Comments on: A Physics Party Trick that Sucks… Liquid

By: Nick Chira

Nick Chira — Sat, 31 Oct 2009 16:15:20 +0000

Generally speaking, the first video was correct, and the 2nd video is not. In a hydrocarbon combustion reactions, more molecules of oxygen are consumed than molecules of carbon dioxide that are produced, thus dramatically lowering the pressure. As others have noted, the remaining oxygen usually forms liquid water. The reduction in overall air molecules is the dominant force of this physics trick.

As a matter of fact, the change in temperature does have an effect, but it is opposite to the observed overall effect. That is, as you enclose the flame with the glass, the air in the glass heats up, which increases the pressure, thus forcing liquid OUT of the glass. It is then true that the air cools down substantially when the flame goes out, but it is still hotter than the air was previously, and is thus always higher pressure than the surrounding atmosphere, so it still acts to force liquid out, but when the candle goes out the temperature effect can no longer appreciably affect the effect from the number of air molecules in the enclosed space.

It should further be noted that temperature will never affect the overall density of a closed system in any way, shape, or form. As a matter of fact, hot air is generally less dense than cold air as they have more pressure for a given number of molecules.

By: Brian

Brian — Fri, 31 Jul 2009 14:59:09 +0000

I have a problem with the statement: “Oxygen is being “consumed” by the fire, that produces Carbon Dioxide ”

Fire is heat and light energy produced by the chemical reaction that “consumes” the reactant O2 while also producing the product CO2 (among other things).

I don’t think this is trivial because fire is not a tangible thing. It’s a name for the combination of types of energy our sense organs can detect during this exothermic reaction.

By: Dr.CSBR.Prasad

Dr.CSBR.Prasad — Sat, 28 Mar 2009 17:57:28 +0000

Heat from the candle flame also displaces air around it. When it goes off, low pressure is created in addition to the explanations offered above.

By: james yeatman

james yeatman — Thu, 15 Jan 2009 22:16:16 +0000

Hey. im doin a project and i was wondiering if hypotonic transportation would be simular to this expanation . the project im doing is just to demonstrate hypotonic transportation.?

By: frenchemist

frenchemist — Sun, 02 Nov 2008 19:17:56 +0000

hi, i have just discovered your blog today, nice job, good idea to propose these home experiments but i still not confortable with the explanation. So, I reppeat the experiment two, tree times and after observing some small drops inside the glass, finally i can propose to you another explanation:
candles are made of hydrocarbon, it means atoms of Carbon and atoms of Hydrogen. When the candle are burning C atoms react with O2 molecules to give CO2. So, starting from one molecule of O2, you create another molecule (CO2) , the number of gas molecules do not change (same pressure). But in the same time, the H atoms react with some part of O2 to create H2O (water). This water do not remain as a gas but as a liquid. If you take care, you can see small drops at the internal surface of the glass. So finally a part of O2 gas desepears to produce liquid water and then the pressure decrease inside the glass. Your first explanation was not totally wrong. (sorry for my basic english)

By: Neil Pharr

Neil Pharr — Thu, 26 Jun 2008 00:37:22 +0000

“Yes, and only Evolution does that! (Kidding!)”

Very interesting comment Tom (no kidding!)

I remember reading in Readers Digest in about 1963, “Life is an inherent property of matter”. That got my attention.

Could there be a waining of some cosmic binding force on matter that is not only reflected in the generation of life/mind, but a directional flow in bio-evolution toward more suble expression of mind and thus the appearance of (defies the laws of thermodynamics!”) as your comment?

neil in atlanta

By: AVoiceFomGermany

AVoiceFomGermany — Mon, 05 May 2008 13:29:02 +0000

I’d say ‘density’ (or whatever the correct English word is) explains it best, since CO2 weighs more per volume unit than O2. So it’s the volume that’s consumed, not the O2 that’s still there, but now connected to the C Atom.
Hmm, do I hear another experiment coming?

By: Tom S. Fox

Tom S. Fox — Tue, 15 Apr 2008 01:34:36 +0000

“in failing to mention that, I sounded like this experiment defies the laws of thermodynamics!”

Yes, and only Evolution does that! (Kidding!)

By: mooeypoo

mooeypoo — Mon, 14 Apr 2008 00:18:42 +0000

Hm, yes, thank you for the correction. You’re right, I didn’t quite mean it this way, but I sound as if I do. So — I am fixing the post right now. The fire isn’t consuming the molecules! I posted a correction both here and in the video page.

The effect is still about pressure, but matter is definitely not “vanishing” and is not ‘created out of nothing’. Of course.

Apologies.

By: Shane Killian

Shane Killian — Mon, 14 Apr 2008 00:03:58 +0000

Moo, I love you, you’re great, but you’ve got this wrong. Remember that matter cannot be created or destroyed, only change its form. The oxygen atoms are still there; they aren’t being teleported off to Loompaland or anywhere. What’s happening is that the oxygen is now part of other molecules; for example, oxygen combines with the carbon from the candle to make CO2. Oxygen gas is simply replaced with carbon dioxide (and other gases).

The reason why the milk is being sucked into the glass is because the candle has been heating the air around it. As it consumes the oxygen in the glass, it burns less hot, and eventually burns out entirely. As this happens, the density of the air inside the glass decreases, and this pulls in the milk (or, specifically, this is what causes the greater pressure in the air outside the glass to push in the milk).