How Does This Candle Suck Water Up Into a Glass?

Photo: Rhett Allain

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.

If you look at the reaction of methane and oxygen, you get the following.

This says that one molecule of methane plus 2 molecules of oxygen produce one molecule of carbon dioxide and 2 water molecules.

So, let’s consider the gas in the glass before the combustion. It’s approximately 79 percent nitrogen gas and 21 percent oxygen. Then the fire consumes oxygen and produces carbon dioxide and water (vapor). Eventually, there’s not enough oxygen in the glass and the fire goes out.

Since we are talking pressure, we need to talk about some other stuff — the Ideal Gas Law. This model assumes there are bunch of gas particles that are spaced out enough that you can treat them as particle that just bounce off each other. With that, there are 4 things that can change:

• The volume of gas (V).
• The temperature of the gas (T).
• The gas pressure (P).