There are many incorrect explanations for this experiment. In order to understand what is really going on you need to know what chemistry is taking place. The combustion reaction occuring is:2O2 + CH4 → Co2 + 2H2O

, Charles’s Law is a piece of the ideal gas law that says that the volume of a gas will change proportionally with temperature (and vice versa) when outside pressure and the mass of the gas remain constant. Since the walls of the glass do not change the amount of pressure that they exert on the gas inside, the pressure in this experiment remains constant. The gas cannot enter the glass because it is upside down in the water, creating a seal.


Technically, since the combustion reaction converts two O2 molecules into one CO2 and two H2O molecules, the amount of gas temporarily increases. However, the candle will not keep the temperature inside the glass above 100 degrees Celsius (212 degrees Fahrenheit) and the two H2O molecules will condense into liquid water. This means the amount of gas in the glass effectively decreases ‘’’while the candle burns’’’.Charles’s Law applies in this case only because the amount of gas in the glass remains constant from the time that the candle burns out. Without the combustion, no more gas is consumed or produced. The heat source simultaneously disappears, allowing the gas to cool and decrease in volume., Since the volume of your gas is proportional to the temperature, it decreases as the gas cools. This decrease in volume creates a vacuum inside the glass. The pressure outside the glass remains the same, so this creates a pressure differential., Within any given system, fluids move from the high pressure end to the low pressure end. With air sealed out of the glass, the water is forced from the high pressure end of the system (outside the glass) to the low pressure end of the system (inside the glass) until the pressure inside the glass is equal to the pressure outside the glass.This is a little like opening a faucet. The pressure outside the water line is lower than the pressure inside the line, so the water is forced through the line and out of the faucet (even if it has to go against gravity to do so).
Know what chemical reaction is taking place.
Recall Charles’s Law.
Consider what happens when the gas cools.
Understand pressure differentials.
{"src": "wikihow-all_validation", "title": "How to Make Water Rise3"}