Fire sponge party trick

Classical Mechanics Level 4

Your cool uncle approaches you at a holiday party carrying a plate filled with water, a glass, a napkin, a piece of tape, and a match. He tapes the napkin to the bottom of the glass, lights the napkin on fire, and places it upside down on top of the plate. As the flame starts to burn out, a large volume of the water begins to draw into the glass, emptying the tray.

What happened?

Fire expands air in glass, water rushes in to counteract the high pressure inside the glass. Fire burns almost all the air, leaving little in the glass, water rushes in due to resulting vacuum. Hot air rises, leaving a vacuum at bottom of glass, water rushes in to counteract the vacuum. Fire expands air in glass, pushes some out through the water. Remaining air contracts when it cools.

2 solutions

Arpit Kaushal
Aug 13, 2014

For things to undergo combustion they need air and when the glass with the napkin taped on its bottom and is turned upside down some air is trapped within the volume of glass and before turning it we put it on fire and so it consumes the air in it and because no their is no air so their is no pressure exerted by air because of which water did not enter glass, but now it does.

If this is true, why does the water not move until the fire goes out?

Josh Silverman Staff - 6 years, 10 months ago

A fire doesn't consume 'air'. It consumes oxygen (O2) and produces an equal amount (measured by moles of molecules) of carbon dioxide (CO2) - both of which are gases. If we ignore the difference in O2 and CO2's densities and vapour pressures, there isn't any "air consumed".

What happens is air expands due to heat, with part of the air escaping from underneath the glass to equalize pressure outside and inside the glass. Subsequently, when the air cools and contract, pressure inside the glass drops and hence creates a partial-vaccum, which is why water rushes in to equalize the pressure

K K - 6 years, 10 months ago

Nice explanation.

Josh Silverman Staff - 6 years, 10 months ago

This would be a problem if the LAW OF CONSERVATION OF MASS is accounted for. Combustion your honor is a CHEMICAL REACTION and it follows this rule. What have you done? You annihilated the MASS to turn it into energy? E=MC2?

Sam Tumlad - 6 years, 9 months ago

Sam Tumlad
Aug 31, 2014

Simple Gay-Lussac's Law. :)

Let us assume that the VOLUME of gas inside the glass is constant. Under this gas law it states that under constant volume PRESSURE and TEMPERATURE are directly proportional. So when Uncle McCool here starts a flame it elevated the temperature of those particles and therefore; increased the pressure. As he placed the glass on the plate combustion ceased and the temperature dropped. As it did the pressure also dropped to account for the loss of pressure water is sucked inside the glass.

Fire sponge party trick

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