Passage VI

A science teacher lights a new wax candle (which, if left undisturbed, would burn for several hours) in her classroom. She then shows her students a glass jar and asks them to predict what will happen if she places the jar over the lit candle, completely sealing off the candle from the surrounding atmosphere (see figure).

Four students give their viewpoints about why the candle burns and about what will happen to the burning candle once it is under the jar.

Student 1

The candle's flame results from a combustion reaction. When the candle is lit, the flame that lights the candle heats the candle, which causes the candle to give off fumes of wax. The wax fumes act as a fuel that reacts with the O₂ present in the air. The reaction gives off energy, which heats the surrounding air, producing a flame. If the jar is laced over the lit candle, the candle will soon stop burning because the O₂ , in the air under the jar will be quickly used up.

Student 2

Student 1 is correct except that the fuel reacts with the N₂, not with the O₂, in the air.

Student 3

None of the components of air are involved in the burning of the candle. When the he candle is lit, the energy from the flame causes the fuel (candle wax fumes) to spontaneously decompose. The heat from the decomposition reaction causes the reaction to continue. Thus, if the jar is placed over the lit candle, the candle will continue to burn until all of the fuel is used up.

Student 4

When the candle is lit, the energy from the flame causes the fuel (candle wax fumes) to spontaneously decompose into a massless substance called heat. None of the components of air chemically react with the fuel, but the O₂ present in air is necessary for the sustained decomposition reaction. The O₂ is needed because as the heat is produced, the heat must be absorbed for the reaction to continue, and O₂ is the only component of air that absorbs heat. When O₂ absorbs heat, the O₂, is not chemically altered. If the jar is placed over the lit candle, the candle will soon stop burning because the O₂, in the air under the jar will quickly become saturated with heat.