Example 1: reading a curve
How much potassium nitrate dissolves in 100 g of water at 60 °C?
- Find 60 °C on the x-axis.
- Read up to the curve and across: about 110 g.
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IB DemystifiedMYP Sciences
Solutions and solubility
Sugar vanishes into tea, salt is left behind when sea water dries, and fizzy drinks go flat when warm. Solubility explains how much of a substance can dissolve, how fast, and how temperature changes it.
Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Dissolving: how much v how fast
- Solubility in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- use the terms solute, solvent, solution, soluble, insoluble and saturated
- read and interpret solubility curves
- explain factors that affect how fast a solid dissolves
- calculate simple concentrations and masses of crystals
- explain why gases dissolve less in warm water
- discuss warm river water and salty farmland
Before you start
You will use these skills. If any feel shaky, review them first.
- mixtures and separation (see Pure substances, mixtures and separation)
- particles in solids and liquids (see States of matter)
- reading line graphs
Key vocabulary
- Solute
- The substance that dissolves.
- Solvent
- The liquid that does the dissolving.
- Solution
- The mixture formed when a solute dissolves in a solvent.
- Solubility
- The maximum mass that dissolves in 100 g of solvent at a given temperature.
- Saturated solution
- A solution that cannot dissolve any more solute at that temperature.
- Concentration
- How much solute is dissolved in a certain volume of solution.
Understanding the ideas
-
What is it?
When a solute dissolves, its particles spread out between the particles of the solvent. The solute has not disappeared: the mass of the solution equals the mass of solute plus solvent.
-
Why does it happen?
There is a limit to how much can dissolve: the solubility. For most solids, solubility rises with temperature, sometimes steeply (potassium nitrate) and sometimes hardly at all (salt). Gases do the opposite: they are less soluble in warm water.
-
How do we know?
Solubility curves show these patterns, and let us calculate how many crystals form when a hot saturated solution cools. Stirring, heating and crushing make solids dissolve faster, but do not change how much can dissolve.
-
Why does it matter?
Solubility matters for medicines such as oral rehydration solution, for fish that need dissolved oxygen, and for farmland where evaporation leaves salts behind.
-
What does it connect to?
This topic links to separation methods and crystallisation, to acids and salts later in chemistry, and to water pollution in environmental science.
Dissolving: how much v how fast
- How much dissolves depends on the solute, the solvent and the temperature (the solubility).
- How fast it dissolves depends on stirring, temperature and particle size (surface area).
- Crystals from cooling = mass dissolved when hot − mass that can stay dissolved when cold.
- Gases are less soluble in warm water.
- Mass is conserved: mass of solution = mass of solute + mass of solvent.
Solubility in the real world
Oral rehydration solution must be made at the right concentration to treat diarrhoea safely. Salt-affected soils in parts of the Indus plains show how evaporation leaves dissolved salts behind. Fish kills can happen in hot summers, when warm water holds less oxygen.
Worked examples
Example 2: crystals on cooling
A saturated potassium nitrate solution in 100 g water cools from 40 °C to 20 °C. What mass crystallises?
- At 40 °C: 64 g dissolved. At 20 °C: 32 g can stay dissolved.
- 64 − 32 = 32 g of crystals.
Assessment tips
Solubility questions often use curves and simple calculations. Expect to:
- Read solubility curves and compare substances.
- Calculate masses of crystals and concentrations.
- Explain dissolving using particles.
- Plan fair tests on dissolving.
Common mistakes: saying sugar melts in tea; mixing up how fast with how much; forgetting that gases are less soluble when warm; and reading the wrong curve.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: growing crystals
Partially guided investigation · about 40 minutes, then observe over several days · pairs
- Research question
- Does cooling a hot saturated solution slowly or quickly give larger crystals?
- Scientific background
- A hot saturated solution holds more solute than it can when cold, so crystals form as it cools. Slow cooling usually lets fewer, larger crystals grow.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Potassium alum or copper sulfate, hot water (about 60 °C), two beakers, stirring rod, filter paper, a warm cloth or box for slow cooling, an ice bath for fast cooling, hand lens.
- Method
- Dissolve the solid in hot water, stirring, until no more will dissolve (saturated).
- Filter the hot solution into two identical beakers.
- Cool one slowly (wrapped in cloth) and one quickly (in an ice bath).
- After a day, compare the size and number of crystals with a hand lens.
Safety. Wear eye protection and gloves. Copper sulfate is harmful: do not touch it with bare hands, and wash hands afterwards. Take care with hot solutions.
Then evaluate: how could you measure the crystal sizes fairly?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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