Compounds vs Elements vs Mixtures
In 1783, French chemist Antoine Lavoisier proved that water was not an element but a compound of 2 hydrogen atoms and 1 oxygen atom — overturning a 2,000-year-old Greek theory in a single experiment.
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Q1 · A glass of orange juice with pulp, a copper wire, and a jar of pure table salt. Which of these is "just one substance" and which is "two or more things mixed together"?
Q2 · Salt is written as NaCl (sodium + chlorine). Why can you eat salt safely, even though sodium reacts violently with water and chlorine is poisonous?
● Know
- An element has only one type of atom
- A compound has 2+ elements chemically bonded together
- A mixture has 2+ substances physically combined (not bonded)
● Understand
- Why a compound has properties different from its starting elements
- Why mixtures can be made in any ratio, but compounds have fixed ratios
- The difference between an element's symbol (Na) and a compound's formula (NaCl)
● Can do
- Identify an element, compound or mixture from a particle diagram
- Use a formula like H₂O to count the atoms in a compound
- Classify everyday substances (air, water, salt, copper) as element, compound or mixture
- Element
- Compound
- Mixture
- Symbol
- Formula
- A group of element codes like H₂O
- A substance made of only one type of atom
- Substances physically combined, not bonded
- A one or two-letter code for an element
- Two or more elements bonded in a fixed ratio
Snap a piece of copper wire and look at the break — every part of it, right down to a single atom, is still copper. You can't break it into anything simpler. That's an element: a substance made of only one type of atom. There are 118 known elements on the periodic table, and every other substance in the universe is built from combinations of them.
How an element looks in a particle diagram: all circles the same colour and size.
| Element | Symbol | Common form |
|---|---|---|
| Oxygen | O₂ | The gas in the air you breathe (paired atoms). |
| Iron | Fe | The grey metal in nails, steel beams, train rails. |
| Gold | Au | The yellow metal panned for at Bathurst. |
| Carbon | C | Pencil "lead", charcoal, diamond. |
You can't split an element into something simpler using chemistry. (Splitting an atom itself is nuclear physics, not chemistry — and is way outside Year 7.)
An is a substance made of only type of . There are known elements on the periodic table.
A compound is two or more elements that have been chemically bonded together in a fixed ratio. The two famous examples:
- Water (H₂O) — every water unit has 2 hydrogen atoms and 1 oxygen atom. Always.
- Salt (NaCl) — every salt unit has 1 sodium and 1 chlorine atom. Always.
Here's the crucial idea: a compound's properties are completely new. They do NOT match the elements that went in.
| Element | Property on its own | Combined into compound |
|---|---|---|
| Hydrogen (H) | Explosive gas | Joins with O to make H₂O — water (puts fires out!) |
| Sodium (Na) | Soft metal that catches fire in water | Joins with Cl to make NaCl — table salt (safe to eat) |
| Chlorine (Cl) | Poisonous green gas | Joins with Na to make NaCl — safe to eat |
In a particle diagram: different coloured circles joined together in a repeating pattern.
A mixture is two or more substances physically combined, with no chemical bond between them. Each part keeps its own properties.
- Air = mostly nitrogen (78%) + oxygen (21%) + other gases. Any ratio works — the percentages change with altitude.
- Saltwater = water + salt mixed in. You can add more salt or less salt — still a saltwater mixture.
- Beach sand = silicon dioxide grains + tiny shell fragments + bits of organic material. A clear heterogeneous mixture.
- Brass = copper + zinc (an alloy — a metal mixture).
Key differences from compounds:
| Question | Compound | Mixture |
|---|---|---|
| Are the parts bonded? | Yes — chemically | No — just mixed |
| Fixed ratio? | Yes — always the same | No — any ratio |
| Easy to separate? | Hard — needs chemistry | Easy — physical methods |
| New properties? | Yes — completely new | No — each part keeps its own |
In a particle diagram: different shapes/colours floating freely, not joined together.
Symbols and formulas are the shorthand of chemistry. Three rules to remember:
- First letter is capital, second is lowercase. So Cobalt = Co (not CO, which is the compound carbon monoxide!).
- The subscript number tells you how many of that atom. H₂O = 2 H + 1 O. CO₂ = 1 C + 2 O.
- No number means one. NaCl = 1 Na + 1 Cl.
| Formula | Name | Atom count |
|---|---|---|
| H₂O | Water | 2 H, 1 O = 3 atoms total |
| CO₂ | Carbon dioxide | 1 C, 2 O = 3 atoms total |
| NaCl | Sodium chloride (salt) | 1 Na, 1 Cl = 2 atoms total |
| CH₄ | Methane (natural gas) | 1 C, 4 H = 5 atoms total |
Notice that swapping the capital and lowercase changes the meaning entirely. CO = carbon + oxygen (the toxic gas carbon monoxide). Co = cobalt, a metal element. One small typo, a totally different substance.
The formula H₂O tells us that one water unit has hydrogen atoms and oxygen atom. The small number is called a . The total number of atoms is .
Wrong: "Saltwater is a compound because salt and water are bonded." Saltwater is a mixture — the salt particles spread out among water molecules but are not chemically bonded. You can add more salt or less salt and it's still saltwater.
Right: Saltwater is a mixture. The salt dissolves in the water but the two are not chemically bonded. The ratio is not fixed.
Wrong: "H₂O and O₂ are both elements because they both contain oxygen." O₂ is an element (only oxygen atoms). H₂O is a compound (oxygen AND hydrogen, chemically bonded). They are very different.
Right: An element has only ONE type of atom. O₂ qualifies. H₂O has two types — so it is a compound.
Wrong: "Air is a compound because it's a gas mixture you can't see the parts of." Just because the parts are invisible doesn't make it a compound. Air has variable composition (more nitrogen at sea level, less oxygen at altitude) and the gases are not chemically bonded.
Right: Air is a mixture of gases — mostly nitrogen and oxygen. It has no fixed ratio and the gases are not chemically bonded.
Someone shows you a particle diagram with 6 white circles and 12 black circles all randomly mixed and floating freely (not joined to each other). Then a second diagram shows 6 units, each made of one white circle stuck to two black circles. Predict: which diagram shows a compound and which shows a mixture? Explain in 1–2 sentences, then reveal.
How close was your prediction?
Nice — you spotted that "joined together in a fixed ratio" is the compound signature.
Good — particle diagrams take practice. The "joined vs not joined" check is the trick.
At the start of this lesson you were asked: Hydrogen burns. Oxygen lets things burn. Combine them — and you get water that puts fires out. Why? How does that feel now?
Now write a full explanation using the words element, compound and properties. Explain why a compound can have completely different properties from the elements that made it.
Q1. Define element, compound and mixture. Give one example of each. (3 marks)
Q2. Classify each as element, compound or mixture, with a brief reason: pure copper wire, salt water, water (H₂O), air. (4 marks)
Q3. A student says "Compounds and mixtures are basically the same — both have more than one element." Evaluate this claim. Use the fixed-ratio idea and one specific example to argue your point. (4 marks)
Answers
▾MCQ 1
B — An element is made of only one type of atom. A is a mixture. C describes a compound. D is unrelated.
MCQ 2
D — CO₂ is a compound (2 elements bonded in fixed ratio). Iron is an element. Air and beach sand are mixtures.
MCQ 3
C — CH₄ = 1 carbon + 4 hydrogens = 5 atoms in total.
MCQ 4
A — When elements bond into a compound, they take on completely NEW properties. Salt does not behave like either of the dangerous starting elements.
MCQ 5
C — Free-floating (not bonded) + any ratio = mixture. Two different particle types means it is a mixture of two elements.
Short Answer 1
Model answer: An element is a substance made of only one type of atom — example: gold (Au). A compound is two or more elements chemically bonded in a fixed ratio with new properties — example: water (H₂O). A mixture is two or more substances physically combined with no chemical bond and no fixed ratio — example: air.
Short Answer 2
Model answer: Pure copper wire = element (only Cu atoms). Salt water = mixture (salt and water are not bonded; ratio varies). Water (H₂O) = compound (H and O bonded in fixed 2:1 ratio with new properties). Air = mixture (mostly N₂ and O₂ gases, not bonded, variable ratio).
Short Answer 3
Model answer: The claim is wrong. Both compounds and mixtures can contain more than one element, but that's where the similarity ends. In a compound, the elements are chemically bonded in a fixed ratio — every unit of water is exactly 2 H bonded to 1 O. In a mixture, the substances are physically combined only, and can be made in any ratio — saltwater can have a little salt or a lot. Compounds also have completely new properties (like salt being safe to eat even though sodium and chlorine are dangerous), but each part of a mixture keeps its own properties (you can still taste the salt in saltwater). So compounds and mixtures are very different.