Why can't water put out a chemical fire?

Answer 1

Some chemicals will react with water to cause the reaction that is releasing the heat and flames. If you use water on these it would just increase the fire. You must always make sure you know if your chemical reacts with water before using water to put out the fire. If you don't know, use a fire extinguisher instead of water.

As you are obviously well aware, water can put out flames. However, the effectiveness (whether it works effectively, does not work as good or it doesn't work at all) depends on what chemical is burning.

Some chemicals that don't require a catalyst such as heat (example: using a match to start a Bunsen burner flame) to create a fire (An exothermic reaction between the reactants) and react on contact are very hard to to stop because a starter is not needed and the substances will react easily. Because of this, it is probably impossible to stop a gas readily reacting with another gas because the reactants are basically mixing (and then reacting). It is also hard to stop a liquid reacting with a gas because you are basically just mixing water with the liquid reactant. However, it might be possible to stop a solid reacting with a gas by putting the solid under water. That way, the gas won't reach the solid easily (but some of the gas are in the water). However, the effectiveness of this is also determined by my next reason of why water cannot put out a chemical fire.

Sometimes, water can also react with the reactants. A good example are the alkali metals (most especially francium lol). This obviously means that you should never add water to stop a flame when one or more of the reactants are known to react with water. It is only common sense ;)

From this point in this answer to the question, that answers the question. But read on in order to know why water stops flames in the first place... That should give a slightly deeper understanding in the reason. If you are lazy, just read the summary of the paragraph.

Some substances require a starter (catalyst) to start the flame. An examples include: another flame and a spark (electric current). In this case, once you stop the reaction, the reactants won't react anymore even if the they came back in contact. In order to stop the flame (the reaction) with water, the water has to totally block the contact with the reactants. This is easy when you have a solid burning with a gas because the water can quickly and temporarily cover the solid, thus stopping the reaction. Stopping the other states of matter to burn would be harder, such as gas reacting with gas (example: Oxygen and Ethane) because splashing water into a burning cloud of gas that has already mixed is not going to do much. This is because the substance that is burning only comes in contact with some of the reactants but are still reacting with the surrounding particles (any physical state, most commonly with gases, namely Oxygen gas). In the case of liquids, it requires it to be extremely saturated with water to stop the flame. To sum that chunk of paragraph up: when a burning solid or liquid + gas requires a starter to react, water puts out the flame (stop the reaction) by displacing the gas, thus suffocating the burning substance.