Pure substances & mixtures

Take a look around: the air you breathe, the juice in your glass, even the metal of your bike. Almost none of it is a single, pure substance. Everyday life is made of mixtures, and once you understand them, a lot of chemistry suddenly makes sense.

Pure substance or mixture?

A pure substance consists of only one kind of substance, for example distilled water (H₂O) or pure sugar. It has the same properties throughout and a fixed profile. A mixture, on the other hand, contains several substances side by side: each keeps its own properties, just mixed together. Tap water is already a mixture, because minerals and gases like CO₂ are dissolved in it.

Homogeneous or heterogeneous

In a homogeneous mixture you can no longer see the individual components, not even with a magnifying glass: salt water, air, or alloys like bronze look like a single substance. In a heterogeneous mixture you can still spot the components: in muesli you see flakes and raisins, in sand and water the sand visibly settles to the bottom.

An important special case is the solution: a substance like salt or sugar spreads so finely through water that a homogeneous mixture forms. Nothing is lost in the process, because the solution weighs exactly as much as its components together: 5 g of salt in 95 g of water gives 100 g of salt water.

Why the distinction matters

This classification is not just for its own sake: it tells you how to separate a mixture again. Heterogeneous mixtures can often simply be sorted, sieved, or filtered. For homogeneous mixtures you need cleverer tricks, because you cannot even see the components. That is exactly what the next topic is about.