Getting the facts right

Oliver Knill, 9/24/2006 (last edit: january 2014)

Besides pedagogy or psychology, it can be also relevant just to get the facts right. The Water-Candle experiment is an illustrative example. It is a situation where many different effects play together and where it is hard to figure out which ones really matter. My own perspective about this experiment has shifted several times and comments of some who wrote me added valuable insight. Please look also towards the end of this page where some interesting links are added and information like why the great Lavoisier himself replaced the experiment since it was too subtle.

Experiment: Cover a burning candle with a pitcher so that the candle is in an air-tight room sealed by the water at the ground. Observations: After some time, the candle dims and goes out. Just before the candle dies, the water level rises to almost 1/10 th of pitcher height. No air bubbles are seen. The water level stays up for many few minutes more.
The chemical aspect: oxygen O2 and paraffin Cn H2n+2 react. The burning produces water H 2O and carbon dioxide C O 2. For n=1 we balance the equation as follows:
2 O2 + C H 4 = C O 2 + 2 H 2 O
Because twice as much oxygen is burned than carbon dioxide released, the air volume decreases.
The physical aspect: the candle heats the air and expands it. This cancels the depletion of the oxygen temporarily and the water level stays down. When the oxygen is depleted, the candle goes out and the air cools. The volume of the air decreases and the water rises. The temporary temperature change delays the rise of the water. As several readers have pointed out, also the water condensation should be mentioned. While water is initially gas, it condenses and helps to delay the effect.
Summary: There are two different effects. Both a chemical and a physical reasoning are needed to explain what we can see. Both physics and chemistry matter. The initial cancellation effect can confuse the observer. Mathematics plays a role when the chemical equations are balanced.

Photos of the experiment

Photos: Oliver Knill, September 19, 2006.

An exhibit of explanations

Many explanations on the web contradict each other (September 2006). Actually, the primary hits in a search engine all lead to pages, which difficult. Here are some pitfalls: