Describe the preparation, separation and purification of soluble salts by reaction of an acid with: (a) an alkali by titration (b) excess metal (c) excess insoluble base (d) excess insoluble carbonate

Acids, bases and salts – Preparation of salts

1️⃣ Reaction with an alkali by titration

Think of the acid and the alkali as two friends who need to meet at the exact same time. When they meet, they neutralise each other and form a salt and water.

Balanced equation (example with hydrochloric acid and sodium hydroxide):

\$HCl + NaOH \;\longrightarrow\; NaCl + H_2O\$

Procedure:

  1. Fill a burette with the alkali solution (e.g. 0.1 M NaOH).

  2. Add the acid (e.g. 0.1 M HCl) to a conical flask with a few drops of phenolphthalein.

  3. Titrate until the colour just fades – that’s the equivalence point.

Separation & purification:

  • Evaporate the solution to dryness.

  • Re‑dissolve in a small amount of hot water.

  • Cool slowly to crystallise the salt.

  • Filter, wash with cold water and dry.

Exam tip: Write the balanced equation before you start the titration. It shows you the stoichiometry and helps you calculate the amount of acid needed.

2️⃣ Reaction with excess metal

Imagine the metal as a superhero that can donate electrons. When it reacts with an acid, it gives up electrons to form a salt and hydrogen gas.

Example: zinc + hydrochloric acid

\$Zn + 2HCl \;\longrightarrow\; ZnCl2 + H2\uparrow\$

Procedure:

  1. Add a measured amount of zinc metal to a flask.

  2. Pour the acid slowly; you’ll see bubbles of H₂ forming.

  3. Once bubbling stops, the reaction is complete.

Separation & purification:

  • Filter to remove any unreacted metal.

  • Evaporate the filtrate to dryness.

  • Re‑crystallise from hot water or a suitable solvent.

Exam tip: Remember that the metal is the reducing agent and the acid is the oxidising agent. Write the oxidation states to check your equation.

3️⃣ Reaction with excess insoluble base

Think of the insoluble base as a sponge that absorbs the acid but doesn’t dissolve. The acid reacts with the solid base to give a soluble salt.

Example: calcium carbonate + hydrochloric acid

\$CaCO3 + 2HCl \;\longrightarrow\; CaCl2 + CO2\uparrow + H2O\$

Procedure:

  1. Add the insoluble base (e.g. CaCO₃) to a flask.

  2. Pour the acid slowly; you’ll see fizzing from CO₂.

  3. Stir until fizzing stops.

Separation & purification:

  • Filter to remove any unreacted base.

  • Evaporate the filtrate to dryness.

  • Re‑crystallise the salt from hot water.

Exam tip: Write the gas evolution as a separate line in your equation. It helps you remember that the reaction is not just a simple neutralisation.

4️⃣ Reaction with excess insoluble carbonate

Carbonates are like the “slow‑release” bases. They react with acids to give a salt, water and carbon dioxide.

Example: potassium carbonate + nitric acid

\$K2CO3 + 2HNO3 \;\longrightarrow\; 2KNO3 + CO2\uparrow + H2O\$

Procedure:

  1. Add the carbonate to a flask.

  2. Pour the acid slowly; watch the CO₂ bubbles.

  3. Once bubbling stops, the reaction is finished.

Separation & purification:

  • Filter out any leftover carbonate.

  • Evaporate the clear solution to dryness.

  • Re‑crystallise the salt from hot water.

Exam tip: Carbonates give CO₂ – write it as a separate product. This will help you spot the reaction type quickly.

General Tips for Separating & Purifying Soluble Salts

  • Always filter to remove insoluble impurities.

  • Use evaporation to concentrate the solution.

  • Re‑crystallise from a hot solvent and cool slowly.

  • Wash the crystals with cold water to remove surface impurities.

  • Dry in a low‑temperature oven or air‑dry.

Remember: the key to a clean salt is purity of reagents and careful handling during each step.