So we were at a fancy dinner party the other night, and so naturally that was a perfect moment for kitchen chemistry. You may know the rule put down by my overbearing spouse — no chemistry in the kitchen. Which is what leaves me no other outlet but my friend’s kitchens or at fancy company dinner parties.
We took a burning candle and placed it on a saucer full of water. It was a mostly burnt out votive candle, so it floated nicely. I then put an inverted water glass over the candle. The flame went out, and water was drawn up into the glass.
This is a fun thing to do any time you are around candles and water glasses.
Here is what’s going on: Wax is a hydrocarbon — which means it is made of polymers composed of two hydrogen atoms and one carbon atom. Well, that isn’t entirely true — it is approximately true. For example, octane is a molecule that is a hydrocarbon with a formula C8H18. Dodecane has the formula C12H26. As the molecules get larger, they approach the 2:1 ratio. So to simplify our stoichiometry we’ll say that hydrocarbons are basically CH2. Molecular oxygen is O2. And this is how a hydrocarbon burns:
All off those molecules are gas (water is a gas at the temperature of combustion). So why does the water level go up? Two reasons: 1) The water vapor will condense and 2) The carbon dioxide may dissolve in the water. Thus the net reaction is consuming more gas than it produces, reducing the pressure inside the glass. Water is pulled up into the glass by the greater pressure exerted from the outside atmospheric pressure.
10 replies on “The old candle in the glass trick”
“This is a fun thing to do any time you are around candles and water glasses.”
Uh … yeah. As you well know, Andy, the rule at my house is a little looser than Becka’s: no biology in the kitchen (chemistry, physics, etc. — okay). Perhaps I will have to reconsider.
Thanks for the nifty explanation, though.
i thoroughly and completely deny the description “overbearing” applies to me.
One of the self-quizzes at the end of a chapter in my biology book had a question like this:
CH2 + 2 O2 –> where you were supposed to know what it turned into. How do you know that CH2 + 2 O2 –> CO2 + H2O? I don’t understand chemistry. Probably never will, no matter how hard I try.
Oh, one clarification. The wax hydrocarbon is not a gas. And it probably would be better to designate it as N(CH2) where N is the size of the polymer (for wax, N is probably 20 or so).
When does “overbearing” really mean “cautious due to personal experience?”
Don’t worry, Andy, soon, I promise, we can do science in my kitchen. My first lab requires a microscope on campus, but after that….
Kitchen chemisty is only allowed in my house when it creates a good dessert………so the rule must have something to do with genetics……….though the parent genetic medium seems to have less of an aversion to it than the subsequent offspring
my kitchen has occasionally looked like an abatoir and a mad scientist’s laboratory. i only disallow things that produce toxic smoke and products that make the dog sick.
said spouse and spouse’s sister are responsible for their own disasters in my kitchen as well, even if they weren’t “official” science experiments.
there has been a great deal of debate as to whether cooking qualifies as science. andy is allowed to cook, but sometimes his products do lean to the science experiment realm. this whole rule stems from a particular incident in which i found petrie dishes incubating something on top of the butter dish in my fridge.
I assume that it wasn’t a new kind of blue cheese…………..
I need to see this one for myself, but probably not at home!