What are Pure Substances

Pure substances are made up of only one type of molecule. This molecule can be made of different atoms, but the substance cannot have different types of molecules.

Element

One type of pure substance is elemental. The molecules of this substance have to be made of the same element.

Elements cannot be broken down into simpler substances. They are the smallest building blocks of the universe.

For example, O₂ is an elemental substance, because it is only made of oxygen. CO₂ is not, because it has two types of atoms: carbon and oxygen.

Most elements naturally exist as single atoms, like Na, Au, Zn, etc. However, there are seven diatomic elements, which only naturally exist in pairs of two (like O₂). Here’s a mnemonic device to remember them ↴

Have – Hydrogen (H₂)

No – Nitrogen (N₂)

Fear – Fluorine (F₂)

Of – Oxygen (O₂)

Ice – Iodine (I₂)

Cold – Chlorine (Cl₂)

Beer – Bromine (Br₂)

Compound

Compounds are pure substances made of multiple types of elements/atoms that are chemically bonded together. CO₂ is a compound because it is made of multiple elements, carbon and oxygen. NaCl is a compound, because it is made of sodium and chlorine.

Compounds can be separated into simpler substances: the atoms that compose them. For example, you could separate NaCl into Na and Cl.

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