Pure substances have only one kind of atom

Question

I read in a book recently that no substance is ever totally pure, even if we take great care to separate it from other substances. And also that pure substances contain only one kind of matter. But I actually have some doubts related to both of the notes. I would rather number...

1. Pure substances contain only one kind of matter? True for elements but what about compounds? Are compounds not pure substances? They are homogenous at composition, right?
2. No substance is totally pure? Reactive elements are found in combined state, but what about less reactive or maybe radioactive?
3. As well, can't we obtain any substance with 100% purity?

I did surf the questions but did not get any reasonable explanation. Also, please do correct me where I am wrong.

Answer 1

Most of the issues can be resolved with precise definitions.

Let's start with purity. Purity is relative. In many cases (eg table salt) 99% is pure enough. What this means is that for all practical purposes we don't care about the remaining impurities. Chemists tend to describe something as "pure" when there is less of any contaminant than matters for what they are doing with the chemical. Sometimes purity of very high levels matters. Semiconductors behaviour depends on very precise doping with very small amounts of other substances. The wrong contaminant even at levels of 1 atom per million is a problem. So the industry have worked out good ways of making extremely pure silicon, for example. These techniques have been used to make some of the purest substances ever (the silicon sphere now being used to redefine the Kg is pure to parts per billion atoms.) Strictly speaking it isn't pure in the sense of having absolutely 100% of all its atoms as silicon, but that is as pure as it needs to be for its purpose. So, strictly, nothing is ever absolutely pure, but it doesn't matter.

The word "substance" is a bit vague. In chemistry it could be a compound an not just an element. Purity for a compound means that all the molecules in the substance are the same (to the level of purity that matters). Having more than one type of atom doesn't make it impure as long as all the molecules are the same.

Then there are some homogeneous mixtures that are hard to separate by any simple means, some metal alloys for example. We wouldn't normally describe these as "impure" but chemists might use special terminology to describe them (alloy rather than compound for example).

And as for "Pure substances contain only one kind of matter" it depends what you mean by "matter". Chemists would prefer the term "molecule" to "matter". Matter can be interpreted different ways (does it mean atoms or molecules: it's unclear). "Pure substances contain only one kind of "molecule" would be an unambiguous statement chemists would agree with.

Answer 2

The Wikipedia article might be a good reference to start.

1. A pure substance cannot be separated by physical means. This means in principle, so if in practice, you cannot separate physically, but you theoretically could, that's still not a pure substance. A compound would then be pure because you cannot separate out its components by physical means.

2. That's true, but "pure" here does not refer to purity as you using the word. It refers to purity of essence in that the substance is not constituted from multiple parts by simple mixing.

3. We can't, but again, that's not what pure here is supposed to imply. Oxygen is a pure substance whether it's mixed or not because it is an element. A mixture of oxygen and something else is not pure because it is a physical mixture.