A physics question ....?

Question

my little brother did the experiment about the candle that stops burning if u close on it with some tube ( the candle is standing in a vase of water), after a while the candle stops burning and the water level rises a little. So the oxygen is consumed by the candle but why the water level rises???

Answer 1

You are on the right track by stating that the candle consumed the oxygen or air. Well oxygen has mass and a volume, so when that volume is consumed it must be replaced by something else (Law of Energy Conservation, you can't destroy the energy). Since water is slightly expandable, it then rises to replace the consumed air's volume. (Now there is less air, so the water can move up to take it's place). This all boils down to a pressure question. You have a change in pressure inside the jar, which causes the water to rise.

Answer 2

Burning the candle in the jar removes oxygen, but produces carbon dioxide and water. The water is gaseous, but when it comes into contact with the cold water it condenses causing a rise in water level.

Another factor is air pressure, removing the oxygen and fixing it into water decreases the pressure inside the jar, hence there is less air pressing down on the water surface, allowing it to expand slightly, causing the water to rise.

Answer 3

This has nothing to do with conservation of energy or "expandable" water. It's about replacement of some gaseous O2 molecules with molecules of combustion products and what happens to those products. Let's refer to the number of moles of gas in the container as N, initially = Ni.
Some of the atmospheric O2 participates in combustion, leaving in its place H2O and CO2, and only trace amounts of other oxygen-containing compounds like CO and organics. The ratio of product molecules to O2 molecules used in their production is 2 for H2O and 1 for CO2, so depending on the proportions of H and C in the wax, there are between 1 and 2 times the number of participating O2 molecules existing as gaseous product, so N > Ni. We know there is a significant proportion of H in wax and therefore of H2O after combustion, almost all of which condenses. This leaves a gas containing fewer CO2 molecules than there were participating O2 molecules. So condensation alone reduces N to less than Ni. In addition a significant amount of the CO2 dissolves in the water resulting in a still greater reduction in N.
About the water rising: At constant temperature T, the water level always is at an equilibrium point that satisfies the gas law PV = NrT (r being a constant, P pressure, V volume, N moles of gas and T temperature). If N decreases in a container that is closed at the top and has a liquid column underneath, P*V decreases, so the water level will rise until a new equilibrium between volume V and pressure P is reached. In other words, the water level naturally rises if N decreases.

Answer 4

The heated gas cools down and contracts thus leading to the water rising.