A mixture is two or more substances mingled together but not chemically joined. Each keeps its own properties, the amounts can vary, and the parts can be separated by physical means. A compound is the opposite: atoms joined in a fixed ratio, with new properties.
You breathe in oxygen and nitrogen with every breath, yet only the oxygen keeps you alive. In air, oxygen still behaves exactly like oxygen.
Elements are one type of atom; compounds are different atoms joined. This lesson looks at substances that are only mixed.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: oxygen in air
ExploreTry it on the board and compare what happens with your prediction.
Explore: what's in a mixture?
Click particles, or press Inspect next particle. Are any of them joined to each other?
Salt water is a mixture too. Are the salt particles changed, or just spread out?
Try it yourselfNothing is saved: start again as often as you like.
Inspect every kind of particle in air.
Use the board. This ticks when you get there.
Make a mixture: dissolve some salt in water.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: mixtures and compounds
In your book: draw particle diagrams of an element, a compound and a mixture. Under the mixture, write two ways it differs from the compound.
Mixture or compound?
Each part keeps its own properties
New properties, unlike its elementsSalt is not poisonous like chlorine.
The amounts can be anythingWeak or strong cordial is still cordial.
A fixed ratio of atomsWater is always H₂O.
Separated by filtering, evaporating or distilling
Separated only by a chemical reaction
Element, compound or mixture?
Oxygen
Water
Air
Salt water
Carbon dioxide
Copper
Three kinds of mixture. Match each to an example and what it does.
Solution
Suspension
Colloid
Elaborate TypicalUse the same idea in a new context.
Elaborate: any amount
Try it yourselfNothing is saved: start again as often as you like.
Inspect the mixture with lots of hydrogen.
Use the board. This ticks when you get there.
Now the mixture with lots of oxygen.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: humid air
Try it yourselfNothing is saved: start again as often as you like.
Inspect every kind of particle in humid air.
Use the board. This ticks when you get there.
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
A mixture is two or more substances mingled together but not chemically joined. Each keeps its own particles and properties.
A mixture's parts can be in any amounts; a compound always has a fixed ratio (H₂O).
Mixtures can be separated by physical means (filtering, evaporating, distilling); compounds only by chemical reactions.
Air and salt water are mixtures; water and carbon dioxide are compounds.
For teachers
Students define a mixture, explain that its parts keep their properties, vary in amount and can be separated physically, and distinguish mixtures from compounds (AC9S8U06, node 4). About 24 minutes.
I can sort substances into elements, compounds and mixtures.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It comes after Why isn't salt poisonous? (compounds) and links back to Year 7 separating mixtures.
Not a hinge node, but the element–compound–mixture distinction is where Year 8 students most often slip: "air is a compound", "a mixture has fixed amounts", "anything with two kinds of atom is a compound" (humid air). The prediction and the Wider step target these.
Board notes: gases in a box, enlarged and in 2D; air's proportions are rounded (about 78% nitrogen, 21% oxygen, 1% argon, a trace of carbon dioxide). The dissolve board's salt particles stand for sodium and chloride ions.
Running the lesson
Engage: tally the predictions. 'Turned into a new substance' is the misconception to watch.
Explore: ask whether any particle on the air board was joined to a different kind.
Explain: the two sorts are the success criteria.
In your own words: the three particle diagrams are the evidence.
The hard part (on purpose)
Telling a mixture from a compound when both contain two kinds of atom. Ask 'are the different atoms joined in one particle, or in separate particles?' and let the student look before answering.
When students get stuck
A student says air is a compound. Ask: Is any nitrogen atom joined to an oxygen atom on the board? Fix: No: they are separate particles, mingled. Different substances not joined make a mixture.
A student thinks a mixture has a fixed recipe. Ask: Is weak cordial still cordial? Fix: Yes. A mixture's parts can be in any amounts; only compounds have a fixed ratio.
A student says humid air is a compound because it contains water. Ask: Is the water joined to the nitrogen? Fix: No. It is a mixture that happens to include a compound.
Stretch and support
Stretch: Brass is a mixture of copper and zinc in different amounts for different uses. Why can it be called a mixture, even though it looks like one metal? (The atoms are mingled, not in a fixed ratio, and each metal's atoms are unchanged.)
Support: Give the three questions as a checklist: Joined? Fixed amounts? Separated physically?
Where it leads
Next: Symbols, formulas or models? (representations, node 5).
In class: iron filings and sulfur mixed (a magnet separates them) versus heated to iron sulfide (it doesn't); making and evaporating salt water.
What each step is for
Engage: oxygen in air: Students who think air is a compound (or that mixing makes a new substance) pick those options; 'each keeps its properties' is the idea to build.
Explore: what's in a mixture?: Inspecting air, then dissolving salt, shows mixtures at the particle level: different particles side by side, none changed.
Explain: mixtures and compounds: Contrasting mixtures with compounds on three tests (properties kept, amounts vary, separate physically) makes the distinction a reasoned one.
Elaborate: any amount: Two mixtures of the same gases in different proportions show variable composition, the clearest contrast with a compound's fixed ratio.
Elaborate: humid air: A mixture that includes a compound (water vapour in air) checks that 'mixture' is about how substances are combined, not what they are.
Now you can
Explain what a mixture is.
Tell a mixture from a compound.
Sort substances into elements, compounds and mixtures.