Question

Bubbles while boiling..

Now i understand the concept of boiling.. and i know why bubbles are formed.. one due to the dissolved air coming out.. and secondly, the water inside gets converted to vapour because of the heat.. and it can now pop out cause the vapor pressure can counter atm pressure.. I hope thats right..

what i don't understand is.. when u decreases the pressure so much, that the water can boil even without heating ( that is the boiling point of water is below the room temperature)... then why bubbling happens? i mean.. why would the bottom part of water now become steam?

Answer 1

I don't think the bubbles are due to steam, so much as it's due to the same process as when you boil water with normal atmospheric pressure. The lower pressure means bubbles form at lower temperature.

I think it's more that, at atmospheric pressure, you need extra heat to cause bubbles to form, because they need to overcome that extra atmospheric pressure. At lower pressure, bubbles form more easily since there's less atmospheric pressure.

Answer 2

http://scienceline.ucsb.edu/getkey.php?key=3197

Answer 3

ok .. these bubbles are basically H20 only right?? if m boiling water??..

Answer 4

i read that... it also says water vapors are formed inside the volume of the liquid.. thats what i meant by steam :P.. isn't steam = water in vapor form?

Answer 5

Mostly, but some of it is dissolved air. But yeah i see what you meant about steam. The H2O molecules can expand to fill a bigger space and cause a bubble.

Answer 6

but.. what i don't get is.. for these bubbles to form, water must get converted into vapor.. and that means the bonds that keep the water a liquid must be broken.. so regardless of what pressure it is.. the breaking bonds must require energy.. which it clearly doesn't get at room temperature..

Answer 7

The bonds (as in the weak bonds holding the H2O in water form) require energy to break. The vapour pressure also requires energy to overcome the atmospheric pressure...

Don't you think if you reduce atm pressure, you're lowering the energy required to create steam?

Answer 8

Room temperature is not absolute zero, the water has some heat energy - just not enough to overcome atm pressure.

Answer 9

i understand that as i lower the pressure.. i am making it EASY for the vapors to escape .. and so i make boiling EASY.. .but i am not able to get a good gut feeling for why lower the pressure makes it EASY to FORM water vapour?.. :(

Answer 10

They're the same thing. If you can make it easier for vapour to escape, why wouldn't it be easier to form vapour bubbles?

Answer 11

The reason bubbles don't form at room temp/pressure is because atm pressure is too great and prevents them from forming...

Answer 12

i don't get how are the two connected..

how is forming vapor has anything to do with the energy they require to escape?!.. what bothers me is that i m not able to understand how pressure effects how much energy is required to BREAK THE BONDS ?

Answer 13

Because they HAVE the energy to escape if the atm pressure is low enough. The pressure pushing down on the surface prevents enough molecules from escaping for it to boil.

Answer 14

The pressure doesn't affect the energy. It just means they need more energy, to overcome the pressure.

Answer 15

If you remove vapour pressure entirely, that doesn't mean you can boil water at 0 Kelvin.

Answer 16

hmmm.. oki.. i ll accept that.. and think more about it :)

i have another doubt.. if you suck all the air from a jar containing water.. then it ll boil.. but boiling requires energy right? so is that energy obtained from whatever energy the water already has? because as u are saying .. now since i decreased the pressure, its easier to boil.. ??


OR is it that it somehow makes the surrounding cooler?!

Answer 17

It's the energy the water molecules already have. At room temperature, water molecules have energy... or else... they'd form ice.

Answer 18

exactly. so m on the right track


so i don't get one experiment..

in that experiment.. he puts acetone and water.. and decreases pressure.. since acetone boils very easily.. acetone starts boiling.. and because of that the water freezes.. !! how is that?? because as u are saying the acetone boiling shouldn't be cooling the water right?

Answer 19

Energy = movement of molecules.

Answer 20

It sounds like acetone is absorbing energy from the water, as it boils. This causes the water to lose energy, and thus freeze. But you didn't describe much of the experiment...

Answer 21

yea yea.. as i figured.. but why can't the same happen with water when i boil it at lower pressure??.. why can't i make it cool the surrounding.. why wouldn't water absorb energy from the surrounding ? :D.. thanks fr being so patient man !!

Answer 22

It could possibly absorb energy from it's surroundings

Answer 23

so.. somehow we can use this system as a cooling agent?! like taking water and pumping air out?!

Answer 24

Well... how are you going to lower the pressure? That requires energy. You can't break the laws of thermodynamics and get more energy out than you put in.

Answer 25

no no.. m not breaking law of thermodyanics at all.. ofcourse u need energy to pump out air.. entropy is definitely not decreasing..

but i was thinking of this as a substitute for a refrigerator.. more like a mobile mini refrigerator.. or does something lke this even exist?..

i could use battery powered pump?!

Answer 26

It's probably a crappy refrigerator and inefficient.

Answer 27

lolz.. thanks a lot .. appreciate the help!..

Answer 28

http://www.realsimple.com/food-recipes/tools-products/appliances/how-does-refrigerator-work-00100000087962/index.html