Gas Properties
Part 1: Break the simulation
A. Google ​
phet gas ​
to find the simulation. Then, play around to ​
find as many ways as you can to make the lid pop off the gas container.​
This will introduce you to many aspects and capabilities of the simulation. B. When you’re done, talk to another group to see if you found all the ways they found, and vice versa. Gloating is permitted. Part 2: Microscopic reasoning about an ideal gas
A. A pressure meter in the simulation—and in real life—measures how much the gas molecules are banging against the walls of the container. Think about what happens to the molecules and how they interact with the wall when the gas is being compressed at constant temperature. i.
Based on the “story” you just came up with, do you think the pressure of the gas increases, decreases, or stays the same? Again—don’t use remembered facts or equations, just use common­sense physical reasoning. ii.
Now try it out with the simulation. Note the​
hold temperature constant​
button in the upper right corner. Be sure to check not only your predicted ​
answer​
, but also your predicted ​
explanation​
by looking at what’s happening with the molecules. B. What happens to the pressure of the gas when you raise its temperature (without changing the volume)? Please go through the same cycle as in part A, first telling a “story” about what’s happening with the molecules, then using that story to predict what happens to the pressure, and finally trying it out with the simulation to check both your answer and your explanation. C. What happens to the pressure when you add more gas molecules to the container? story­prediction­try... D. What happens to the pressure if you hold the temperature constant while both doubling the volume AND doubling the number of gas molecules in the container? Part 3: What does the math say?
A. Now you’ll re­answer the questions from part 2, but this time using the ideal gas law, pV​
= ​
NRT​
(or whatever version you’re most comfortable with). i.
According to the ideal gas law, what happens to the pressure of an ideal gas when it is being compressed at constant temperature? ii.
Did you reach the same conclusion in Part 2A above? If not, try to reconcile the discrepancy. B. What happens to the pressure of an ideal gas when you raise its temperature (without changing the volume)? Like you just did, answer with the idea gas law, then check your answer against your microscopic explanation from Part 2B above. C. What happens to the pressure when you add more gas molecules to the container? D. What happens to the pressure if you hold the temperature constant while both doubling the volume AND doubling the number of gas molecules in the container? E. A student who did this tutorial last year told us, “That was a waste of time, answering the same questions twice. Once I had the answer, what was the point of answering it again?!” Do you share the student’s frustration? Why or why not. Please be honest. Part 4: Adiabatic compression and expansion
A process is “adiabatic” (ay­dee­uh­bat­ic) when no heat flows into or out of the gas. A. Is an adiabatic compression the same thing as a constant­temperature compression? In other words, if you do an adiabatic compression, not allowing heat to flow into or out of the gas, will the gas stay at the same temperature? i.
Make a prediction based on what’s going on with the molecules. ii.
Make sure Temperature is not held constant in the simulation, and compress the gas. What happens to the temperature? iii.
Now hold the temperature constant and do the same compression. Is heat flowing into or out of the gas? B. OK, now it’s time to dig deep for explanations, if needed. Why does the temperature increase during an adiabatic compression? In playing around with the simulation, pay particular attention to the interaction between the moving wall and the gas molecules the wall runs into. C. Now let’s compare an adiabatic compression to a constant­temperature (“isothermal”) compression. In which case, if either, will the gas ​
pressure​
increase by a greater factor? i.
Make a prediction by building on your molecular reasoning above. Give as a complete an explanation as you can, in terms of what’s happening with the molecules. ii.
Now check your answer and explanation using the simulation. D. In which case, a constant­temperature or an adiabatic expansion, will the gas pressure decrease by a greater percentage? Predict, explain, test it… Part 5: Bringing math into the mix: Adiabatic processes
A. Consider an adiabatic compression of the gas to half of its original volume. a. Using the ideal gas law alone, can you predict the final pressure? Explain why or why not. b. Do you think the ideal gas law applies to an adiabatic compression? In other words, do you think ​
pV​
equals ​
NRT​
before, during, and after the adiabatic compression? Explain. Part 6 (if you have time). Taking a breather
The writers of this tutorial got into a discussion about a mundane observation: ​
when you inhale and then exhale on a cold winter day, the gas coming out of your mouth from the exhale is MUCH warmer than the gas going into your mouth during the inhale. ​
We couldn’t fully figure out how the gas gets so much warmer so quickly, though we’re pretty sure it involves both biology and physics—the stuff from parts 1 and 2 above isn’t enough to explain it. Please join us in trying to figure this out.