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.
This practical activity takes around 30 minutes.
Per pair or group of students:
White tile or piece of white paper
Lead nitrate (TOXIC, DANGEROUS FOR THE ENVIRONMENT), 1 crystal
Potassium iodide, 1 crystal
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
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.
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
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.
The experiment is also part of the Royal Society of Chemistry’s Continuing Professional Development course: Chemistry for non-specialists