ACARA v9 CONTENT DESCRIPTION “use a particle model to describe differences between pure substances and mixtures and apply understanding of properties of substances to separate mixtures”
Builds on the particle model of matter, where every substance is pictured as tiny moving particles. Here that model answers two questions: what is the difference between a pure substance and a mixture, and how can the parts of a mixture be separated again.
Pure substances and mixtures, seen as particles
A pure substance is made of only one kind of particle, the same throughout. A mixture is two or more substances jumbled together, their particles intermingled but not joined. Because the substances in a mixture keep their own properties, the mixture can be taken apart again, which is something a pure substance cannot do.
Pure or mixed?
Zoom in to the particles. What makes a substance pure, and what makes it a mixture, is what those particles are.
A pure substance is made of only one kind of particle, all the same throughout. With nothing else mixed in, there is no other substance to separate out.
Separating a mixture, one property at a time
To separate a mixture we look for a property that one part has and the others do not. Iron is magnetic, so a magnet removes it. Sand does not dissolve and its grains are large, so filtering catches it. Salt dissolves but has a far higher boiling point than water, so evaporating the water leaves the salt behind. Each step targets a single component.
Separating a mixture
Iron filings, sand and dissolved salt are all in water. Each tool removes just one of them, by using a property only that part has.
A magnet, a filter and evaporation each use a different property. Apply them in any order and watch what is left after each one.
Matching the method to the property
No single method separates everything. The right tool depends on how the parts differ: magnetism, particle size and being insoluble, or boiling point. Choosing the method that fits each property is the heart of separating mixtures, and the order can be chosen to remove one substance at a time.
Each method uses a different property
Separation works because the parts of a mixture differ. Pick a method to see which property it relies on.
Magnet works by using magnetism, a property that only the iron filings has. Because each component differs in some property, we can target them one at a time.
Solutions and suspensions are not the same
When a solid dissolves, it splits into particles far too small to see and spreads evenly, so the liquid stays clear and never separates on its own. That is a solution. A solid that does not dissolve stays as visible bits held in the water, making it cloudy. That is a suspension, and if you leave it to stand the bits settle to the bottom.
Solution or suspension?
Stir two different solids into water. One dissolves and one does not. Then leave each to stand.
A substance that dissolves breaks up into particles far too small to see and spreads evenly, so the liquid stays clear and mixed. Left to stand, it never separates on its own.
Recovering the water as well
Evaporation leaves the salt behind but loses the water to the air as vapour. To keep the water too, distil the solution: heat it so pure water boils off as vapour, then cool that vapour against a cold surface so it condenses back to liquid water and is collected. The salt, with its much higher boiling point, stays in the flask, and no particles are lost.
Distillation recovers the water too
Evaporation alone keeps the salt but loses the water as vapour. Add a cold surface to catch that vapour and you keep both.
We start with salt dissolved in water. Evaporating alone would leave the salt but let the water escape as vapour. Distillation catches that vapour so both are kept. Press Heat to begin.
Why this matters
Separating mixtures is everywhere, from making clean drinking water to refining metals and recycling materials. Once you can picture a mixture as different substances keeping their own properties, you can work out which property to use to pull each one out.
Quick self-check
1. At the particle level, what makes a substance pure?
2. Which method removes the iron filings from the mixture?
3. Filtering separates the sand from the salty water because the sand...
4. How is the dissolved salt finally recovered from the water?
5. Why can the parts of a mixture be separated, but a pure substance cannot?