Robert Krampf's Experiment of the Week: #483 Why Wet Things Don't Burn?

This week's experiment was inspired by my recent science video about how to hold a balloon in a candle flame without popping it. It started me thinking about why water is so effective at stopping fires.

*WARNING* This experiment involves the use of fire. Be safe, and be sure that there is an adult with you to help.

To try this, you will need:

paper towels
kitchen tongs or pliers
a lit candle or a lighter
a bowl of water

Begin by tearing a strip of paper towel about an inch wide and a couple of inches long. Hold it in the tongs and place it over the flame of the lighter. What happens? It quickly catches fire, just as you would expect. Put the burning paper into the water to put out the flames.

Then tear another strip of paper towel the same size. Hold it with the tongs, but this time dip in into the water first. Once it is wet, hold it in the flame. What happens this time? The paper turns black, but it does not burn. Even the black color does not come from the paper. Instead, it is carbon soot that comes from the flame. As long as the paper is wet, it will not burn. Why?

Understanding the Science

Some things are so common that we just accept them without wondering why. Water puts out fires, so of course wet things don't burn. But why? There are several things that add together to give water its fire quenching ability.

First, water does not burn. Why not? After all, it is made up of hydrogen and oxygen, both very flammable gases. When you burn fuel, it combines with oxygen. Last week, we saw that when we burned iron, it bonded with oxygen to become iron oxide. The carbon found in many fuels will bond with oxygen to become carbon dioxide. The hydrogen in water is already bonded to oxygen, so it is the same as if it had already been burned.

Second, water absorbs a lot of heat. Every material has a property known as specific heat. That is the amount of energy that it takes to raise the temperature of one gram by one degree. Water has a high specific heat, which means that it can absorb a lot of heat energy before it gets hot enough to boil. At the point where it boils, it absorbs even more heat, to give the molecules enough energy to change from a liquid to a gas. With the water absorbing all that heat, the wet fuel does not get enough heat for it to burn. That is how the balloon in the video was protected from the flame. It is only when the water has evaporated that the fuel can finally get hot enough to catch fire.

Water also separates the fuel from the oxygen it needs to burn. A coating of water provides a barrier to keep oxygen away from the fuel. Without that oxygen, the fuel won't burn.

All these things combine to explain why wet things don't burn.

Safety
With that said, it is important to mention that there are some fires that you should never use water on.

Never put water on an electrical fire. Water conducts electricity, and it could provide a connection between you and the electric wires.

Never use water on grease or oil fires. Water is heavier than the oil, so instead of floating on top, where it would block the oxygen, it sinks to the bottom. There it absorbs enough heat to change into steam. The rapidly expanding steam throws the burning oil in all directions, spreading the fire instead of putting it out.

That is why you should always have an approved type ABC fire extinguisher in your kitchen. They are inexpensive and can save your kitchen and your life.

Have a wonder-filled week!