Why is hot wax thrown into cold water a dangerous combination?

[quasi426]

Hi, recently I found something out by playing around with candles. Basically I heated some candle wax until it was completely out of its solid state (at least through the first approximation of my eye.) Then I continued to heat the wax until the liquid wax ignited and started to burn. Usually the wick on the candle is only burning wax vapor. Then I proceeded to dump the burning hot candle wax into cold water to my surprise, a very exothermic process took place (basically a mini explosion.) Can anyone explain the chemical reason behind this. My hypthesis so far is that wax is hyrophobic and does not like to interact with water. But when it is so hot and the water is so cold, heat must travel from hot to cold and this forces the wax to interact with the water. When the heat transfer is over, then the wax wants to leave away from the water very quickly and releases some energy as it does this.

 

[DaveC426913]

I don't suppose it's as simple as the very hot wax vapourizing the water it comes into contact with?

 

[quasi426]

I thought that is what it might be initially, but there was a mini explosion of flames. So if I were to pour some cold water in a steel pot at the same temperature, the water would vaporize, but large flame would occur.

 

[DaveC426913]

Ooh!

I read the experiment over again. Now I understand.

Remember, combustion requires exactly 3 ingredients: fuel, oxygen and heat. Without anyone of them, a fire stops. The corollary is that if you provide two of those ingredients in abundance, but not the third, no combustion.

So, you've provided wax for fuel and added lots of heat, but the only place the wax mixes with oxygen is at the wax/air interface. You get surface burning, but that's all.

Then you added water. The water turned instantly to vapour, expanding and expelling itself and lots of wax in a cloud of very small droplets (this would probably be called aerosolization). Now you have lots of super-heated wax, rapidly interfacing with lots of oxygen over a huge surface area (the wax envelope around every tiny steam bubble) - suddenly, you have all three ingredients for combustion in ample supply.

BOOM!

 

[quasi426]

Interesting. Thanks for the explanation.

 

[Moonbear]

Congratulations! You've discovered the principle behind why one should never attempt to extinguish a grease fire with water. What you did is exactly what my high school chemistry teacher did to demonstrate to us this very same thing (there are probably rules against doing things like that in schools now). DaveC has given the appropriate explanation. Pretty amazing to watch, isn't it? You're lucky you didn't burn yourself!

 

[quasi426]

Yes I was quiet lucky not to get burned. Especially since I had no idea such an exothermic reaction was going to take place. I was just curious to ignite the liquid wax once I found out that the fuel used by the wick was actually the wax vaper. Nevertheless it was definately fun and exiting (maybe a little dumb.)

 

[Ouabache]

Congratulations! You've discovered the principle behind why one should never attempt to extinguish a grease fire with water. What you did is exactly what my high school chemistry teacher did to demonstrate to us this very same thing (there are probably rules against doing things like that in schools now). Pretty amazing to watch, isn't it?

Your and my high school chemistry teachers must have graduated from the same college. Mine also used dramatic demonstrations, like dropping pure Potassium and Sodium, in turn into water, and watch what happens. If I recall correctly, both undergo spontaneous combustion, K bursts into flame and Na just explodes What lesson did we learn?? (Besides chemistry can be exciting). Elements in the same family, (in this case 1A - alkali metals) have similar properties. (Don't try this at home, **both these demonstrations were done outdoors, with observers standing far away**)

Relating back to candle-wax thread, I have attempted making my own (blocks of paraffin, tinting, scenting, various molds) but never got paraffin as hot as quasi's (just heated over water bath). The trickiest bit is how much scent to add, and when to add it.. If you add it too early, most will vaporise and very little scent will be left in the candle.

 

[Francis M]

Congratulations! You've discovered the principle behind why one should never attempt to extinguish a grease fire with water. What you did is exactly what my high school chemistry teacher did to demonstrate to us this very same thing (there are probably rules against doing things like that in schools now). DaveC has given the appropriate explanation. Pretty amazing to watch, isn't it? You're lucky you didn't burn yourself!

I thought the main reason you didn't put out a grease fire with water was because most people use a stream of water and it just splashed the grease all over the place, so instead of having one isolated fire you have several smaller ones spread over a larger area