pure substance

A pure substance is matter made up of only one kind of particle throughout — nothing else is mixed in, so every part of it behaves identically.

• This is a stricter idea than the everyday sense of "pure." A product labelled pure, like milk or ghee, is really a mixture of many different substances — to a scientist, it only counts as pure if it truly contains just one.
• Because a pure substance can't be split into simpler kinds of matter by physical means, separating one always means using a chemical change instead — splitting water apart into hydrogen and oxygen, for instance.
• Every pure substance is either an element or a compound — nothing in between, and nothing part-mixture.
• Checking whether something is genuinely pure matters beyond the lab too — adulteration, adding cheaper substances to a product, is exactly the kind of impurity that can make food or medicine unsafe.