Shell Topic 3 – Separating Mixtures
Demonstration 3.2b
"Golden Rain" Precipitation
Solubility changes with temperature, not only is it easier to dissolve salt in hot water, more
salt will actually dissolve. When the solution is cooled down it can no longer dissolve the
extra material and it turns back into a solid, this is called a precipitate. Solids can then be
separated from liquids by filtration or decanting.
Safety
Lead Nitrate solid – Toxic down to 0.01 M may cause harm to the unborn child. Harmful if
inhaled or swallowed. Wear Gloves
Dangerous to the Environment - very toxic to aquatic organisms
Equipment
Conical flask
Potassium iodide solution
Bunsen burner
Lead nitrate solution
Cold water
Buchner funnel and paper
Technician Notes
Dissolve 0.15g of lead nitrate in 50cm3 of distilled water – Low Hazard but wear
gloves when handling as it is only just under Toxic.
Dissolve 0.15g of potassium iodide in 50cm3 of distilled water
Add a few drops of 1M hydrochloric acid to the lead solution to prevent cloudiness
Method
1. The teacher will pour two clear solutions together to make a new chemical which is
insoluble in water.
Lead Nitrate + Potassium Iodide --> Lead Iodide + Potassium Nitrate
2. The solution is then heated so that the solid dissolves
Shell Topic 3 – Separating Mixtures
Demonstration 3.2b
3. The solution is then crash cooled by putting it under cold water and then left for the
full effect to develop.
4. Afterwards the solution can be filtered under vacuum using a Buchner funnel
Diagram
Questions
1. Write a word equation for the reaction you saw
2. What is the chemical symbol for Lead?
3. What is the chemical symbol for Potassium?
4. Why does the solution turn yellow?
5. Why does the solution turn back to colourless when heated?
6. What happens when you leave the flask for a short time after the precipitate is
formed?
7. The solid forms as very small particles, what would happen if you let the heated
solution cool down slowly over a few hours?