For a reaction to take place three ingredients are required: oxygen, fuel, and heat. In the case of wax melted down, only the top surface has access to Oxygen, so the fire progresses slowly. But when water is added to the wax, two things happen.
1) The water being more dense, sinks to the bottom of the container.
2) The water instantly vapourises. (Liquid to gas causes a 1000 fold increase in volume.
The water expands violently, and throws the hot wax layer above it into the air as small droplets. The wax now has a much bigger surface area exposed to oxygen so combustion takes place very quickly.
Do not try this, be careful....
For similar reasons - Water should never be used to extinguish burning grease or fat.
One should instead use BAKING SODA to extinguish a wax fire.

source - wikipedia.
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