[Julie Steiner]

Yup, gypsum and lime are definitely different. I know because a few months ago my kids wanted to make limewater to use as a carbon dioxide indicator (limewater precipitates calcium carbonate within about 15 seconds in the presence of carbon dioxide).

The instructions in the book we were using (Oxidation by TOPS Learning Systems--we absolutely love their whole series) were, "Prepare limewater from hydrated lime (also called calcium hydrate, calcium hydroxide, and garden lime). Purchase in as small a quantity as possible from garden supply or farm stores. Lab grade purity is not required. The solubility of this powder in water is very low. Stir in a level teaspoon (4 mL) into a quart (or liter) jar of water, then close with a lid and allow the chalky white mixture to settle about 24 hours. (Limewater left exposed to carbon dioxide in air forms a heavy surface film of calcium carbonate.) Pour off the clear liquid into a second storage jar with lid and label it LIMEWATER (CaOH2). Add more water to the sediment in your original jar, and set aside to resupply your labeled jar as needed."

Alas, San Diego's soil is so incredibly alkaline that none of the garden supply stores here carry lime as a soil supplement. I couldn't order less than a 50-pound bag from elsewhere, which I refused to do for my kids because I'm just a bad parent, I guess. The local home improvement stores had plenty of gypsum, which would not have worked because that's calcium sulfate, not calcium hydroxide.

Say! If any of YOU out there in the 'Sphere have acid soil, I'd be willing to pay you to send us a teaspoon or so of lime. My daughters are still bummed out about skipping the set of experiments involving limewater, because the other experiments in that book are so much fun. Just this week the kids went to a friend's house and repeated the pyromaniacal steel-wool-and-clothespin sparkler demo (with parental supervision, of course.)

Julie Stoner