Cut one or two bars of Ivory soap so that each group gets about 1/4 of a bar. Do a demonstration to introduce the concept of a precipitate – formation of a solid from the reaction of two liquids.

A few drops of dilute sodium hydroxide solution react to form a white precipitate with aluminium ions, calcium ions and magnesium ions. However, if excess sodium hydroxide solution is added ...

If you time how long it takes until you can no longer see the X due to the formation of the precipitate, you can calculate the rate using the equation. rate = 1 ÷ (time taken for X to be obscured ...