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Diffusion in Liquids

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Fig 1: Note the lead iodide solid nearer the lead nitrate
Fig 1


Potassium iodide and lead nitrate crystals react in a Petri dish.


This practical demonstrates how diffusion occurs in a liquid. Small crystals of potassium iodide and lead nitrate are placed apart in a petri dish which is then filled with deionised (distilled) water. The two substances react to form a yellow crust of solid lead iodide at the point they meet in the dish.

Students should be able to see that the solid lead iodide forms closer to the lead nitrate crystal. This is because the iodide ions are smaller than the lead particles and so move faster.

Equipment required (per set)

  • Deionised (distilled) water
  • Small lead nitrate crystal (toxic)
  • Small potassium iodide crystal
  • Petri dish
  • Forceps (for handling the crystals)

The formula for this reaction is below:

Pb(NO3)2 + 2KI 2KNO3 + PbI2


 Wear eye protectionWear eye protection.

Lead nitrate is toxic – use forceps when handling the crystals.

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