Lesson focus: solubilityHow much of a substance can dissolve in a certain amount of liquid. is a capacity under stated conditions, not the speed of dissolving.
Interpret solubility data and calculate crystallisationGetting solid crystals out of a solution by letting the liquid evaporate slowly, like making salt from salty water. on cooling.
Build on earlier learning
SolutionsExtend solution language to saturation and quantitative capacity.
Practical
How much solute dissolves
- Add one spatula of salt at a time to 50 cm³ of water, stirring until it dissolves.
- Stop when solid remains: the solution is saturated.
- Repeat with warmer water and compare the number of spatulas.
Record: A table of temperature against spatulas dissolved and a conclusion about solubility.
You will be successful when you can…
- Define solubility and saturation.
- Read a solubility curve.
- Distinguish rate from capacity.
- Calculate crystals produced on cooling.
Words that carry the science
- solubility
- Maximum solute mass dissolving in a stated solvent amount under stated conditions.
- saturated solution
- Solution containing the maximum dissolved solute under stated conditions.
- unsaturated solution
- Solution able to dissolve more solute under stated conditions.
- crystallisation
- Formation of solid crystals from a solution.
- dissolving rate
- How quickly solute enters solution.
Read capacity from axes, units and temperature
01Units matter
A common unit is grams solute per 100 g water at a stated temperature. Read temperature and solvent mass before using data.
02Saturated is a boundary
Points on the curve are saturated; below are unsaturated. Above implies excess solid or supersaturation, depending on conditions.
03Rate is not solubility
Stirring and smaller grains can increase rate without changing final capacity. Temperature may change both, so experiments must measure them separately.
Solubility curves
Add solid to 100 g of water. Below the curve it all dissolves; on the curve the solution is saturated; any more stays as solid. Press play to cool the solution and watch crystals form.
Controls
Data from standard solubility tables. Sodium chloride hardly changes with temperature, so cooling gives almost no crystals.
Crystals on cooling
solubilityHow much of a substance can dissolve in a certain amount of liquid. is 80 g/100 g water at 60 °C and 30 g/100 g water at 20 °C. A saturated solutionA solution that cannot dissolve any more of the solid at that temperature. contains 100 g water.
- At 60 °C, 80 g is dissolved.
- At 20 °C, only 30 g remains dissolved.
- Difference = 80 − 30.
- 50 g crystals can form, assuming no water loss.
How to make the reasoning visible: Use the same solvent mass at both temperatures.
Solubility dataset
Values are illustrative and use g solute per 100 g water.
| Temperature / °C | Solubility / g | 40 g added | Status |
|---|---|---|---|
| 20 | 30 | 40 g | 30 g dissolved; 10 g excess |
| 40 | 50 | 40 g | Unsaturated |
| 60 | 80 | 40 g | Unsaturated |
| 80 | 110 | 110 g | Saturated |
At 20 °C, how much remains undissolved from 40 g?
Does stirring change the 20 °C solubility?
Use the dataset. What happens to the solubility of this solid as the temperature rises?
Check your understanding
Answer from memory, then check against the suggested answer.
What is solubility?
What is saturated solution?
What is unsaturated solution?
What is a saturated solution?
Does stirring change how much solute can dissolve?
A hot saturated solution cools without water loss. Why do crystals form?
How many grams crystallise if 50 g water cools between the example temperatures?
A hot saturated solution is cooled. Why do crystals appear?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
The solubility of a salt falls from 70 g to 25 g per 100 g of water as a solution cools. The saturated solution contains 200 g of water.
Calculate the mass of crystals that form. Show your working.
(4)Total for this question: 4 marks
Command word: Write the capacity change first, then scale for solvent mass.