IB Chemistry HLTopic 1 — Particulate Nature of MatterPaper 1 & 2Core idea~10 min read
Elements, Compounds and Mixtures
Everything around you is made of tiny particles. What makes one substance different from another is simply which particles are there and how they are put together. Get that idea straight now and a surprising amount of the rest of chemistry falls into place.
📘 What you need to know
An element contains only one kind of atom. You cannot break it down into anything simpler by a chemical reaction.
A compound contains two or more different elements chemically bonded together in a fixed ratio.
A mixture is two or more substances just jumbled together, not bonded, in any ratio you like.
Elements and compounds are pure substances. Mixtures are not.
A molecule is any group of atoms bonded together — it can be an element (O2) or a compound (H2O).
Mixtures can be separated by physical methods. Compounds need a chemical reaction.
A compound has completely different properties from the elements it is made of.
Start with the particles
Take any lump of stuff and imagine cutting it in half, then in half again, over and over. Eventually you reach particles you cannot cut any further with chemistry. Those are atoms.
There are only about a hundred kinds of atom, and every substance in the universe is built from them. So the interesting question is never “what is it made of?” — it is always how many kinds of atom, and are they joined up? Those two questions give you the three categories.
Box 2 is the one that catches people out. Two atoms are joined, so it is a molecule — but they are the same kind of atom, so it is still an element.
A molecule is not the same thing as a compound
This is the single biggest muddle in this topic, so it is worth ten seconds of care.
Molecule describes the shape of the particle: two or more atoms bonded together. Compound describes the content: more than one element present. They are answers to two completely different questions, and a substance can be one without being the other.
Substance
Is it a molecule?
Is it a compound?
Why
Ne (neon)
No
No
Single atoms of one element
O2 (oxygen)
Yes
No
Two atoms bonded, but both are oxygen
H2O (water)
Yes
Yes
Two different elements bonded
NaCl (salt)
No
Yes
A giant lattice of ions, not separate molecules
Seven elements go around in pairs. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine all exist as H2, N2, O2 and so on. They are called diatomic. Every one of them is still an element.
How to tell which is which
Notice that a mixture can be made of elements, of compounds, or of both. Being a mixture is about whether things are bonded, not about what they are.
Everyday language and chemistry disagree about the word “pure”. Pure orange juice is a mixture of water, sugars, acids and colour. To a chemist the purest thing in your kitchen is probably the salt.
Compounds have a fixed ratio. Mixtures do not.
This is the sharpest difference between the two, and the one exam questions lean on most.
Fixed composition
Every sample of a compound has the same elements in the same ratio by mass
Water is always two hydrogens to one oxygen. Not sometimes three. Rain, tap water, water from a puddle in Peshawar or a glacier in Iceland — all of it is H2O.
Now compare that with a mixture. You can make salty water with one spoon of salt, or five, or half. It is still salty water. Nothing forces the ratio, because nothing is bonded.
WORKED EXAMPLE
Compound or mixture, from the numbers alone
Two samples of the same colourless liquid are broken down completely. Sample A gives 2.0 g of hydrogen and 16.0 g of oxygen. Sample B gives 3.0 g of hydrogen and 24.0 g of oxygen. Is the liquid a compound or a mixture?
Step 1: Find the mass ratio in each sampleSample A: 2.0 : 16.0 = 1 : 8Sample B: 3.0 : 24.0 = 1 : 8Step 2: Compare them
Both samples have exactly the same ratio, even though the amounts are different.
Step 3: What that tells you
A mixture could have been 1:8 in one sample and 1:15 in another. A fixed ratio means the atoms are bonded.
Step 4: Check which compoundn(H) = 2.0 ÷ 1.01 = 1.98 mol; n(O) = 16.0 ÷ 16.00 = 1.00 molratio 2 : 1A compound — and it is water, H₂Osame ratio every time is the fingerprint of a compound
The classic test: iron and sulfur
Nothing shows the difference better than this experiment, and it comes up constantly.
Mix grey iron filings with yellow sulfur powder. Nothing happens — you just get a speckled grey-and-yellow powder. Hold a magnet over it and the iron jumps out, leaving the sulfur behind. It is a mixture.
Now heat that same powder strongly. It glows, and when it cools you have a single dull dark solid. No grey specks, no yellow specks. Hold the magnet over it and nothing happens. The iron has reacted with the sulfur to make a new substance, iron sulfide (FeS). It is a compound.
The atoms are exactly the same before and after. What changed is that they are now bonded — and that is enough to give a completely different substance.
An even better example: sodium is a soft metal that explodes in water, and chlorine is a poisonous green gas. Bond them together and you get sodium chloride — the salt you put on your food. A compound is genuinely a new substance, not a blend.
Two kinds of mixture
Mixtures come in two flavours, and the difference is just whether you can see the separate bits.
Homogeneous — the same all the way through. Salt water, air, brass, vinegar. Even under a normal microscope you cannot see the separate parts.
Heterogeneous — you can see the different parts. Sand in water, oil and water, a fruit salad, granite.
🧩 How to classify anything in four questions
Is there more than one substance in it? If yes, it is a mixture — stop here.
If it is one substance, how many elements are in the formula? One symbol means an element; two or more means a compound.
Careful with subscripts. O2 still has only one symbol, so it is still an element.
For a mixture, can you see the parts? Yes means heterogeneous, no means homogeneous.
WORKED EXAMPLE
Sorting five everyday substances
Classify each of these as an element, a compound or a mixture: oxygen gas, air, brass, diamond, sea water.
Oxygen gas, O₂
Two atoms bonded, but both are oxygen. One symbol only.
→ element (made of molecules)Air
Nitrogen, oxygen, argon, carbon dioxide, all just floating about unbonded.
→ mixture (homogeneous)Brass
Copper and zinc melted together. No fixed ratio — you can make it richer in either.
→ mixture (an alloy, homogeneous)Diamond
Millions of carbon atoms bonded, but every single one is carbon.
→ elementSea water
Water plus dissolved salts, in whatever amount that stretch of sea happens to have.
→ mixture (homogeneous)2 elements, 0 compounds, 3 mixturesbrass and diamond both have huge numbers of bonds — bonds alone never make a compound
WORKED EXAMPLE
What actually changed when the iron and sulfur were heated?
Before heating, a magnet removes the iron. After heating it does not. Explain what happened, and say whether the mass changed.
Step 1: Before heating
The iron and sulfur are only mixed, so each keeps its own properties — including the iron being magnetic.
Step 2: During heatingFe + S → FeS
Bonds form between iron and sulfur atoms. A new substance appears.
Step 3: After heating
FeS is one substance with its own properties, and being magnetic is not one of them.
Step 4: The mass
No atoms were added or lost, only rearranged.
mass of FeS = mass of Fe + mass of SA chemical reaction happened, but the total mass is unchangeda physical mixture keeps its parts; a chemical reaction makes something new
💡 Exam tip
Count the different symbols in the formula, not the number of atoms. H2 has one symbol, so it is an element.
If a question asks you to justify an answer, mention bonding: “the elements are chemically bonded in a fixed ratio” is the phrase that scores.
Separating a mixture is physical — filtering, evaporating, a magnet. Breaking a compound needs a chemical reaction like electrolysis.
Alloys such as brass, bronze and steel are mixtures, however solid they look.
In a particle diagram, use one colour per element and a line for a bond. Examiners mark exactly that.
“Pure” in chemistry means one substance. It has nothing to do with being natural or healthy.
⚠ Common mix-up
Thinking any molecule is a compound. O2, N2 and Cl2 are molecules of elements.
Calling water a mixture of hydrogen and oxygen. They are bonded, so it is a compound.
Expecting a compound to behave like its elements. Sodium and chlorine are both dangerous; sodium chloride is table salt.
Thinking a compound can have any ratio. The ratio is fixed. Only mixtures are flexible.
Assuming “clear” means “pure”. Salt water is perfectly clear and is a mixture.
Treating alloys as compounds because they are hard solids. They are mixtures of metals.
Using different colours for the same element in a particle diagram. One element, one colour.
Up next: Separating Mixtures — since mixtures are not bonded, you can pull them apart physically. Filtration, evaporation, distillation and chromatography, and how to pick the right one.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
