How Come Firefighters Use Water?

Question

I know there are certain types of fires that, if one uses water on it, will get worse. But as far as I know, firefighters use water tanks and fire hydrants for virtually every fire too large for a fire extinguisher to do the job.

Is it an issue of if the source isn’t large enough or too far away it doesn’t matter? How do they know the kind of fire before reacting to it?

In: 2

Answer 1

> I know there are certain types of fires that, if one uses water on it, will get worse.

This is true when the fuel for a fire is hydrophobic.

Water puts out fire by getting flammable material wet. This prevents contact between the air and the burning fuel. Fire needs access to oxygen to continue burning because combustion is the chemical reaction between a combustible substance and oxygen when exposed to high heat.

Hydrophobic substances are those that repel, and are repelled, by water. Oil (both cooking oil and petroleum) is hydrophobic. If you take a glass and fill it half with cooking oil and half with water, they will not mix. Even if you stir them they will separate.

Of course oil is combustible. If you leave a pan of oil on the stove, it may catch on fire. (A “grease fire”) You do not want to use water to put it out. If you try, the oil will repel the water and the water will be unable to block oxygen. At the same time, the oil (which you recall is on fire) is going to fly out in every direction, repelled by the boiling water now at the bottom of the pan, setting your kitchen on fire.

So my point is, as long as a fire isn’t fueled by a hydrophic substance, it is safe to use water. Part of firefighters’ job is to understand what exactly is on fire so they can be aware of the risks. There are regulations that anyone who stores large quantities of a hazardous material must report it to the local fire department. Unless a fire site happens to store large quantities of hydrophobic materials, they can safely use water. Otherwise there are chemical alternatives, such as potassium bicarbonate which they use in the form of a fine powder.

There may be some other substances I’m not aware of that would make water unsuitable. But if large quantities of such a material are stored, by regulation that material would similarly be reported to the local fire department.