What Is a Pure Substance?
A pure substance contains only one type of particle — either a single element or a single compound — with nothing else mixed in.
A pure substance is made up of identical particles throughout. Because of this, it has fixed, predictable physical properties — a definite melting point, boiling point, and density.
There are two types of pure substance:
- Elements — made of only one type of atom (e.g. copper, oxygen gas O₂). Note that oxygen gas exists as O₂ molecules, but it is still an element because it contains only one type of atom — oxygen.
- Compounds — made of two or more types of atom chemically bonded together in a fixed ratio (e.g. water, H₂O; carbon dioxide, CO₂)
The word "particle" is being used here as an umbrella term — for elements, the particles are atoms (or molecules made of the same type of atom, like O₂); for compounds, the particles are molecules or formula units. The key idea is that a pure substance contains only one kind of particle throughout.
Pure water in the lab
Distilled water is a pure substance. It has been evaporated and then collected again, which removes all dissolved salts, dirt, and micro-organisms. The result contains only water molecules (H₂O). Because it is so pure, scientists actually need to add back certain helpful minerals before it is safe to drink!
Just because something is called "water" doesn't mean it's pure! The word "pure" in science has a very specific meaning — it means only one type of particle present.
What Is a Mixture?
A mixture contains two or more different substances that are physically combined but not chemically bonded. The substances keep their own properties and can be separated by physical means.
In a mixture, particles of different substances are simply jumbled together. They are not chemically joined, which means:
- The proportions of each substance can vary
- The properties are not fixed (e.g. the boiling point changes depending on what is dissolved)
- The substances can be separated using physical processes
Think of a bag of mixed sweets — the different sweets are all in the same bag, but they haven't changed into a new sweet. You can still pick them apart. A mixture is exactly like that: different substances sharing the same space, but not becoming something new.
Students often confuse compounds with mixtures. In a compound, elements are chemically bonded in fixed ratios — you cannot simply separate them by filtering or evaporating. In a mixture, substances are physically combined and can be separated by physical processes.
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