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When gas from a cooking gas cylinder is used, the pressure does not fall appreciably until the last few minutes. Why?

This was a question in one of my reference books and I couldn't answer it. Didn't find anything useful on google too. I am not able to figure out why the pressure in the cylinder doesn't drop below atmospheric pressure and how is it able to supply the gas, till it is almost over. Can anybody help me find the reason for this phenomenon?

Thank you :)

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    $\begingroup$ When you shake the cylinder, you should hear something sounding like a liquid inside ;). That should give you a hint ! $\endgroup$
    – Frotaur
    Commented Mar 14, 2017 at 13:22
  • $\begingroup$ @Frotaur oh right...how dumb of me, its lpg $\endgroup$
    – oshhh
    Commented Mar 14, 2017 at 13:24
  • $\begingroup$ Hey at least you found the answer on your own :p $\endgroup$
    – Frotaur
    Commented Mar 14, 2017 at 13:25
  • $\begingroup$ @Frotaur not really :|, merci beaucoup anyways $\endgroup$
    – oshhh
    Commented Mar 14, 2017 at 13:26
  • $\begingroup$ In your defense it's kinda a trick question. It's a "gas cylinder" with a bunch of liquid in it. Get it together nomenclature. $\endgroup$
    – JMac
    Commented Mar 14, 2017 at 13:28

2 Answers 2

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As you have realised, vapour is in equilibrium with liquid. If the vapour pressure is reduced more liquid is vaporised to replace it and maintain a constant vapour pressure.

Liquified gas is about 1000 times more dense than vapour at the same temperature and pressure. The pressure in the canister will not fall until all of the liquid has evaporated. If the canister holds 1000 litres of gas in liquid form, it is only while the last 1 litre is being dispensed that the pressure will begin to fall.

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    $\begingroup$ This is true as long as the gas is flowing slowly enough that you maintain thermal equilibrium. Rapid evaporation of the liquid pulls heat from the liquid and lowers the temperature (and thus the vapor pressure). $\endgroup$
    – Floris
    Commented May 31, 2017 at 21:36
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As Frotaur hinted in the comment, the gas cylinder contains liquified petroleum gas, which is in equilibrium with its vapour phase. Whenever some gas is removed from the system, the liquid-gas equilibrium shifts in the forward direction and some liquid is converted to vapour. (Le Chatelier's principle is applied to be precise). Hence, the pressure of the gas doesn't vary appreciably when some gas is removed from the system.

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