Robert W. Mellette
Topic
Decomposition of water by electrolysis
Rationale
The Space Shuttle’s cabin atmosphere is generated from the release of oxygen and nitrogen gas which is stored in a liquid state in tanks located in the fuselage of the spacecraft. There is a sufficient supply to meet the needs of the crew for seven day missions. Future missions will be much longer, lasting up to thirty days in duration. The Space Station will be a permanently manned station. The Space Shuttle and the Space Shuttle will both require extended supplies to support their crews for their say in space. The supply of oxygen will be especially critical. Electrolysis of water to produce oxygen and hydrogen is one option being investigated as a source for replenishment of the oxygen supply.
Objective
To demonstrate to students how water can be broken down into its component gases of hydrogen and oxygen by electrolysis.
Materials Required:
Hoffman of similar apparatus
water
weak acid (vinegar)
six volt dry cell or power supply
wood splints
test tubes
bell wire
safety goggles
figure available in print form
Procedure
Set up the Hoffman or Brownlee apparatus as shown in the diagram. Fill the apparatus with water. Add a small amount of vinegar to the water. Connect a six volt dry cell or power supply to the electrodes of the apparatus. The electric current passing through the water will cause bubbles of gas to begin to form at each electrode. These bubbles of gas will rise up and displace water in the vertical collection tubes. Direct students to observe this process carefully. This experiment will take some time to complete. It is best to start this activity at the beginning of the class period so that there will be enough time for the process to occur. Students should observe that the gases are not of equal volume. In fact that one gas collected (Hydrogen) has twice the volume of the other (Oxygen). Challenge the students to explain this phenomena.
CAUTION: The next part of this experiment should be performed with caution. Safety goggles must be worn at all times and extreme care exercised while testing for Hydrogen. Hydrogen gas is highly combustible. Only a small quantity of gas should be tested.
A glowing splint is used to test the gases collected. Using an inverted test tube draw off a small quantity of the gas to be tested. Light the wood splint with a match. Blow out both the match and the wood splint. Place the glowing wood splint into the test tube containing the gas to be tested. If the splint burst back into flame, oxygen is present. If an explosive pop is heard, the presence of Hydrogen is indicated.
