Water Flowing Uphill

DESCRIPTION:
A candle is placed in a crystallizing dish containing water and lit. A beaker is placed upside-down over the candle. The candle will eventually go out and the water level in the beaker will rise.

As the candle burns it consumes some of the oxygen inside the beaker, and it also heats up the gases inside the beaker. When the flame goes out (due to decreased levels of oxygen), the pressure inside the beaker decreases. This causes the water level inside the beaker to rise which equalizes the pressure between the inside and outside of the beaker.

The decreased pressure after the candle is extinguished can be primarily explained by two factors:

1. When the flame goes out, the air cools and contracts, lowering the pressure inside the beaker.
2. As the candle burns, oxygen gas is consumed during the combustion of the candle wax (a mixture of hydrocarbons), producing carbon dioxide (CO₂) gas and water (H₂O). A larger number of moles of O₂ are consumed than the number of moles of CO_2­ produced, thus there are fewer gas molecules inside the beaker after the combustion.  This is evident from the chemical equation of the combustion reaction, where (x + y/4) moles of O₂ are consumed, but only x moles of CO­₂ are produced:

C_xH_{y­ (wax)} + (x +y/4)O_{2 (g)} → x CO_{2 (g)} + (y/2)H₂O _(l)

TOPICS COVERED:
– combustion
– Boyle’s Law
– Charles’ Law
– light

MATERIALS NEEDED:
– crystallizing dish, with water in it
– candle
– match
– tall, skinny beaker

PROCEDURE:
1. Place the candle in the crystallizing dish and light it
2. Place the beaker over the candle

ADDITIONAL COMMENTS:
The candle will likely get wet when the beaker is removed, and may not re-light right away since the wick will be wet.

SAFETY:
Caution should be taken with matches. Safety goggles should be worn at all times.

REFERENCES:
Shakhashiri, B.Z. Chemical Demonstrations; University of Wisconsin Press: Madison, 1985; Vol. 2, pp 158 – 161.

“Magic Candle in the Bottle.” Elmhurst College: Demonstrations. Virtual Chembook, Elmhurst College, Accessed 28 Jul 2010. http://www.elmhurst.edu/~chm/demos/MagicCandle.html 

STORY:
Ask students if they can think of a way to make water flow up.

Handout