Allexperts Question / Ideal Gas Laws, Science Demos, and Pop Rocks
Questioner: R
Subject: Gas LAws
Date Asked: 2011-03-14 17:30:22
Date Answered: 2011-03-14 18:19:03
Question:
QUESTION: Hi
I am showing students demonstrations for gas laws and did soda can shake up- for Boyles law and Can implosion for Charles law...What would pop rocks candy fit under because pressure and temperature are changing in the gas...please help me clarify -Thanks
ANSWER: Hi R! I'm sorry this finds you so late.
I looked at how Pop Rocks were made - turns out they trap 600psi CO2 in hard candy. Cool!
I think you could spin Pop Rocks as the summary combination of both Boyles and Charles' law (with the added bonus of being a tasty treat) or just approximate them as Boyle's. It's likely that by the time your mouth dissolves the candy enough for a pop, it is pretty much at 35-37 °C.
You could also use them as an example of temperature effects on reaction rates: the Pop Rocks dissolve faster in your mouth than submerged in room temperature or iced water.
Either way, very good exam questions: great idea!
Zap me back if I've been unclear!
http://science.howstuffworks.com/innovation/everyday-innovations/question114.htm
http://www.howstuffworks.com/framed.htm?parent=question114.htm&url=http://www.delphion.com/details?pn=US04289794__
---------- FOLLOW-UP ----------
QUESTION: Hi- Thanks for the feedback...would you just clarify how I would spin it for Boyles or Charles Law...If Boyles Law states temp is the same would the pressure in the candy would decrease in our mouth and the volume increase b/c we feel the pop?
Charles Law says the Pressure is the same and volume and temp increase or decrease together....so if pressure is the same in our mouth the temp increases and release the higher popping ? Help :O...I just want to make sure It is correct when I explain -Thanks
Answer:
Overall, what's going on with Pop-rocks is that we have a fixed volume bubble under high pressure, and the outside being low pressure. We hear a pop when the pressure inside overcomes the strength of the candy holding it in. This change in pressure from state 1 to state 2 will have expected effects on the volume of the gas. (big pressure, small volume to big volume, small pressure)
Boyle's law could relate because we're changing the volume, but could model the temperature in your mouth as close to constant. In this case, we're essentially dissolving a hole to release the gas with no change in temperature.
Charles' law doesn't apply alone if we consider temperature effects because the pressure is not constant. As we increase the temperature in the candy bubbles, the gas inside moves about faster, increasing the pressure on the candy walls. It'd like to increase in volume but cannot. What happens? More pressure, which in turn would push harder on the candy walls, increasing the pop. (Bonus round - when the gas expands out in the pop, does it increase in temperature or decrease?)
So in this case, it's most clearly modeled by the ideal gas law pv=nrt, because we have changes in all three variables; temperature, pressure, and volume. The strategy I learned is to list all your variables including unknown variables, and then try to pick your equation to solve the unknown variable. Since n (moles) and r (gas constant) are constant, we can say p1v1/t1=p2v2/t2 and solve for the unknown variable of interest.
Make sense?