustration is by Leyla Sezen
Author: Tugrul Sezen