Preparation of an Insoluble Salt

Aim

To prepare a pure, dry sample of an insoluble salt (lead iodide, PbI2) by a precipitation reaction, mixing solutions of lead nitrate and potassium iodide.

Apparatus

• Two beakers (100 cm3)
• Measuring cylinders (25 cm3)
• Glass stirring rod
• Filter funnel and filter paper
• Conical flask or beaker (to collect filtrate — for disposal)
• Watch glass or filter paper (for drying)
• Wash bottle with distilled water
• Lead(II) nitrate solution (Pb(NO3)2, approximately 0.5 mol/dm3)
• Potassium iodide solution (KI, approximately 0.5 mol/dm3)

Method

1. Using a measuring cylinder, measure 25 cm3 of lead(II) nitrate solution and pour it into a beaker.
2. Using a separate measuring cylinder, measure 25 cm3 of potassium iodide solution.
3. Add the potassium iodide solution to the lead nitrate solution while stirring with a glass rod.
4. A bright yellow precipitate of lead(II) iodide forms immediately.
5. Filter the mixture through filter paper in a funnel. The yellow precipitate (lead iodide) is collected as the residue on the filter paper.
6. Wash the precipitate thoroughly with distilled water from a wash bottle. This removes any soluble impurities (potassium nitrate, excess lead nitrate, or excess potassium iodide).
7. Leave the precipitate to dry on the filter paper in a warm place, or transfer it to a watch glass and place in a warm oven (below 100 degrees C).

Expected results

The reaction is: Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

The ionic equation is: Pb2+(aq) + 2I-(aq) → PbI2(s)

When the two colourless solutions are mixed, a bright yellow precipitate of lead(II) iodide forms instantly. The filtrate (potassium nitrate solution) is colourless. The dried product is a bright yellow powder.

Analysis

This is a double decomposition or precipitation reaction. The lead ions and iodide ions combine to form the insoluble salt lead iodide, while the potassium and nitrate ions remain in solution as spectator ions.

The choice of reactants is determined by the solubility rules. To make lead iodide, you need a soluble source of lead ions (lead nitrate — all nitrates are soluble) and a soluble source of iodide ions (potassium iodide — all potassium salts are soluble).

Other insoluble salts can be prepared the same way:

• Barium sulfate: barium chloride + sodium sulfate
• Silver chloride: silver nitrate + sodium chloride
• Calcium carbonate: calcium chloride + sodium carbonate

Sources of error

• Incomplete washing: If the precipitate is not washed thoroughly with distilled water, soluble impurities (KNO3) contaminate the product. Wash at least three times with small volumes of distilled water.
• Loss during filtration: Some precipitate may pass through the filter paper if the pores are too large, or some may remain in the beaker. Rinse the beaker with distilled water to transfer all precipitate to the filter.
• Excess reagent: Using unequal amounts means one ion is in excess. While this does not affect the precipitate itself, it means more washing is needed. Using stoichiometric proportions minimises waste.
• Drying temperature: Heating too strongly may decompose some salts. Dry at a low temperature or in air.

Exam tips

Examiners typically ask you to identify the precipitate’s colour (lead iodide = yellow, barium sulfate = white, silver chloride = white), write the ionic equation (showing only the ions that form the precipitate and leaving out spectator ions), and explain why washing is necessary (to remove soluble impurities).

A key exam distinction: for soluble salts, you react acid with base and then crystallise; for insoluble salts, you mix two solutions and filter the precipitate. Knowing which method to use for a given salt depends entirely on the solubility rules.

When selecting reagents, always choose soluble starting materials. All nitrates are soluble, all sodium salts are soluble, and all potassium salts are soluble — so these are safe choices for providing specific ions.

Safety note for exam answers: lead compounds are toxic. Mention washing hands after handling and disposing of waste safely. This safety awareness can earn a mark on Paper 6.