Demonstrate that diffusion takes place in liquids by allowing lead nitrate and potassium iodide to form lead iodide as they diffuse towards each other in this practical

In this experiment, students place colourless crystals of lead nitrate and potassium iodide at opposite sides of a Petri dish of deionised water. As these substances dissolve and diffuse towards each other, students can observe clouds of yellow lead iodide forming, demonstrating that diffusion has taken place.

This practical activity takes around 30 minutes.

Equipment

Apparatus

  • Eye protection
  • Petri dish
  • Forceps
  • White tile or piece of white paper

Chemicals

  • Lead nitrate (TOXIC, DANGEROUS FOR THE ENVIRONMENT), 1 crystal
  • Potassium iodide, 1 crystal
  • Deionised water

Health, safety and technical notes

Read our standard health and safety guidance.

Wear eye protection throughout.

Lead nitrate, Pb(NO3)2(s), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a.

Potassium iodide, KI(s) – see CLEAPSS Hazcard HC047b.

Procedure

  1. Place a Petri dish on a white tile or piece of white paper. Fill it nearly to the top with deionised water.
  2. Using forceps, place a crystal of lead nitrate at one side of the petri dish and a crystal of potassium iodide at the other.
  3. Observe as the crystals begin to dissolve and a new compound is formed between them.

A diagram showing a petri dish, with crystals of potassium iodide and lead nitrate at opposite ends

Source: Royal Society of Chemistry

As the crystals of potassium iodide and lead nitrate dissolve and diffuse, they will begin to form yellow lead iodide

Teaching notes

The lead nitrate and potassium iodide each dissolve and begin to diffuse through the water. When the lead ions and iodide ions meet they react to form solid yellow lead iodide which precipitates out of solution.

lead nitrate + potassium iodide → lead iodide + potassium nitrate

Pb(aq) + 2I(aq) → PbI2(s)

The precipitate does not form exactly between the two crystals. This is because the lead ion is heavier and diffuses more slowly through the liquid than the iodide ion.

Another experiment – a teacher demonstration providing an example of a solid–solid reaction – involves the same reaction but in the solid state.