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Thanks to Bob Worley of CLEAPSS for a great idea that really helps students. It has been well documented, and my own experience confirms this, that students really do not seem to clearly understand dissolving an ionic solid in water. Here is the activity that I learned from Bob at Chem Ed 2015. First, I had my students examine the conductivity of a puddle of water the size of a nickel. They checked for conductivity. Then they took a very small amount of sodium carbonate and a fresh puddle of water and pushed in a few crystals from the side. You can still see the crystals in the water but it tested positive for conductivity. They had to explain this. They did the same with a fresh puddle of water and a few crystals of copper (II) sulfate. Again, it tested positive for conductivity but they could still see the blue crystal. Finally, they started again with another fresh puddle of water, pushed a few crystals of sodium carbonate on one side and on the opposite side they pushed in a few crystals of copper (II) sulfate. After waiting five minutes, a solid dull blue precipitate formed in the middle. Also, the drop tested positive for conductivity.
Students had to explain how the solid formed and the clear liquid that tested positive. Bob even has some great leading questions about what makes up the solid. If it is light blue, where does the blue come from? It was a great two day discussion about water, ions and drawing models as my students tried to explain what was happening. I would strongly recommend this activity. Bob has said, and I agree, that when we do the precipitate double displacement reactions with the reactants already dissolved, we are assuming too much about what we think students know. This forces them to discuss and think about the nature of dissolving ionic solids. I would give this activity a thumbs up.