KS3 · Chemistry
Solubility and saturated solutions
Stir sugar into water, spoon after spoon. Each one vanishes, until one stays at the bottom however hard you stir. Where does the water's limit come from?
Slide the dot along the line to about 75 °C. How much sugar can 100 cm³ of water hold?
Why is there a limit?
Reason it through
Why can a cup of water only hold so much sugar?
First link · your turn
When sugar dissolves and seems to vanish, what has happened to the sugar?
Predict, then check
Think about where the limit sits on the curve when the water is cooler.
A sugar solution is saturated at a high temperature. It is left to cool down. What happens to the sugar?
Chemistry · Fair tests
Set up a fair test
You are timing how long 2 g of sugar takes to dissolve in 100 cm³ of water at different temperatures. Sort each quantity, then read why it belongs there.
Still to sort
Independent variable (0)
The thing you change on purpose.
Where the line is: If you choose its values, it is independent. If you read it off an instrument after the test, it is dependent.
Dependent variable (0)
The thing you measure.
Where the line is: It is the result of the test, so it is never set beforehand.
Control variable (0)
Kept the same every time.
Where the line is: A control variable could change the result, so it must not change.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Keep stirring sugar into water and one spoonful will refuse to disappear. That is the water's limit, and we can draw it.
What you need to know
- Solubility is about how well a solute dissolves in a solvent to make a solution. Keep two ideas apart: how fast it dissolves, and how much dissolves.
- At a given temperature there is a limit to how much solute can dissolve. When that limit is reached the solution is saturated: the solvent is full.
- The solvent, its volume and its temperature decide how much can dissolve. Stirring, smaller pieces and a smaller mass of solute only change how fast.
- A solubility curve shows the most solute that dissolves in a stated volume of solvent at each temperature, so every point on it is a saturated solution.
- In a fair test you change one thing, measure one thing and keep the rest the same.
The big picture
At a given temperature there is a limit to how much solute can dissolve in a solvent. A solution that holds that much is saturated. The solvent, its volume and its temperature set the limit, while stirring and smaller pieces only change how fast the solute dissolves. A solubility curve plots the limit against temperature, so every point on the line is a saturated solution, and a fair-test investigation measures how changing one variable affects dissolving.
Key points
Worked example
Problem
A graph shows the solubility curves of three made-up solutes, P, Q and R, in 100 cm³ of water. At 40 °C, curve P reads 60 g, curve Q reads 35 g and curve R reads 20 g. Curve P also passes through 100 g at 60 °C. (a) Which solute is the most soluble at 40 °C, and which is the least? (b) At what temperature does 100 cm³ of water hold exactly 100 g of P?
⚠ Watch out
'Dissolves faster' is not the same as 'dissolves more'. Stirring harder or crushing the sugar gets you to the limit sooner, but it never moves the limit. Only the solvent, its volume and its temperature do that.
Memory hook
Full means no free hands: once every water particle is busy holding sugar, the extra stays in a pile. More water means more hands; hotter water means more energy to pull sugar particles apart. (Just a picture: particles don't have hands!)
Check yourself
A friend says: 'If I stir for long enough, any amount of sugar will dissolve in my cup of water.' What would you tell them, and which three things really decide how much can dissolve?
Flashcards
(15)What does 'solubility' describe?
What happens to the solute's particles when it dissolves?
What is a saturated solution?
Name three changes that only make a solute dissolve faster.
What decides how much solute can dissolve?
How can you raise the limit, and why does each way work?
What happens when a saturated solution cools?
Two ways to make a saturated solution?
What does a solubility curve show?
How do you read a solubility curve?
Several solutes are plotted on one graph. How do you find the most soluble?
Independent, dependent and control variables?
Which apparatus measures mass, volume, temperature and dissolving time?
What is an anomalous result and what do you do with it?
Scatter graph or bar chart?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
Learning with Lightbulb is opening soon
You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.
Keep me postedMore KS3 Chemistry topics
- Acids, bases and alkalis
- Boiling and condensing
- Changes of state: energy and evaporation
- Characteristics of chemical reactions
- Chemical formulae and symbols
- Chromatography
- Combustion
- Composition of the atmosphere
- Compounds and their formation
- Conservation of mass and balanced equations
- Displacement of metals
- Dissolving
How this lesson was checked. This KS3 Chemistrylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 2 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.