When you've already done the activity where pupils add a selection of solids such as sugar, sand and salt to water and sort them into two groups - dissolved and not dissolved - you may want to challenge them to think a little more. Time to bring on the flour and water.
Add the white flour to the cold water, stir it up and discuss whether they think it has dissolved or not. If the conversation is a little slow, start them off with some tales of what some others thought when asked the same question, such as: "It can't have dissolved because the mixture's all cloudy. If the flour had dissolved, it would be clear." "I think it must have dissolved because the flour is all mixed up with the water"; "There's more flour at the bottom than the top. I think the flour and water will separate again. The flour hasn't dissolved".
After they've discussed their ideas for a while, ask groups to come up with their own definition of dissolving. Use their ideas to work towards your own class definition, such as "Dissolving is when a solid and a liquid mix and form a clear solution that will not separate out again if left to stand."
They should now realise that the flour hasn't dissolved, as it separates out when left to stand.
Following on from the work on dissolving comes the work on separating solids and liquids. Usually they pour various mixtures through the filter paper and quickly realise that the soluble solids go through the filter paper while the insoluble solids remain behind.
Pour the flour and water mixture through the filter paper. Unsurprisingly, the flour stays on the filter paper and clear water comes through. Now pour the same mixture through a filter made from a piece of kitchen paper.
Some flour stays on the paper as expected, but some comes through with the water making a cloudy mixture in the beaker below. If perplexed looks follow, ask them to think about their previous work on sieves. At this point someone usually makes the connection and says: "Oh - the kitchen paper is like a sieve with bigger holes than the filter paper. Some of the little bits of flour can get through the bigger holes but all the flour gets stopped by the smaller holes."
Simple flour and water - a cloudy mixture that can help make things just a little bit clearer for pupils.
Anne Goldsworthy is an independent consultant