CHEMISTRY · Stage 8 · Chapter 8: Pure Substances and Solutions
Chemistry 8.7 Solubility — Saturated Solutions and Solubility Curves
5 min read 5 parts Lesson 7 of 10
Lesson summary
8.7 Solubility — Saturated Solutions and Solubility Curves — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define solubility and saturated solution, describe how solubility varies with temperature and interpret solubility data and curves.Solubility = the maximum …
- Unsaturated solution
- Comparison
- Line graphs
- Bar charts
- Points on the graph
- Solubility at a temperature
Section 1 of 5Solubility and saturation
- Solubility = the maximum mass of a substance that dissolves in 100 g of solvent (usually water) at a given temperature.
- Every substance has its own solubility.
- Saturated solution = a solution that contains the maximum mass of solute that will dissolve, so some undissolved solute always remains.
- Unsaturated solution
- a solution that can still dissolve more solute.
- Sugar
- more than 200 g of sugar dissolves in 100 g of water at room temperature, and extra sugar falls to the bottom.
- A 330 cm3 bottle of cola contains about 40 g of sugar, roughly 10 teaspoonfuls.
| Substance | Solubility at 20 °C (g/100 g of water) |
|---|---|
| Sugar | 202 |
| Salt | 36 |
- Comparison — sugar is more soluble than salt because more sugar than salt dissolves in 100 g of water.
Section 2 of 5Effect of temperature
- Solids
- for most solids, solubility increases as temperature rises, so more sugar dissolves in hot water than in cold water.
- The size of the increase is greater for some substances than others.
- Sugar
- solubility rises from 202 g at 20 °C to 476 g at 100 °C per 100 g of water.
- Salt
- solubility changes very little, from 36 g at 20 °C to 39 g at 100 °C.
- Exception
- on the graph of six substances, solubility increases with temperature for all except cerium(III) sulfate.
- Crystals
- a hot saturated solution deposits crystals as it cools, because solubility falls and the excess solute comes out of solution.
- Gases
- their solubility decreases as temperature rises, so a warm fizzy drink goes flat faster and warm lakes hold less oxygen for fish.
Section 3 of 5Solubility curves
- Solubility curve = a graph of solubility (g per 100 g of water) on the y-axis against temperature (°C) on the x-axis, with a separate curve for each substance.
- Line graphs
- they are used because both variables, solubility and temperature, are continuous.
- Bar charts
- they are used to compare substances because the independent variable, the substance, is categoric.
| Temperature (°C) | Potassium nitrate (g/100 g water) | Sodium chloride (g/100 g water) |
|---|---|---|
| 20 | 32 | 36 |
| 40 | 64 | 37 |
| 60 | 110 | 37 |
| 80 | 169 | 38 |
- Points on the graph
- a point on the line is a saturated solution, a point below it is unsaturated, and a point above it means excess solid.
- Drawing
- draw a smooth curve, not dot to dot, and label the axes with units.
Section 4 of 5Using solubility curves
- Solubility at a temperature
- read the value from the curve.
- Dissolving temperature
- find the temperature at which a given mass just dissolves.
- Crystals on cooling
- mass of crystals = solubility at the higher temperature - solubility at the lower temperature.
- Separating mixtures
- potassium nitrate can be separated from sodium chloride by dissolving the mixture in hot water and cooling,
- so potassium nitrate crystallises and sodium chloride stays in solution.
Section 5 of 5Secondary data
1Secondary source2 points
= a source of evidence you have not collected yourself in an investigation,
- such as books, scientific journals, the Internet or another student's results.
- It is used when you cannot do the experiment, for example without a balance.
2Trustworthy sources3 points
- scientific journals and data books are written and checked by experts.
- Evidence on a chemistry website such as IUPAC is more trustworthy than evidence on social media.
- The quality of evidence from the Internet varies.
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