Can we Boil Water in a Paper Cup?

The Idea

Is it possible to boil water over a flame in a paper cup? This project lets you find out why this is possible.

What You Need

• 2 paper cups—most "paper" cups have a thin coating of wax, which can still be used, but if you can get them, uncoated cups are preferable
• water
• flame—a match or a Bunsen burner
• thermometer or digital temperature sensor
• Styrofoam cup
• sand (enough to partially fill a paper cup)
• water balloon
• paper bag

Method

1. Fill the paper cup nearly to the top with water.
2. Hold the cup over the flame.
3. Continue doing this until either the paper burns or the water boils. Optional: measure the temperature as it is heating up.
4. Fill the second paper cup with sand.
5. Hold this cup over the flame and observe the effect of the flame on the cup.
6. Fill the Styrofoam cup nearly to the top with water.
7. Hold this cup of the flame and observe the effect of the flame on the cup, as shown in Figure 93-1.

Expected Results

The water will boil in the paper cup. If the cup is coated with wax, the wax may melt, especially above the waterline. If there is a circular rim on the bottom, it may burn without burning through the cup. The paper cup filled with sand will char, but it won't necessarily burst into flames. The Styrofoam will melt and, where the flame is applied, possibly leave a hole in the side of the cup.

Why It Works

When heat is added to water, its temperature increases until it reaches the boiling point of water at 100°C. The paper doesn't burn because heat is conducted away from the paper before it can reach its kindling point (the temperature where it begins to burn). Paper begins to burn at around 233°C (which is close to the nominal value 451°F for paper, made famous in Ray Bradbury's novel Fahrenheit 451). The water temperature can increase until it boils and still remain well below the kindling temperature of paper.

Sand conducts heat away from the paper. However, unlike the paper, sand does not undergo a phase changes as water does at its boiling point. The temperature increases above 100°C. This is why we see charring in the paper cup containing sand.

The Styrofoam is an insulator. As a result, the water does not conduct heat away from the Styrofoam cup as it does with the paper cup, which conducts heat much more readily. This explains why the flame melts the Styrofoam.

Other Things to Try

An alternative approach is to wrap a piece of paper around a metal pipe and note its response to a flame. In a similar manner, the metal pipe conducts heat away from the paper before it can start to burn.

Fill the paper bag with water and hold it over the flame. The water conducts heat away from the paper at a fast enough rate to keep it from burning.

The Point

Phase changes in matter, such as the transition from liquid to vapor, take place at a constant temperature called the boiling point. A liquid cannot exceed the boiling point until all the liquid has evaporated. Materials such as sand conduct heat much better than air. Some materials such as Styrofoam are much better insulators than other materials, such as paper.

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