Class practical

This class practical shows that diffusion takes place in liquids. Students place colourless crystals of lead nitrate and potassium iodide at opposite sides of a Petri dish of deionised water. As they dissolve and diffuse towards each other they form clouds of yellow lead iodide.

Lesson organisation

This practical activity takes around 30 minutes.

Apparatus Chemicals

Eye protection.

Per pair or group of students:

Petri dish


White tile or piece of white paper


Potassium iodide, 1 crystal

Deionised water

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

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

Potassium iodide, KI(s), - see CLEAPSS Hazcard.


a Place a Petri dish on a white tile or piece of white paper. Fill it nearly to the top with deionised water.

b Using forceps, place a crystal of lead nitrate at one side of the petri dish and a crystal of potassium iodide at the other.

c Observe as the crystals begin to dissolve and a new compound is formed between them.


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.

The demonstration A solid-solid reaction  involves the same reaction but in the solid state.

Health & Safety checked, 2016


This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015