Calcium Hydroxide (Bujha Chuna): Uses and Safety

Calcium hydroxide is slaked lime: the white powder you get when quicklime (calcium oxide) reacts with water. In Hindi it is called bujha chuna, and it is the same basic material behind traditional chuna whitewash.

• Formula: Ca(OH)2
• CAS number: 1305-62-0
• Molar mass: about 74.09 g/mol
• Appearance: white, odourless powder
• Solubility: only slightly soluble, roughly 1.7 g per litre at 20 °C, and less in hot water than in cold

It is a strong base. A clear saturated solution, called limewater, turns milky when carbon dioxide bubbles through it, because insoluble calcium carbonate forms. The same reaction with air is what slowly hardens lime plaster and whitewash on a wall. Heated to around 580 °C it loses water and goes back to calcium oxide.

Azlok sells Calcium Hydroxide Powder in a 200 g pack. The product name doesn't state a grade, so treat it as a general technical grade: right for whitewash trials, garden and soil use, limewater experiments and small workshop jobs.

It is not labelled food grade or pharmacopoeia grade. Don't use it for food, paan or anything that goes in the mouth. Check today's price per kg before ordering.

• Building and home: whitewash for walls, tree trunks and cattle sheds; lime mortar and lime plaster, often mixed with sand.
• Garden and farming: raising the pH of acidic soil, though agricultural lime (calcium carbonate) is gentler and more common for this.
• Water treatment: adjusting pH and softening water in treatment plants.
• School and lab: making limewater to test for carbon dioxide; a cheap base for neutralising acids.
• Industry: leather tanning (removing hair from hides), sugar refining, and flue-gas treatment to capture sulphur dioxide.

Making limewater (for the CO2 test):

1. Add about one level teaspoon of powder to 1 litre of clean water in a bottle.
2. Shake well and leave it to settle overnight.
3. Pour or filter off only the clear liquid on top. That is saturated limewater.
4. Keep it in a tightly closed bottle. Left open, it reacts with air and goes cloudy.

Simple whitewash: stir the powder slowly into water until it is like thin milk, and leave it to stand for a few hours before use. Apply two or three thin coats rather than one thick one. Thick coats crack and flake. Many people add a little common salt or glue to improve adhesion, but recipes vary, so test a small patch first.

Soil pH: test the soil before and after. Use small amounts, mix it in well, and don't add it together with fresh manure or ammonium fertilisers, since lime drives off ammonia.

• It is caustic. Powder in the eye can cause serious damage, so wear safety glasses or goggles when mixing.
• Wear rubber gloves. Wet lime on skin causes burns if left on, especially under clothing or in boots.
• Use a dust mask when pouring dry powder; the dust irritates the nose and throat.
• If it gets in the eyes, rinse with plenty of clean water for at least 15 minutes and see a doctor.
• Keep it away from acids, which react with it and give off heat.
• Store in an airtight container. In open air it slowly turns into calcium carbonate and loses strength.
• Label the jar clearly and keep it away from children and pets.
• Small amounts can be neutralised with dilute vinegar and washed away with plenty of water; ask your local authority about larger amounts.

The 200 g pack is enough for limewater, a school project, a few test patches of whitewash or a small garden bed. It also stays fresher, since an opened pack slowly absorbs carbon dioxide.

For whitewashing whole rooms or treating farm soil you will need much more, and it is usually cheaper to buy in larger quantity. For 25 kg and above, send a bulk inquiry with the quantity and use.

If you need lime for food work such as pickling, buy a product labelled food grade from a licensed seller, not this one.