7.2.2Acids, Bases and Salts
Preparing Insoluble Salts — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 8 Unit quiz
Part 1 of 5The idea of precipitation (HT only)
What happens
- What happens When two solutions containing the right ions are mixed, the ions of an insoluble salt join and appear as a solid.
- That solid, which forms inside the liquid, is called a precipitate.
Conditions for success
- Conditions for success The salt you want must be insoluble in water.
- Both starting reactants must be soluble, so they can be dissolved and mixed as solutions.
- The second product must stay dissolved so it can be washed away.
Choosing reactants
- Choosing reactants To make a sulfate, mix a soluble sulfate with a soluble salt of the metal wanted;
- to make a carbonate, use a soluble carbonate such as sodium carbonate.
Part 2 of 5Method for a pure, dry precipitate (HT only)
1
Dissolve each of the two soluble reactants in separate beakers of water to make aqueous solutions.
2
Pour the solutions together and stir with a glass rod. A cloudy suspension of the precipitate forms.
3
Set up a filter funnel with filter paper and pour in the mixture. The precipitate stays on the paper while the soluble product passes through.
4
Wash the precipitate on the paper with distilled water to remove traces of the other solution.
5
Dry the solid by leaving it in a warm oven or pressing it between sheets of filter paper.
Part 3 of 5Worked example: lead(II) sulfate (HT only)
1Choose reactants
- Choose reactants Lead(II) nitrate and potassium sulfate are both soluble, and lead(II) sulfate is not.
2Mix
- Mix White precipitate of lead(II) sulfate forms immediately.
3Filter and wash
- Filter and wash Distilled water washes away potassium nitrate solution left on the solid.
4Dry
- Dry A pure, dry sample of lead(II) sulfate is left.
Part 4 of 5Equations (HT only)
| Type | Equation |
|---|---|
| Word equation | lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate |
| Balanced symbol equation | Pb(NO3)2(aq) + K2SO4(aq) → PbSO4(s) + 2KNO3(aq) |
| Ionic equation | Pb2+(aq) + SO42-(aq) → PbSO4(s) |
Part 5 of 5Soluble or insoluble salts
Soluble
- Soluble All sodium, potassium and ammonium salts, and all nitrates.
- Most chlorides (except silver and lead) and most sulfates (except barium, calcium and lead).
Insoluble
- Insoluble Most carbonates and hydroxides, other than those of sodium, potassium and ammonium.
- Silver chloride, barium sulfate and lead(II) sulfate are classic precipitates.
Key terms
| Term | Meaning |
|---|---|
| Precipitate | An insoluble solid that forms when two solutions are mixed. |
| Precipitation reaction | A reaction in which ions in solution combine to form an insoluble solid. |
| Filtration | Separating an insoluble solid from a liquid using filter paper. |
| Distilled water | Water with dissolved substances removed, used to wash the precipitate without adding impurities. |
| Spectator ions | Ions that stay dissolved and do not appear in the ionic equation. |
Avoid these mistakes
1
Trying to make an insoluble salt with an acid and an excess of solid. That method only works for soluble salts.
2
Washing with tap water, which adds dissolved impurities. Use distilled water.
3
Writing the precipitate as (aq). Insoluble products must be shown as (s).
4
Leaving out the washing step, which means the dried solid is contaminated by the soluble by-product.
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