Solutions, Solute, Solvent, Concentration
You'll be able to label the solute and solvent in any solution you're shown.
In 1884, Australian chemist Archibald Liversidge measured Sydney Harbour water and found it held approximately 35 g of dissolved salts per litre, the first precise seawater concentration measurement recorded in Australia.
Printable Worksheets
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Q1 · When you stir sugar into tea and it disappears, where do you think the sugar actually goes? Is it gone forever?
Q2 · A bottle of "concentrated cordial" tells you to add 1 part cordial to 4 parts water. What does "concentrated" mean here, and what would "dilute" mean for the finished drink?
● Know
- Solute = what dissolves; solvent = what does the dissolving
- Dilute vs concentrated; saturated solution
- An aqueous solution has water as the solvent
● Understand
- Dissolving at the particle level, solute particles slip between solvent particles
- Why dissolving is a physical change, not a chemical one
- Why hot water dissolves more sugar than cold water
● Can do
- Label the solute and solvent in any solution you're shown
- Describe a solution as dilute, concentrated or saturated
- Draw a quick particle-level sketch of dissolving
- Solute
- Solvent
- Aqueous
- Concentration
- Saturated
- How much solute is dissolved in a given amount of solvent
- The substance that dissolves
- Holding the maximum solute at that temperature
- A solution where water is the solvent
- The substance that does the dissolving
Stir a spoonful of sugar into warm water and watch, the white crystals slowly disappear until the liquid is completely clear. The sugar hasn't vanished; it's spread among water molecules in a new kind of mixture called a solution. Every solution has two roles:
- Solute the substance that dissolves (usually the smaller amount).
- Solvent the substance that does the dissolving (usually the larger amount).
| Solution | Solute | Solvent |
|---|---|---|
| Salt water | Salt (NaCl) | Water |
| Sweet tea | Sugar | Water (tea) |
| Fizzy drink (before going flat) | CO₂ gas + sugar | Water |
| Brass (alloy) | Zinc | Copper |
| Air | Oxygen + other gases | Nitrogen |
When the solvent is water, the solution is called aqueous. Water dissolves so many different substances that it's nicknamed "the universal solvent". Most of school chemistry happens in aqueous solutions.
Text description
- Solute: what dissolves, here salt.
- Solvent: what it dissolves in, here water.
- Solute + solvent → solution, a homogeneous mixture.
- Dilute: few solute particles. Concentrated: many solute particles.
- Concentration = the amount of solute in the solution.
In a solution, the is what dissolves and the is what does the dissolving. When the solvent is water, the solution is called . Water dissolves so many substances it is nicknamed the universal .
The sugar doesn't disappear when it dissolves, it just gets spread out so much you can't see it.
- The solid sugar grain is packed full of sugar particles held tightly together.
- Water particles bump into the sugar grain from all sides.
- The water particles knock individual sugar particles loose, one at a time.
- The freed sugar particles slip into the gaps between water particles.
- Eventually every sugar particle is surrounded by water, they're now spread evenly through the whole drink.
This is why dissolving is a physical change, not a chemical one. Each sugar particle is still a sugar particle, it's just no longer stuck to its neighbours. If you boil the water away, the sugar particles will come back out and form crystals again.
Text description
- Before stirring: water particles and a sugar crystal of eight particles sitting together.
- After stirring: the eight sugar particles are spread between the water particles.
Three words you have to keep straight:
- Dilute only a small amount of solute dissolved in lots of solvent (weak cordial).
- Concentrated a lot of solute dissolved in not much solvent (strong cordial straight from the bottle).
- Saturated the solvent is holding the maximum amount of solute it can dissolve at that temperature. Any extra solute will sit undissolved at the bottom.
- Supersaturated a special, unstable case that holds more solute than the normal maximum. You make one by saturating hot water, then cooling it gently so the extra solute stays dissolved for a while.
| Solution (100 mL water) | Sugar dissolved | Type |
|---|---|---|
| A | 1 g | Very dilute |
| B | 50 g | Concentrated |
| C | ~200 g (at 20°C, max ~204 g) | Almost saturated |
| D | 250 g added, but about 46 g sits at the bottom | Saturated (with excess undissolved) |
Concentration is about amount; saturation is about reaching the limit.
How do you get a supersaturated solution? Hot water dissolves more solute than cold water, so you saturate the solution while it is hot, then cool it slowly without knocking it. The extra solute stays dissolved, but the moment a tiny seed crystal drops in or the glass is bumped, the excess crystallises out all at once. This is how rock candy is grown and how some reusable hand-warmers release their heat.
Wrong: "When sugar dissolves, it disappears and is gone." The sugar is still there, it's just spread out as individual particles between the water particles. Taste the water and you'll still taste sweetness; boil it off and the sugar comes back as crystals.
Right: Dissolved sugar is still sugar, it's just too spread out to see. Dissolving is a physical change, so the solute can always be recovered.
Wrong: "Concentrated and saturated mean the same thing." Concentrated just means there's a lot of solute, but you might still be able to add more. Saturated means you have hit the limit and no more can dissolve.
Right: Concentrated = a lot of solute. Saturated = the maximum the solvent can hold. A saturated solution is always concentrated, but a concentrated solution isn't always saturated.
Wrong: "Water can only be a solvent for solids." Water dissolves solids (salt), liquids (vinegar) AND gases (oxygen, that's how fish breathe). Don't limit "dissolving" to solids only.
Right: A solvent can dissolve solids, liquids and gases. Fizzy drinks contain dissolved CO₂; rivers contain dissolved oxygen.
Most solids dissolve more in hot water than in cold water. At 20°C, about 204 g of sugar dissolves in 100 mL of water. At 80°C, about 362 g dissolves. That's nearly double.
The particle-level reason: in hot water the water particles are moving much faster. They bump into the sugar grains harder and more often, so they knock solute particles loose more easily, and they carry the freed particles away from the grain sooner, which keeps fresh water in contact with the surface that is still dissolving.
Speed and total are two different questions, and it is worth keeping them apart, because students who blur them get caught out in Lesson 10. Harder, more frequent collisions make the sugar dissolve faster. Stirring and crushing do the same thing: they get you to the finish sooner. Heating does something they cannot, which is to move the finish line itself, and the evidence for that is a measurement rather than an argument — 100 mL of water holds about 204 g of sugar at 20 °C and about 362 g at 80 °C. Why a higher temperature raises that limit is a genuinely harder question than it looks, and the honest answer is that it depends on how strongly the sugar particles are held by water compared with how strongly they are held in the crystal, which is Year 11 chemistry. Be careful of the tempting shortcut that hot water has "more room" for sugar: warming water from 20 °C to 80 °C expands it by under 3 per cent, which is nowhere near enough to explain the speed or the limit. The speed is the collisions. What you need here is the fact, the measurement behind it, and the habit of not confusing faster with more.
This is why a barista can pack a lot more sugar into a hot coffee than an iced coffee, and why iced drinks are usually sweetened with syrup (already dissolved) instead of grains.
Warning this rule is for solid solutes. Gas solutes behave the opposite way (you'll meet that in Lesson 10).
A student dissolves 50 g of sugar in 100 mL of hot water. They then let it cool to room temperature without stirring. Predict: what do you think will happen to the dissolved sugar as the water cools? Will it stay dissolved or come back out?
How close was your prediction?
Nice, you saw that the cold solvent can't hold as much, so the extra solute crystallises out.
Good, this is the surprise at the heart of saturation. Solubility depends on temperature.
Earlier you were asked: Where does sugar go when it dissolves? What do "concentrated" and "dilute" mean?
Now write a fuller answer using the words solute, solvent and particles.
Q1. Define solute, solvent and aqueous solution. Use one everyday example to illustrate each. (3 marks)
Q2. Explain at the particle level what happens when sugar dissolves in water. Use 3–4 sentences and refer to both solute and solvent particles. (4 marks)
Q3. A student says: "If I keep adding sugar to my coffee, it will keep dissolving forever." Evaluate this claim. Use the words saturated and temperature, and describe what would happen if the coffee then cooled. (4 marks)
Answers
▾MCQ 1
B Water (tea) is the solvent (the larger amount, doing the dissolving). Sugar is the solute.
MCQ 2
D Saturated. Dilute = small amount of solute; aqueous = water solvent.
MCQ 3
A Dissolving is a physical change. Sugar particles spread out between water particles but stay as sugar. Boil the water away and they reappear.
MCQ 4
C20 g per 100 mL = 200 g per litre. That's far more solute per unit solvent than any other option.
MCQ 5
B In hot water, particles move faster. They knock solute particles loose more often and carry the freed particles away from the grain, so fresh water keeps meeting the surface and the sugar disappears sooner. Note the question is about speed. The total the water can hold rises with temperature as well, but that is a separate mechanism, and it is not extra room between the water particles.
Short Answer 1
Model answer: A solute is the substance that dissolves (e.g. salt). A solvent is the substance that does the dissolving (e.g. water). An aqueous solution is one where water is the solvent, for example, salt water or sugary tea.
Short Answer 2
Model answer: When sugar is added to water, the water particles bump into the sugar grain from all sides. Each collision knocks individual sugar particles loose from the grain. The freed sugar particles slip into the gaps between the moving water particles. Eventually every sugar particle is surrounded by water, spread evenly through the cup, the sugar is dissolved.
Short Answer 3
Model answer: The claim is wrong. At any given temperature, the solvent can only hold a certain maximum amount of solute, once you reach that limit the solution is saturated and no more sugar will dissolve. Extra sugar will sit undissolved at the bottom. However, raising the temperature increases the limit, so hot coffee can hold more sugar than cold coffee. If the coffee then cools, the limit drops and some of the dissolved sugar will crystallise back out of solution.