Why Does Water Put Out a Fire and How?

Before we get any further, note well: Water must never be used on an electrical fire or on an oil or grease fire.

Reasons: Water conducts electricity and can lead it elsewhere, perhaps to your very own feet. And because water won’t mix with oil or grease, it just scrambles it around and spreads the fire.

Fire needs three things to survive: fuel, oxygen, and, at least initially, a temperature high enough to ignite the fuel and get the combustion reaction to begin. After that, the reaction gives off more than enough heat to keep things going.

Obviously, the first thing to do would be to remove the fuel. Nothing to burn, no fire. But water can’t do that, so it attacks the other two essentials: the oxygen and the temperature.

A deluge of water from a bucket or hose can smother the fire as if it were a blanket, simply by blocking out the air. A thin layer of water, even for a short time, can do the trick. No air, no oxygen, no fire.

Water can also lower the temperature of the material that’s burning. Every combustible material has a minimum temperature that it has to reach before it will ignite and burn. If the water cools the material below that temperature, voilĂ ! no more burning. Even hot water is well below the temperature at which most things can burn.

A deluge isn’t necessary. Water from a sprinkler can put out a fire, even though it leaves a lot of oxygen available between the rain drops. So it must work by lowering the temperature. Remember how cooling it is to run through the lawn sprinkler?

A sprinkler lowers the temperature in two ways. First, water in fine droplets tends to evaporate quickly, and evaporation is a cooling process. Second, water has a peculiarity that makes it much better than any other liquid for dousing fires: It is a heat sucking glutton. Water has a huge appetite for heat. A pound of water absorbs 252 calories of heat before its temperature goes up by a single degree Fahrenheit.

Is that a lot? Well, contrast it with the amount of heat it takes to raise the temperature of a pound of several other substances by one degree Fahrenheit: Mercury requires only 8.3 calories; benzene, 63 calories; granite rock, 48 calories; wood, 106 calories; and olive oil, 118 calories.

The moral is that a little bit of water can take away a lot of the fire’s heat before boiling away and leaving the premises as steam. Water is therefore an extremely effective cooling agent. That’s why it is used in automobile cooling systems. Of course, being cheap doesn’t hurt.