Charles' Law and exploding hairspray cans, a question

[Ber Rabbit]

Am working on my Charles Law lecture (NAUI instructor training)and need some help with my "attention gaining step." Was wondering if I could use the analogy of an empty aerosol can being discarded in a fire as an example of what Charles Law does.

Here's what I do know:
Aerosol cans say "do not puncture or incinerate"
An empty aerosol can, when placed in a fire turns into a rocket after a period of time (KIDS DON'T TRY THIS AT HOME, YOU HAVE NO CONTROL OVER WHERE THE CAN GOES AND CAN BE SERIOUSLY INJURED OR KILLED!)

Here's what I want to know:
Does the aerosol can explode because the temperature of the fire has heated the remaining gas inside the can beyond the "burst limit" of the can? (Just like leaving a scuba cylinder baking in the hot sun?) Or is this explosion a result of the flammability of whatever residue may remain in the can?

I have to give the lecture Wed. afternoon for a grade, any comments are welcome! If I can't use this as my attention gaining step does anyone have any other ideas?

Thanks!
Ber

 

[Northeastwrecks]

Dear Ber Rabbit:

As a kid, I did try this at home when the neighbor was doing a controlled burn of the underbrush behind his house. It was pretty spectacular.

I used a can of aerosol deodorant, which I believe to have been nonflammable and which, therefore, probably did not combust.

It exploded rather spectacularly after about 10 minutes in the fire.

Good luck.

 

[jbd]

I see both Charles law and Amonton' law in this scenario.

Charles law deals with constant pressure and has volume as the variable. It makes the presumption that the gas is free to expand.

Amonton's law deals with a constant volume and has pressure as the variable. The assumption here is that the volume can not change.

The scenario starts out more like Amonton's law and ends with the bursting of the can becoming Charles law with the rapid expansion of the released gas IMHO.

Any burning of the contents at the instant the can bursts is due to the combining with O2. Again IMHO.

In reality there is some Charles law effect happening just before the can bursts.

 

[jbd]

talk about a hot air balloon?

 

[Shaun]

With regard to the flammable expellant used with the container… the bursting cannot be due to the explosion of the expellant within as it is well above its upper flammable limits. The expellant can only ignite after the container has burst and its contents mixed with the surrounding air and been diluted sufficiently to bring the mixture within its upper and lower flammable limits.
As to why the container bursts… I agree with the previous postings.

Regards,

Shaun

 

[Walter]

Forget Charles' Law, it has no application in diving. You need to explain Gay Lussac's Law which deals with temperature and pressure relationships. The siutuation you describe relates to Gay Lussac's Law.

 

[Ber Rabbit]

Unfortunately my assigned lecture is Charles' Law. The way I was taught its only use in diving is when you are dealing with hot fills or leaving your scuba tank in the sun. The basic relationship I was taught was that as temperature increases, pressure increases (the molecules move faster). If this is wrong some enlightenment would be appreciated. This was always used as the explanation for burst disks blowing when the tank sat in the sun. None of my SCUBA books including my ancient copy of the NOAA Diving Manual (price $8.55) mention Gay Lussac's Law but have Charles' Law listed for volume/pressure/temperature relationships. All help is appreciated, physics is a very weak point for me and I'm trying hard to pick this stuff up.

What is Gay Lussac's Law?

Any other suggestions for attention gaining steps for this type of lecture?

Thanks,
Ber

 

[Walter]

Gay Lussac's Law is the law most people think is Charles's Law. I'll check when I get home, but I believe the NAUI Nitrox text covers Gay Lussac's and Charles' Laws correctly.

If you have to cover Charles' Law, discuss hot air balloons, not diving.

 

[jbd]

has to do with volumes of combining gases although he did much work supporting Dalton theories.

Amonton's Law is the one that deals with the relationship between temerature and pressure of a fixed volume of gas.

This is according to web searches.

Charles law is best related to a hot air balloon in which the given amount of air expands and fills the balloon which results in a lower density and allows the balloon to become positively bouyant in the air.

 

[Ber Rabbit]

OK, make sure I'm understanding this correctly, I'm a bit of a slow learner but once I've got it I keep it.

A hot air balloon is filled with air like a scuba tank, not helium, etc.
The balloon rises because heat is applied to the air causing it to expand (the molecules are moving faster and farther apart)
The expanding air inflates the balloon
Continuing to apply heat makes the molecules move even faster (are there now fewer molecules? Is that why the density decreases?)
Hot air balloon is now lighter than air and takes off.
Air in balloon starts to cool so balloon slowly sinks unless air is re-heated.

Am I getting there?
Ber