Mixtures and Solutions
Stir sugar into water and it vanishes. It has not gone anywhere, and being able to say where it went is most of this topic.
What a mixture is
A mixture is two or more substances together that are not joined chemically.
Nothing new has been made, so the parts keep everything that made them what they were and can always be taken apart again. Tip sand and salt into the same jar and you have not made a third substance. You have sand and salt, in a jar.
That covers far more than it sounds like it should. Air is a mixture of gases, sea water is a mixture of water and dissolved salts, and almost everything a child can point at in a room is a mixture rather than a pure substance. Pure things are the rare case.
What dissolving actually does
Stir a spoon of sugar into warm water and watch it go. It gets smaller, then cloudy, then there is nothing there at all, and every child who sees that concludes the sugar has been destroyed. Given the evidence in front of them, that is a fair conclusion. It is also wrong.
What actually happened is that the sugar particles came apart from each other and slotted into the gaps between the water particles, spread so evenly and so thinly that no eye can pick one out. Every last one of them is still in the glass. Taste it. That sweetness has to be coming from somewhere, and weighing the glass before and after settles the argument for good.
Three words worth getting right
The solute is what dissolves, the sugar.
The solvent is what does the dissolving, the water.
Together they make a solution, a mixture in which everything is spread so evenly that the parts cannot be seen at all.
Soluble and insoluble
A substance that dissolves in a liquid is soluble in it. One that does not is insoluble.
- Salt and sugar are soluble in water.
- Sand, chalk and flour are insoluble in water. Stir them in and they cloud the water, then settle.
- Solubility depends on the liquid. Some things that will not dissolve in water dissolve easily in other solvents.
What makes something dissolve faster
Warmth, stirring and smaller pieces, all three working the same way by giving solute particles more chances to meet solvent particles. Be careful with what that means, because it is a favourite exam trap: those three change how FAST something dissolves, not how much of it can dissolve in the end.
Separating a mixture
Nothing chemical happened when the mixture was made, so nothing chemical is needed to undo it. Three methods cover almost every question asked at this level, and choosing between them comes down to a single question.
Choosing the method
One question decides it
Did it dissolve?
No, so it is still solid bits floating about: filter it. The paper traps the bits and the liquid passes through.
Yes, and you only want the solid back: evaporate the liquid and the solid is left behind.
Yes, and you want the liquid back too: distil it, so the vapour is cooled and collected.
Get that first question wrong and everything after it goes wrong too. Filtering sea water is the classic example: it feels like the obvious method, and it achieves absolutely nothing, because dissolved salt slips through the paper alongside the water. More marks are lost to that one answer than to anything else on this topic.
Common mistakes
1. Saying the sugar disappeared
Saying the sugar disappeared, or worse, that it melted away. It spread out between the water particles and stayed there. Weigh the glass before and after and the mass has gone up by exactly the mass of the sugar you added, which is not what happens to things that have been destroyed.
2. Trying to filter a solution
Reaching for filter paper whenever a question mentions separating. Filtering works by trapping bits that are bigger than the holes in the paper, and dissolved particles are nowhere near big enough, so they sail through with the water. If it dissolved, filtering is not the answer.
3. Confusing dissolving with melting
Using melt for both, which everyday speech encourages. Melting needs heat and turns a solid into a liquid version of itself, so melted ice is still water and melted chocolate is still chocolate. Dissolving needs a second substance to spread into, and what comes out is a mixture. Ice melts. Sugar dissolves.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
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For parents
Dissolve a spoon of salt in a little warm water, pour it into a saucer, and leave it on a sunny windowsill for a few days. Nobody has to do anything else. The water leaves, and the salt comes back as crystals your child can scrape up with a fingernail. After that, the claim that dissolving destroys nothing is not something they have to take your word for.
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