Try the Hamilton Trust website - it definitely exists on there! They have some ideas you may want to use. I have just finished this unit of work and in our last session we made sherbet. This was to show that an irreversible change occurs when the dry mixture is mixed with water (and more importantly as far the the children were concerned, saliva on their tongue!) The bubbles that are formed are CO2, showing a chemical change which cannot be reversed. Ingredients needed are: icing sugar (1 teasp) bicarbonate of soda (1/2 teasp) and citric acid crystals (1 teasp). You will have to ask for citric acid at your local chemist shop as I've never seen it in the supermarket. I sometimes end this unit also with looking at chemical hazzard symbols. This time round I ran out of time and had to cut it short. Also look at the Northumberland grid for learning at their science self assessment sheets, there is one for this unit which I have just used ( I think they're called Aiming High in Science).