6 min read · Aug 6, 2020
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It’s a classic demo and one that you can try yourself. You light a candle and put it in some water (I used blue dye in my water). Next, cover the candle with a glass. As the flame burns out, the water gets “sucked” up into the glass. It’s pretty cool.
Let me start off with the “sucked” part. No, the water isn’t actually sucked up. Actually, the pressure inside the glass is reduced while the pressure outside the glass (due to the atmosphere) stays constant. Since the outside pressure is greater than the inside pressure, the water gets pushed up into the glass.
Check it out.
But why does the pressure inside the glass decrease? It’s mostly due to the chemical reaction between the wax and oxygen. We often call this type of reaction “burning stuff”.
The key to most fire reactions is the oxygen in the air reacting with some type of carbon. Technically, this is called a combustion reaction — but I like to call it fire. Also, it can get pretty complicated to fully explore combustion of wood and wax, so let’s consider a very simple version — burning methane. The main part of the reaction is the same.
