IB Chemistry SLTopic 1 — Particulate Nature of MatterPaper 1 & 2Core idea~7 min read
Elements, Compounds & Mixtures
Every material around you is built from atoms — but how those atoms are joined (or not joined) decides whether you’re looking at an element, a compound, or a mixture. Get this distinction clear and the rest of the topic falls into place.
📘 What you need to know
An element contains only one kind of atom and can’t be broken down into anything simpler by chemistry.
A compound is two or more elements chemically bonded in a fixed ratio.
A mixture contains two or more substances that are not chemically joined and can be in any ratio.
The parts of a mixture keep their own properties, so they can usually be separated by physical means.
Mixtures are either homogeneous (uniform throughout) or heterogeneous (not uniform).
What is an element?
Think of an element as the simplest “pure” substance a chemist can have. It’s made from just one type of atom, and no chemical reaction can split it into anything simpler.
Elements are the building blocks that make up everything else.
When elements react, their atoms join in fixed whole-number ratios so each atom ends up with a full outer shell of electrons.
Examples: sodium (Na), oxygen (O2), iron (Fe), carbon (C).
Picture a box of identical LEGO bricks, all the same colour and shape. That box is an “element” — one kind of building block, nothing else mixed in.
What is a compound?
When atoms of different elements bond together, you get a compound. The key word is bonded — the atoms are chemically locked together in a set ratio, and it takes a chemical reaction to break them apart again.
A compound always has the same elements in the same fixed ratio — water is always H2O, never H3O.
A compound’s properties are usually completely different from the elements that made it.
Sodium and chlorine react to form sodium chloride — a compound with totally different properties from either element.
What is a mixture?
A mixture is what you get when substances are simply mixed together but not chemically bonded. Because nothing has reacted, each substance keeps its own properties and can be separated out again by physical methods.
The parts can be present in any ratio — unlike a compound’s fixed recipe.
Each component behaves the same as it would on its own. Air, for example, is a mixture of nitrogen, oxygen and other gases, each keeping its own character.
Oxygen in air still supports combustion, exactly as pure oxygen would.
⚠️ Common mix-up
Compound vs mixture: in a compound the elements are chemically bonded in a fixed ratio; in a mixture they’re just physically together in any ratio.
A compound needs a chemical reaction to separate; a mixture only needs a physical method like filtering or evaporating.
Homogeneous or heterogeneous?
Mixtures come in two types, and the difference is simply whether the mixture looks the same all the way through.
A homogeneous mixture has a uniform composition — you can’t pick out the separate parts. Examples: air, or bronze (an alloy).
A heterogeneous mixture is not uniform, so different parts look different. Examples: concrete, or orange juice with pulp.
Quick test: if you can see (or could pick out) the separate bits, it’s heterogeneous. If it looks like one smooth substance throughout, it’s homogeneous.
Particle diagrams make the difference obvious: elements, compounds and mixtures at the particle level.
Why the properties change
Here’s the idea students most often miss: when elements form a compound, the product’s properties have nothing to do with the starting elements. Sodium is a reactive metal, chlorine is a poisonous gas — yet bonded together they make ordinary table salt.
In a mixture, the opposite is true: because nothing bonds, each substance keeps its own properties. That’s exactly why mixtures can be pulled apart again physically.
WORKED EXAMPLE
Classify each of the following as an element, a compound, or a mixture: (a) copper wire, (b) carbon dioxide (CO2), (c) sea water.
(a) Copper wire → elementMade of only one kind of atom (Cu).(b) Carbon dioxide → compoundCarbon and oxygen chemically bonded in a fixed ratio (1 C : 2 O).(c) Sea water → mixtureWater with salts dissolved in it, not bonded and in no fixed ratio — separable by evaporation.
💡 Exam tip
Look for the word bonded or a chemical formula — that signals a compound.
If something can be separated physically (filtering, evaporating, a magnet), it’s almost certainly a mixture.
A single chemical symbol with no other element attached (Fe, O2, He) points to an element.
Up next: Separating Mixtures — since the parts of a mixture keep their own properties, we can use those differences (solubility, boiling point, magnetism) to pull them apart.
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