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chemistry igcse acapprinted 97 · PDF 97
145 pagesrevision 46934
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1. Measure the alkali into a conical flask and add a few drops of indicator (e.g., methyl orange or
thymolphthalein).
2. Fill a burette with acid and record the initial volume.
3. Slowly add acid from the burette to the flask until the indicator changes color.
4. Record the final burette reading to find the volume of acid used.
5. Repeat the procedure without the indicator, adding the same volume of acid to the alkali.
6. Heat the solution to partially evaporate water, forming a saturated solution.
7. Let the solution cool and form crystals, then decant excess liquid and dry the crystals.
Preparing Insoluble Salts
Insoluble salts can be made through a precipitation reaction. In this method, an insoluble salt is
formed as a solid precipitate, so it’s essential that the salt product is insoluble in water, while the
reactants are soluble.
Using Two Soluble Reactants
Method:
1. Dissolve each of the soluble salts in water separately in two beakers.
2. Combine both solutions in a single beaker and stir. An insoluble salt will form as a precipitate.
3. Filter the mixture to separate the solid precipitate from the solution.
4. Rinse the residue with distilled water to eliminate any leftover traces of other solutions.
5. Dry the precipitate by placing it in an oven, leaving behind the purified insoluble salt.
Example: Preparing Pure, Dry Calcium Sulfate Crystals
1. Dissolve calcium chloride (CaCl₂) in water in one beaker.
2. Dissolve sodium sulfate (Na₂SO₄) in water in another beaker.
3. Pour the two solutions into a single beaker, stirring to create the precipitate, calcium sulfate
(CaSO₄).
4. Filter the mixture to separate the calcium sulfate precipitate.
5. Rinse the precipitate with distilled water to wash away any sodium chloride (NaCl) solution.
6. Dry the calcium sulfate by leaving it in an oven.
CaCl2 (aq) + Na2SO4 (aq) → CaSO4 (s) + 2NaCl (aq)