Why do people say water is a pure substance?

Question

A pure substance is something that cannot be further broken. But water CAN be broken down into hydrogen and oxygen. Help???

Answer 1

A pure substance is something that we know exactly what is in it. Therefore, it i an element or a compound. It is not a mixture because we do not know the ratio of the igredients.
Water that has been distilled is pure because it is only water. H2O

Answer 2

"Pure" in the sense that it can be broken down only by chemical, rather than simple physical, means. It's a matter of degree: a mix of sand and sawdust could be separated by a wind, but the hydrogen and oxygen in water are so tightly bound that it takes an electric current or energetic photon to dissociate them. The difference is the binding energy.
There are some very weakly bound chemical that could be considered mixtures, depending on your point of view. Water of hydration can easily be driven from gypsum with moderate heat, producing plaster of Paris, and vice versa; is that much different from wetting and drying a paper towel? It depends on how much energy is needed.

Answer 3

Notwithstanding your valid argument about breaking water down into H2 & O, how does one define PURE water? No matter where you go the water has something in it, whether its minerals, bacteria, chlorine or radioactive compounds.

Scientifically speaking "PURE" water would contain nothing but H2O, no minerals or anything that would give it any taste or colour.

Don't know where you'd find "Pure" water unless you make it yourself.

Answer 4

Actually, a "pure substance" means that there is nothing else in it. So, if you have water that has nothing else in it, it's "pure water".

As another example, if something is said to be "pure nonsense", it means that there is only nonsense and no sense to be found anywhere in it.