Why Styrofoam Cups Won't Burn With Water

It is possible to use a small flame to boil water in a styrofoam cup without burning or melting the cup. The heat is absorbed by the water before the styrofoam gets hot enough to melt. If a flame is too hot, however, the cup will melt; this trick works better with a paper cup.

Specific Heat

Specific heat, or heat capacity, describes the amount of energy which must be put into the substance to increase its temperature. Water has a relatively high specific heat--it takes one calorie, or 4.2 joules, to increase the temperature of one gram of water by one degree Celsius. The same amount of energy would heat up a piece of metal much faster. Because water has a high specific heat, it is able to quickly absorb the heat of the flame and keep the styrofoam cool.

Thermal Conductivity

Thermal conductivity is a measure of how quickly a heat difference transfers from one side of an object to the other. Styrofoam has a very low thermal conductivity, which is why it is used to insulate coolers and coffee cups. Paper has a higher thermal conductivity, which is why it is even easier to boil water in a paper cup without burning it.

Boiling Point

Water boils at 100°C or 212°F at normal air pressure at sea level. The melting point of styrofoam varies depending on the exact type, but it is usually around 250°F or higher. As long as the flame is small enough that the heat has time to be transfer from the styrofoam to the water, the water will boil before the cup melts.

References

• Engineering toolbox: Thermal Conductivity of some common materials
• Davidson College: Calorimetry