Preparation, Separation and Purification of Soluble Salts by Reaction of an Acid with:

(a) An Alkali – by Titration

This method is used when both the acid and the base (alkali) are soluble. It involves accurately measuring and mixing the acid and alkali to obtain a neutral solution, which can be crystallised to give the salt.

1. Use a pipette to add 25 cm³ of alkali to a conical flask.
2. Add a few drops of indicator (e.g. phenolphthalein or methyl orange).
3. Fill a burette with the acid.
4. Slowly add the acid while swirling the flask until the indicator shows neutralisation.
5. Repeat the titration to get consistent results (concordant readings).
6. Repeat the titration without indicator using the average volume of acid.
7. Evaporate the resulting salt solution and crystallise to obtain pure, dry crystals.

Example Equation:

\( \text{NaOH (aq)} + \text{HCl (aq)} \rightarrow \text{NaCl (aq)} + \text{H}_2\text{O (l)} \)

(b) Excess Metal

This method is suitable for preparing salts of reactive metals (e.g. magnesium, zinc). The metal must be more reactive than hydrogen. It is added in excess to ensure complete reaction and then removed by filtration.

1. Add excess metal (e.g. magnesium) to dilute acid (e.g. sulfuric acid).
2. Allow the reaction to finish (bubbling stops).
3. Filter off the unreacted metal.
4. Evaporate the filtrate to crystallise the salt.
5. Filter and dry the salt crystals.

Example Equation:

\( \text{Mg (s)} + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{MgSO}_4 (aq) + \text{H}_2 (g) \)

(c) Excess Insoluble Base

This method is used when the base is insoluble (e.g. copper(II) oxide). It is added in excess to ensure all the acid is neutralised.

1. Warm dilute acid gently in a beaker.
2. Add the insoluble base a little at a time with stirring.
3. Continue until no more base dissolves (excess remains).
4. Filter off the unreacted base.
5. Evaporate the solution to obtain salt crystals.
6. Filter and dry the crystals.

Example Equation:

\( \text{CuO (s)} + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{CuSO}_4 (aq) + \text{H}_2\text{O (l)} \)

(d) Excess Insoluble Carbonate

This method is used for metal carbonates such as calcium carbonate. The reaction gives off carbon dioxide gas.

1. Warm dilute acid in a beaker.
2. Add the insoluble carbonate gradually while stirring.
3. Continue until no more bubbles are seen (reaction is complete).
4. Filter to remove excess carbonate.
5. Evaporate the filtrate to crystallise the salt.
6. Filter and dry the crystals.

Example Equation:

\( \text{CaCO}_3 (s) + 2\text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + \text{CO}_2 (g) + \text{H}_2\text{O (l)} \)

Preparation of insoluble salts by precipitation

Precipitation is a method used to prepare insoluble salts by mixing two aqueous solutions. When the ions from the solutions react, an insoluble solid (called a precipitate) forms and can be filtered out.

Key steps:

1. Select two soluble salts such that one provides the desired cation and the other provides the desired anion.
2. Mix the solutions — the insoluble salt will precipitate out.
3. Filter the mixture to separate the solid precipitate.
4. Wash the precipitate with distilled water to remove soluble impurities.
5. Dry the solid between filter papers or in a warm oven.

Why this works: You are using a double displacement (ionic exchange) reaction where the product is an insoluble salt.

Common examples of insoluble salts:

• • Silver chloride – \( \text{AgCl} \)
  • Lead(II) sulfate – \( \text{PbSO}_4 \)
  • Barium sulfate – \( \text{BaSO}_4 \)