कैल्शियम कार्बोनेट / खड़िया (Calcium carbonate)

Calcium carbonate is a white, odourless mineral powder. It is the main part of chalk (khadiya), limestone, marble and seashells. It hardly dissolves in plain water, but it fizzes and dissolves in acids, giving off carbon dioxide.

• Formula: CaCO3
• CAS number: 471-34-1
• Molar mass: 100.09 g/mol
• Solubility: about 0.013 g per litre of water at 25 °C, so for practical purposes insoluble
• Heating: breaks down to quicklime (calcium oxide) and carbon dioxide at roughly 825 to 840 °C

Don't confuse it with chuna (slaked lime, calcium hydroxide) or quicklime. Those are strongly alkaline and burn skin. Calcium carbonate is mild.

Azlok sells calcium carbonate as a fine powder in a 100 g pack. The listing doesn't state a grade, so treat it as a general technical grade for DIY, craft, garden and lab demonstration work.

It is not labelled food grade or pharmacopoeia grade, so don't use it as a calcium supplement, in food or in toothpaste you plan to sell. Check today's price per kg for current rates.

Home and DIY

• Chalk paint for furniture, giving a flat, matt finish that sands smoothly
• Mild scouring powder for steel, glass-top stoves and tiles
• Making sidewalk chalk and modelling mixes with plaster of Paris

Garden

• Raising the pH of acidic soil and potting mix (agricultural lime is mostly calcium carbonate)

Lab and education

• Producing carbon dioxide with dilute hydrochloric acid
• Showing the limewater test and the action of acids on carbonates

Industry

• Filler and whitener in paints, putty, plastics, rubber, paper and sealants
• Raw material for cement and lime, and a mild abrasive in cleaning powders

Chalk paint. Mix 2 tablespoons of calcium carbonate with 1 tablespoon of water into a smooth slurry with no lumps. Stir it into 1 cup (about 250 ml) of water-based emulsion paint. Apply thin coats and let each dry for an hour or two. Sand lightly with fine sandpaper between coats. Mix only what you'll use the same day.

Scouring. Make a paste with a little water or liquid dishwash, rub with a soft cloth, then rinse well. It's softer than sand but can still scratch polished or coated surfaces, so test a hidden spot first.

Carbon dioxide in the lab. Add a spatula of powder (2 to 5 g) to a flask, then pour in dilute hydrochloric acid slowly. Lead the gas into limewater and it turns milky. Do this with a teacher or trained person present, and wear safety glasses. Use dilute acid only; never concentrated acid for a school demonstration.

Acidic soil. Test the soil pH first. Mix the powder evenly into the top soil and water it in; it acts slowly, over weeks. Don't add it to plants that like acidic soil.

• The fine powder is a nuisance dust. Pour gently, and wear a dust mask when mixing large amounts.
• Wear safety glasses when working with acids. The fizzing can splash.
• Keep it away from strong acids in storage; it reacts with them and releases gas.
• Store in a closed, dry container. Damp powder forms lumps.
• Label it clearly and keep it away from the kitchen and food.
• Keep away from children.
• Small leftover amounts can go out with dry household waste.

The 100 g pack is enough for a few chalk paint batches, school experiments or a small set of pots. For painting large furniture, or for trial batches of putty or filler, buy several packs so the powder comes from the same lot.

If you need calcium carbonate for food, medicines or cosmetics you sell, look for a product labelled with that specific grade. Workshops and factories that need 25 kg or more can send a bulk inquiry.