09:43 – Barbara just gave me a haircut and is vacuuming now. Today we’ll pack the next-to-last group of chemicals for the biology kits: Barfoed’s reagent, Benedict’s reagent (qualitative), bromothymol blue, fertilizer concentrate B, glycerol, neomycin sulfate solution, Seliwanoff’s reagent A, sodium borate, and turmeric reagent.
As I was making up the Benedict’s reagent yesterday, I thought what a great demonstration it’d be for kids. It’s made up as two solutions that are subsequently mixed with constant stirring. The first solution is sodium carbonate and sodium citrate, which is colorless and water-like. I made up 1.7 liters of that. The second solution is copper sulfate in about 200 mL of water. It’s a beautiful blue color, about the same as Crest Pro Health mouthwash. As I poured the second solution into the first, I thought about having kids watch this part. They’d all assume, naturally enough, that the 200 mL of medium blue solution would be diluted by the 1700 mL of water-like solution to yield a much paler blue solution. In fact, the opposite occurs. The more of the copper sulfate solution you add, the deeper the blue color becomes, until the final 1900 mL of solution is a much, much deeper blue than the original copper sulfate solution.
As to why, most people who know a bit about chemistry think copper(II) ions are blue, but in fact they’re colorless. It’s hydrated copper(II) ions that are blue, because they form an intensely blue coordination complex with water molecules. That’s why copper(II) sulfate pentahydrate crystals are blue, while anhydrous copper(II) sulfate is colorless.
Nearly all copper coordination complexes are blue or green of some sort, but the exact color and intensity depends on the ligand. With water as the ligand, one sees the familiar blue of copper(II) sulfate. With citrate as the ligand, the color shifts to a shorter blue wavelength, and the color becomes much, much more intense.