Can we boil water in a paper cup?

Classical Mechanics Level 1

When we place a paper cup on an open flame, it immediately catches fire and burns.

Is it possible to boil water in a paper cup on an open flame?

Yes No

5 solutions

Rohit Gupta
Apr 7, 2017

When an empty paper cup is put on a burner, it receives a lot of energy from it but can not pass the same to the surroundings as the air is a bad conductor of heat. Due to this, the energy of the paper cup increases rapidly and so thus its temperature. The paper cup catches fire as soon as its temperature is increased beyond its ignition temperature which is around $233 ^\circ C$ .

Now, when we fill the cup with water and put it on the burner, the paper being very thin, conducts most of the energy directly to the water. Water is a good conductor of heat and thus it absorbs the heat rapidly and warms up.

Water boils at $100 ^\circ C$ which is much smaller than the ignition temperature of the paper. Also, during the phase change from the liquid state to vapors, the temperature of the water remains fixed at its boiling point. Water boils and does not allow the temperature to rise beyond $100 ^\circ C$ and hence the paper cup does not catch fire.

Brian Charlesworth
Mar 29, 2017

Seeing is believing .

Is it possible to boil oil in it?

Mehdi K. - 4 years, 2 months ago

Oil isn't as good a conductor of heat as water, and it's boiling temperature is much higher, (about 300 C, depending on its type and purity). The flashpoint of paper is only 233 C, so based on these facts I would would expect the paper cup to ignite, (or at least disintegrate), prior to the oil boiling.

Brian Charlesworth - 4 years, 2 months ago

To solve this we need to know the thickness of the paper, the volume of the water, the area of the bottom of the cup, the flash point of the paper, boiling point of the water, temperature of the flame, distance from the flame, and the heat transfer rate of the paper. It is true that water is a great (perhaps the best naturally occurring) conductor of heat, and so it should be very good at pulling the heat from the paper, but the cup must first xfer the heat from the outside surface to its inside surface. Since it is far less efficient at doing this than the water is, the outer edge of the cup will heat at a more accelerated rate than the inside edge. Depending upon the thickness of the paper, there must be conditions in which either outcome is possible. So we could use a mathematical representation of these values as they pertain to this cup to explain both why the given answer applies and which conditions must be altered to get the opposite outcome.

Jonathan Fitch - 4 years, 1 month ago

Robert Gallenberger
May 2, 2017

If in doubt, take the cup of water to the top of Mt. Everest where the boiling point of water is about 72C.

Matthew Grier
Apr 26, 2017

If you hold it above the flame, but not so close that it burns, you could boil water.

A Former Brilliant Member
Apr 17, 2017

Since paper cup conduct heat to some extent it allow heat flow from hotter fire to colder water until both attains common temp I think. Water is not conductor of heat as ice fully immersed and kept there by some means it does not melt although water starts to boil.(convection involved) Heat is constantly transferred to colder water at bottom. Hence it absorbs heat and starts to boil. Thus paper doesn't catch fire until water is wetting it! I think

Can we boil water in a paper cup?

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