Thar She Blows!
Objective: This activity will involve changing the amount of air space in a film canister to determine the relationship between the volume available for a confined gas and its pressure.

Time required:  1 day.

Group Size: 4

Materials Needed:
• 1  graduated cylinder (25 mL capacity)
• 1 film canister with a tightly fitting lid
• 3 alka seltzer tablets
• styrofoam cup filled with water
• 1 pipet
• stopwatch
• TI-83 graphing calculator.
Prior knowledge: The student should be able to fit a set of data to a linear regression  curve.

Procedure:
1. Fill the film canister completely with water.  Pour the water into a graduated cylinder.  This amount of water represents the volume ( total capacity) of the film canister.   Record the volume of the canister.
2. Measure  28 mL of water and put it into the canister.  Add ½ a tablet of alka seltzer.  Quickly replace the lid securely on the canister and begin timing.  Step away from the apparatus.  When the lid blows off, stop the timer, and record the time.
The difference between the total capacity of the canister and 28 ml is the volume of space available for the gas to build up.  Find the difference and record it in the data table.
3. Repeat step 3, starting with 24.0 ml of water in the canister.  Again, add ½ tablet of alka seltzer, and record the time required before the pressure builds to the point where the lid blows off.  Record the data.  Calculate and record the amount of space available for the buildup of gas.
4. Repeat, using  20.0 mL, then 16.0mL, then 12.0 mL and 8.0 mL of water.  As the amount of water lessens, more space becomes available for the gas generated by the reaction.
Data Table:

Total capacity of  film canister = ___________ mL
 Volume of H2O (mL) in canister Volume of airspace (mL) in canister Time required to blow lid off (sec) 28.0 24.0 20.0 16.0 12.0 8.0

Analysis:
1. Enter the volume of airspace in the canister in list 1, and the time values in list 2.  Prepare a plot of volume (x-axis) versus time (y-axis).
2. Using the STAT, CALC feature of the graphical calculator, determine if the relationship between the volume of a gas and its pressure fits a linear or a ln function. Fit the data to its best regression pattern. Paste the equation for the line of best fit into Y=.
3. Record the line of best fit.

The gas generated in the reaction is carbon dioxide. The experiment was designed to gradually increase the volume available for the gas to expand within. The result is that it takes longer and longer for sufficient pressure to build up.  Eventually, the pressure builds to a sufficient degree to blow the lid off the canister.
1. Consider a large amount of gas collecting in a small space.
1. Will the pressure exerted by the gas be relatively large or small?
2. If the same amount of gas is placed in a larger space, will it exert a greater amount of pressure or less pressure than in part a?  Explain.

2. Think about what would  happen if ¼ of an alka seltzer tablet were added to 24 mL of water in the canister rather than ½ a tablet.
1. Do you think enough gas would be generated to blow off the lid?
2. If so, would it require more time or less time before it blows?

E Saylor

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