Question

A paper cup placed among hot coals will burn if empty (note burn spots at top of cup in Fig. 14–14), but won’t burn if filled with water. Explain. Forget the marshmallows.

FIGURE 14-14 Question 29.

Short answer

In the case of an empty cup, the self-ignition temperature is reached easily and it will start burning easily. When the cup is filled with water, it will achieve its ignition temperature after a long time, and it will not burn.

Step-by-step solution

Understanding the ignition temperature of the paper cup

The ignition temperature is the smallest temperature of the cup. The cup is heated to initiate the self-combustion process of the cup without any burning or heating source.It is the minimum temperature of the cup before it starts burning. It is also known as the autoignition or self-ignition temperature of the paper cup.

Explanation for why the empty paper cup will burn easily but will not burn when it is filled with water

The water will absorb more heat energy since the specific heat capacity of water is very large. The heat added to the paper cup is transferred to the water by the conduction process.

Due to the presence of water in the cup, the cup's ignition temperature is not reached easily, and it will take time to attain that temperature.So, the cup has a lower temperature, and it will not burn.

In the empty cup, the ignition temperature of the cup is reached in a very small amount of time. Moreover, the empty paper cup’s temperature will increase very quickly, and it will burn easily.