Mixtures and separation · Model
Dissolving and solutions
Sugar stirred into water vanishes without a trace. Where has it gone, and what decides how much of it can go there?
Start here
Stir sugar into water and it disappears. Nothing else in science does that.
100 g of water in a beaker on a balance. 10 g of sugar tipped in. Stir until the last grain vanishes and you cannot see any sugar anywhere.
What does the balance read now?
110 g. Every gram of sugar is still in the beaker — you just cannot see it, because it has been broken up into particles far too small to see and spread evenly through the water. Nothing was destroyed and nothing was made. Dissolving hides a substance; it does not remove it.
The substance that dissolves is the solute. The liquid it dissolves in is the solvent. Together they make a solution — a mixture, spread so evenly that every spoonful is the same.
A solute that dissolves is soluble; one that does not is insoluble. And there is a limit: keep adding solute and eventually no more will go in. The solution is then saturated.
Predict first
The bench below has four dials: what you are dissolving, how hot the water is, whether you stir, and whether the solid is a lump or a powder. Two of them change how fast it dissolves. One of them also changes how much can dissolve at all.
Which dial changes how much will dissolve?
Your turn · the dissolving bench
100 g of water. Four dials. Watch which readout moves.
What you are dissolving
Water temperature
Stirring
The solid
How much dissolves in 100 g water
36.4 g
Changed only by the solute and the temperature.
Time for 10 g to disappear
90 s
Changed by temperature, stirring and how finely ground it is.
The solution
clear
Transparent, and coloured only if the solute is coloured.
In the beaker, particle by particle
Salt on the bottom of the beaker, unchanged. Stirring lifts it up; it settles again.
Salt is soluble in water. Sodium chloride. Almost the same amount dissolves however hot the water is. Leave it standing as a lump and it still gets there — it just takes longer.
You have run all three temperatures. Stirring and grinding changed the clock and never changed the grams. Heating changed both — and it changed the grams by a lot for sugar and hardly at all for salt, which is a warning against the phrase "hot water dissolves more". How much depends on the solute and the temperature. Nothing else on this bench moves it.
Key fact
Dissolving spreads a solute through a solvent as particles too small to see. Stirring and grinding change how fast. Temperature changes how much — and by how much depends on the solute. For gases it runs the other way: warm the liquid and less gas stays dissolved, not more.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“The sugar melted into the water and turned into liquid.”
It went in as a solid and now there is only liquid. Commit before you read on.
Melting and dissolving are different events with different causes. Melting needs heat and needs nothing else present: sugar melts at about 186 °C, and it goes brown and turns into caramel while it does. Dissolving needs a solvent and happens perfectly well in cold water, at nothing like 186 °C.
And the sugar has not become water. Boil the solution dry and the sugar comes back, as sugar, weighing what it weighed. That is the difference between a mixture and a reaction: nothing new was made, so everything can be got back. You will get it back on purpose two lessons from now.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Salt is stirred into water until it disappears. Which is the solvent?
Rung 2 · The one that catches people
A saturated sugar solution has undissolved sugar on the bottom. A student stirs harder to get it in. What happens?
Rung 3 · Explain
10 g of sugar is stirred into 100 g of water and disappears completely. A student says the sugar has gone. Use the balance reading and the particle model to explain what has actually happened, and how you could get the sugar back.
Rung 4 · Take it somewhere new
A fizzy drink goes flat much faster when it is warm, and a river in a heatwave can suffocate its fish. Both are about a gas dissolved in water. Explain what these two facts have in common, and why "hot water dissolves more" is a rule you should not trust.
Key note
A solute dissolves in a solvent to make a solution — a mixture spread so evenly you cannot see the solute at all. Nothing is destroyed: the mass stays the same and the solute can be got back. Stirring and grinding change how fast it dissolves; temperature changes how much can dissolve, and by how much depends on the solute. When no more will go in, the solution is saturated.
Going further
Gases dissolve too, and they break the rule you have just learned. Warm a fizzy drink and it goes flat faster, because carbon dioxide becomes less soluble as the water gets hotter — the opposite of sugar. Cold water holds more dissolved oxygen than warm water, which is why a heatwave can kill the fish in a shallow river without anything being added to it at all.
So "hot water dissolves more" is not a law of nature; it is a rough rule about solids that has exceptions in both directions. Salt barely cares about temperature. Calcium sulfate gets slightly less soluble as it warms, which is why it plates out inside boilers and hot-water pipes. Any real solubility claim has to name the solute, the solvent and the temperature — which is exactly what the numbers on the bench were doing.
Before this lesson
At GCSE this becomes
- Solubility curves you read values off, and concentration in grams per cubic decimetre and in moles.
Where to next
Ask Mr Badmus AI
Still not sure why stirring does not get more sugar in?
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