By Kevin Leland
If you ask anyone if they know how to boil water in a paper cup, supposing it’s a brain teaser, the quick thinking person will answer “yeah, in a microwave.” To this response you should answer, “No. Without using a microwave, could you do it?” How about over a Bunsen burner with the flame directly on the paper cup filled with water?
Most people deduce that this would be impossible by putting the cup of water over a flame until it starts to boil. Logically, because everyone has witnessed what happens to paper when you put a flame to it, they think that a cup will catch fire if put over a flame.
The truth is, it WILL NOT! A paper cup filled with water and put over a flame can’t burn. The water will boil, and as long as it is kept over the flame it will continue to boil until all the water turns to steam…And then the cup will ignite in flames. How is this possible?
The simple explanation is that the water keeps the cup cool, even the boiling water. Because paper doesn’t catch fire until around 500 degrees Fahrenheit, and water boils at 212 degrees fahrenheit, the paper will keep transferring the heat to the water as it boils. What happens if the boiling water gets hotter and hotter and isn’t able to keep the cup cool enough so that it doesn’t burn?
That can’t happen. Enthalpy of transformation is the fancy explanation for why it is impossible for boiling water at atmosphreric pressure of 14.7 PSI to get any hotter than 100 degrees Celsius or 212 degrees Fahrenheit. It’s the same case with melting ice. As an ice cube is heated past the point of freezing, 32 degrees farenheight, no matter how much heat is added, the ice cube will stay at exactly 32 degrees farenheight, until it is completely transformed from a solid to a liquid. The two enthalpies are:
1. Solid to a liquid- Latent heat of fusion: Melting
2. Liquid to a gas- Latent heat of vaporization: Boiling
Definition of terms:
Thermodynamics – is the science of energy conversion involving heat and other forms of energy, most notably mechanical work. It studies and interrelates the macroscopic variables, such as temperature, volume and pressure, which describe physical, thermodynamic systems.
Enthalpy of transformation – A thermodynamic function of a system, equivalent to the sum of the internal energy of the system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.
The expression latent heat refers to the amount of energy In physics, energy absorbed by a chemical substance during a change of state that occurs without changing its temperature, meaning a phase transition such as the melting of ice or the boiling of water. The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden.
Don’t ever bet anyone they can’t boil water in a paper cup!
Watch it done: Demonstration of Latent Heat of Vaporization
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