How can something with different atoms be a pure substance and not a mixture?

Question

Shannon Gibbs

Chemistry

31 August, 20:10

How can something with different atoms be a pure substance and not a mixture?

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Answers (2)

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Holly Bell · Answer 1

A pure substance is a substance with a definite composition. This means that its composition does not vary. Examples of this are sugar and salt. Whether it is a mixture or a compound, what characterizes a pure substance is that its composition does not vary.

Layla · Answer 2

1) There are two kinds of pure substances: elements and compounds.

2) Elements are constituted by one kind of atom. The elements are differentiaded by the atomic number (number of protons), and so they are classified in the periodic table.

There are only 118 known elements and you can look at them in the periodic table.

3) The other type of pure substances are the compounds.

The compounds are the chemical combination of two or more kind of atoms (elements), always in the same ratio. This is, compounds have a unique, unvariable composition. That is why you can represent a compound with a chemical formula. For example, water, a pure substance, is a compound in which every molecule consists on two atoms of hydrogen bonded to one atom of oxygen.

Any molecule of pure water that you find in your home, in your city, in the sea, or even in other planet, will have the same composition: H₂O.

There are infinitely many compounds: sugar, salt, pvc, glucose, carbon dioxide, ... each of them is a different pure substance.