Is water a pure substance? I had a question on a quiz and it said

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Is water a pure substance? I had a question on a quiz and it said which is not correct. I answered "Water is a pure substance" and got it wrong. The correct answer was "Elements occur only in the form of atoms."


Explanations please, thank you.
 
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Hi. Water, group of single kind of molecules H20, is pure substance.
Not only molecules that consists of same kind of atoms like H2 O2 but also those that consist of many kind of atoms like H20 CO2 NH3 CH3COOH　are called pure substance as far as it consists of single kind of molecule.
 


... pure H2O would be "pure" - well, I said that in the name didn't I? However, water at room temp is not just H2O molecules - it is also H+ and OH- in equal mixture (giving it the characteristic pH. Also some O2 dissolved in there ... all that even if you started out with just H2O you would end up with a mixture of different stuff.

So - when the multi-choice says "water" it is unclear if they mean pure H2O or the us kind of mix above or just plain old tap water. Still, it may be the best answer: you could argue either way really.

In your case, unfortunately, there was a better answer in the list.

"Elements occur only in the form of atoms" is pretty much the definition of "element" and so it trumps the previous, merely "arguable", option.

You have to watch for these.
In multi-choice there is usually one clearly wrong answer and two "could be" answers.
 


Trivia quizzes do not allow for deep chemistry nor deep philosophy. I actually sympathize more with your answers than the ones that were marked correct.

Purity is a matter of convenience and degree. No sample of water has ever been prepared that did not have sodium ions present in femtomolar (10–15 M) quantities, and at least a dozen other ions present in more than attomolar (10–18 M) quantities.

And even as an ideal concept, pure water cannot be a pure substance. The definition of a pure substance involves a collection of identical molecular entities, or a perfectly regular network-bonded lattice. But ideally pure water contains 10–7 M hydrogen ions and a similar concentration of hydroxyl ions.

"Elements occur only in the form of atoms" is, on the other hand, largely true. It really depends on whether you regard an ion of an element as an atom of that element, because elements are frequently present in the form of ions.

But the problem with the way this question would normally be interpreted is that chemists have adopted the habit of a loose use of the word "element" when they really mean "simple substance".

Dioxygen gas is a simple substance which contains only the element oxygen, as also does the simple substance ozone. Carbon dioxide is a compound which also contains the element oxygen. It does not contain dioxygen gas nor ozone. Neither dioxygen gas nor ozone is "a form of" the element ozone. But an alpha particle, He2+ is a form of the element helium, even though it is not a helium atom.