I know you're going to say that you already know this experiment.
I know you do!
But I'd bet you that the explanation you believe to be right
is at least partially wrong.
Want to bet on it?
The experiment consists in putting a candle under a glass.
After a few seconds, the candle goes out and the level of the water rises.
Here are the explanations we commonly hear:
1- "The candle goes out when it has used up all of the oxygen that was kept under the glass."
2- "The water comes to replace the space the oxygen used to take up."
It's no wonder we all believe them to be right, considering that they're being repeated all over the Internet.
Today, we'll demonstrate why those conclusions are false,
and which conclusions are right.
Let's start off with the reason why the flame goes off.
Hyp. n°1: "The candle goes out because all of the oxygen has been used up."
Let's assume this is true. When I place two candles under the glass,
they indeed go out before the one candle does.
Now, let's see what happens with 2 candles of a different length.
The taller candle goes off BEFORE the shorter one.
How would you explain that the tall candle has consumed all of the oxygen if the short candle is still burning...?
Well, you wouldn't. Because it hasn't.
Remember the 3 indispensable elements to make a fire: fuel, oxygen and a source of heat.
The candle goes out when one of those three elements is lacking.
Here, it's the inability to access the oxygen that puts it out.
I did say "inability to access it"; that's different from it being all used up.
Let me explain.
This is what happens when the candle burns:
oxygen + wax => carbon dioxide + vapor + heat + light.
The fact that the taller candle goes out first gives us a hint on what happens.
The candle produces CO2. It is hot and therefore travels toward the upper part of the under-glass area.
This forces the oxygen to retreat toward the lower part of the area.
But when in contact with the glass, the CO2 molecules cool down
and start travelling back downwards until it keeps the oxygen from reaching the flame.
You can reproduce this phenomenon by mixing baking soda with vinegar, which together produce CO2.
When you "pour" the gas onto the flame, it goes out even though there still is oxygen in the room.
The candle does not go out because it has used up all of the oxygen,
but rather because the CO2 keeps the oxygen from reaching it.
Now let's investigate why the water level rises.
Hyp. n°2: "The water rises because it has to replace the empty space left by the used-up O2."
Let's assume this is true. Since the oxygen is being consumed as the candle burns,
the water should rise progressively.
However, as you can see, the water keeps on rising even after the candle has gone out.
The hypothesis is partially false because it misses one element.
If you look back to our equation, you'll see that the space the oxygen leaves
is mostly compensated by the products of the reaction.
But there is a physical phenomenon acting on our observations, too.
The water rises because the pressure is different inside and outside of the glass.
The air that surrounds the candle is already warm when you place the candle under the glass.
The air cools as it comes in contact with the glass.
As it cools, the air sort-of shrinks and takes up less space under the glass.
The pressure diminishes, creating a difference with the outside pressure.
This is why the water rises.
Let's sum it up: the candle goes out because it produces CO2
which keeps the oxygen from reaching the flame;
the water level rises because of a pressure difference
induced by the cooling of the air that's under the glass.
Have I won the bet? ;)
Since we're playing with candles and now's the Halloween season,
here are some easy and fun experiments to try for yourself.
This one allows you to light a candle again without touching the wick! That's hours' worth of fun right there.
(Lumos!)
Another one is a way to make your own candle with very simple household items:
Roll some toilet paper, stick it in your loaf of butter and there you have it - a great candle that'll burn for hours.
If you'd like to know more about these last 2 experiments, go check out Kidiscience (francophone blog).
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