Cup on fire

Classical Mechanics Level pending

You have a paper cup filled with water and you heat the bottom with the flame from a bunsen burner. What happens to your system?

Assumptions

We are thinking about the period in which the cup still contains water (it hasn't all been lost due to evaporation, or some other process).

The water boils and the cup stays intact. The water heats up a little until the cup catches fire. The water absorbs all the heat and the flame goes out. The water stays at the same temperature and the cup catches fire and the water falls out.

11 solutions

Shamik Banerjee
Mar 6, 2014

This is due to the reason that he heat given by the flame is quickly transferred from the paper cup to the water. As a result, the temperature of the paper does not reach the ignition temperature and hence is not burned. The heat gained by water raises its temperature and it will eventually start boiling.

But how do we know for sure whether the cup has sufficient heat-conducting capacity, so that it itself does not catch fire before water reaches its boiling point?

Vaishnavi Gupta - 7 years, 3 months ago

Its due to the fact that water prevents it from reaching ignition temp. Remember its a paper cup so it will be saturated with water. Its not a plastic cup.

Bala Tweakbytes - 7 years, 3 months ago

why are we assuming that the ppr cup will be moist(or saturated with water to say)

Bhupendra Dewangan - 5 years, 9 months ago

The paper cup absorbs the molecules of water, which take up the heat of the flame and start boiling. The molecules of paper remain untouched.

Debeshwar Chand - 7 years, 3 months ago

In case of paper conduction it is Directly proportionate with thickness . more of thickness have much resistance in transmiting heat on other side. But as thickness is much low as of 40 micron so it is relatively much better conduction . Therefore Heat is transferd to water imidiatly..so the temperature of paper does not exeeds 100 degree celcius.

Gorakh Prasad - 7 years, 2 months ago

it depends the rate of heat transfer.. if heat is transferred to quickly, the cup will reach ignition temperature , without distributing the temperature inside. if slowly heated it will not burn the system will distribute the heat not allowing the cup to reach ignition temp. which is obviously greater than boiling temperature.

Prakhar Gupta - 7 years, 3 months ago

BUT how could the water boils before the paper get burned?

Ahmed Amiry - 7 years, 3 months ago

Boiling fixes the temparature at 100 degrees Celsius.Much lower than the ignition temparature of paper .Boiling actually causes cooling preventing the temparature from rising above 100 degrees Celsius.

Amit Chopra - 7 years, 3 months ago

Check out this demonstration of the phenomena:

IMAGE ALT TEXT HERE

The water draws heat away from the paper until the water reaches $100\mbox{ deg C}$ while the paper doesn't burn until $\sim 200+\mbox{ deg C}$ . Why doesn't the paper heat to its combustion temperature after the water starts to boil (after it has already reached $100\mbox{ deg C}$ )?

Josh Silverman Staff - 7 years, 3 months ago

Why don't you use a smaller paper cup and a bigger flame?

Thích Thảo - 7 years, 3 months ago

This was the best video I could find on YouTube. If I can get my housemates to let me start trying this experiment at home, I can try with a smaller cup/bigger flame, definitely.

Josh Silverman Staff - 7 years, 2 months ago

The paper cup will instantly burn if placed on stove and not on Lab Lamp; the temperature of fire generated by burning lab fuel which is most often spirit is way too less than the temperature of stove fire.

Fred Burger - 5 years, 10 months ago

After it reaches 100*c the water becomes paper's cooler

Romaans Tomcenko - 5 years, 9 months ago

I actually dont go with this.. as it cant be said surely that cup will stay intact till water gains its temperature high enuf to boil... before that cup will start burning as cup is a bad conductor of heat and till water gains its boiling temperature cup will start burning.

Pritish Plawan Paul - 7 years, 3 months ago

but how is the heat quickly tranferred?is the material of cup a good conductor of heat??

Souradeep Roy Choudhury - 7 years, 3 months ago

Umm, once the water boils the cup will burn. Hence, it's none of the answers.

Cody Johnson - 7 years, 3 months ago

That's true in the long run. I'll change the wording to disallow that possibility.

Josh Silverman Staff - 7 years, 3 months ago

happens in prisons

Mostafa Ahmed - 7 years, 3 months ago

Care to expand?

Josh Silverman Staff - 7 years, 3 months ago

actually at boiling temperature, the cup cannot stay intact.... at some 60-65 degree C only the cup starts burning...

Anuj Modi - 7 years, 3 months ago

Have you watched the video demonstration I posted?

Josh Silverman Staff - 7 years, 2 months ago

water have specific heat as 1. so all temperature transmitted to water as soon the paper cup receives heat from flame. in such transaction paper temprature remains remains equal to water temperature. therefore it stays upto boiling . even after boiling temperature the water emits latent heat in vapor so paper container remains as of boiling temperature..

Gorakh Prasad - 7 years, 3 months ago

Maybe it's because the paper cup get drenched and that water in it surface prevent it from burning, otherwise i don't know how the cup can stay intact

Izízero Carvalho - 7 years, 3 months ago

People that are questioning, just get a paper cup filled with water and hold it on a flame...

Artur Zanon - 7 years, 3 months ago

Water will be evaporating while it is heated by the flame, so the level of the water decrease. Then the paper will catch fire and start burning a bit....

Thích Thảo - 7 years, 3 months ago

The kindling point or auto ignition point of paper (dry) is 218 to 246 deg C,and in this case, the paper cup is saturated with water that makes the temperature even higher than 246 deg C. Compare this with 100 deg C as the boiling point of water. The answer is right, the water will boil , and evaporates all water content until the cup is practically empty before the paper cup will starts burning!

Joselito Panis - 7 years, 3 months ago

i thought that boiling temperature should be higher than the ignition temp of the paper cup. so i chose the option water boils until the cup catches fire.

Manan Trivedi - 7 years, 3 months ago

Is there live demonstration of this thing ?

Beyond Zero - 3 years, 2 months ago

Vinay Sheoran
Mar 8, 2014

water won't let paper heat over it's boiling temperature, which is well below paper burning temperature. However if the paper is thick enough it will burn.

Robear Wagih
Mar 8, 2014

The heat is given to the paper cup by fire. The heat is lost from the paper cup by water. The cup doesn't have enough heat to burn. The water absorbs enough heat to boil. TADAAAAAAA

This trik Will only work using a small candle or a tea-light... a bunsen burner will make the paper burn inmediatly even if the cup is fullfilled with ice... due To The high temperature of The butane flame. ...

Nicholas Vara - 3 years, 2 months ago

Izaz Ali Khan
Mar 8, 2014

Water has the highest HEAT CAPACITY of 4.2 J/g.C. It has the capacity to absorb a large amount of heat.It undergoes the temperature rises very much slowly.Due to highest heat capacity The water absorbs all heat and the cup remains in safe and sound condition as long as the water is there.

Similar is the case with heating water in balloon!

Good answer from Wiki!

Joselito Panis - 7 years, 2 months ago

Srinjoy Ghosh
Mar 10, 2016

The heat given by the flame is absorbed by the water so, the temperature remains stable and does not remains the ignition point of the paper and it is not burned.While the water's temperature rises and starts boiling.

Hadia Qadir
Jul 28, 2015

A body can absorb heat without change in its temperature whenever (A )it is melting( B) it is boiling (C) both A and B(C )it is evaporating

Dibyasingh Rautaray
Mar 13, 2014

Before the cup reaches to ignition temperature the heat energy transfer to the water and the water starts boiling. once all the water get evaporated then the cup start burning.

Valentin Bogatu
Mar 13, 2014

Paper burns at 500 degrees Fahrenheit, water boils at only 212 degrees Fahrenheit. When you heat the paper, it does not reach 500 degrees F (because the heat is transferred to the water) before water boils inside. The paper will not burn, instead the water inside the cup will boil.

Saurav Roy
Mar 13, 2014

since we are heating the entire system the paper that is in contact with the fire doesn't catch fire and since that the given condition is that if water is already there then the heat generated will be taken by water and thus it will prevent the paper from boiling. hence since the water absorbs the heat thus then the paper doesn't catch fire................................

Tunk-Fey Ariawan
Mar 8, 2014

It should be when the water reaches its boiling point, the cup cannot stay intact. You can try a similar experiment by using plastic bag filled by water.

$\text{\# }\mathbb{Q.E.D.}\text{ \#}$

Shin Satomi
Mar 8, 2014

The reason behind the paper cup can handle the heat and boil the water inside it was this. By using a paper cup with a flat bottom (no edge ring) with water on it. Placing it on the wire mesh over the flame of the Bunsen burner . The water inside the cup will start to heat due to the temperature of the flame. As a result the heat well make the water boil but the paper cup will not ignite. Unless it will stay for 233° C on where the paper cup will ignite in flame.

Cup on fire

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