Making Bleaching Powder — The Chlorine Chemistry That Made White Fabric Affordable

This experiment generates chlorine gas — one of the most dangerous gases in chemistry. Work ONLY outdoors with wind at your back, or in a powerful fume hood rated for toxic gases. Wear a full-face respirator with chlorine/acid gas cartridges (P100/OV/AG), chemical splash goggles, heavy-duty nitrile gloves, and a full lab coat. Have a large bucket of water nearby for emergency skin decontamination. If you smell a strong, sharp, swimming-pool-like odour at any point, you are being exposed to chlorine — move upwind immediately.

Weigh 15 g of pyrolusite ore (manganese dioxide, MnO₂) — a heavy, black, granular mineral. MnO₂ is the oxidising agent that converts hydrochloric acid into chlorine gas. This is the same reaction Carl Wilhelm Scheele used in 1774 when he first isolated chlorine — he called it 'dephlogisticated muriatic acid.' The manganese is reduced from Mn⁴⁺ to Mn²⁺ as it transfers oxygen to the chloride ions, oxidising them from Cl⁻ to Cl₂.

Add the manganese dioxide to a round-bottom flask. Fit the flask with a two-hole rubber stopper: one hole for a dropping funnel (or thistle tube) to add acid, and one for a glass delivery tube to carry the chlorine gas out. Connect rubber tubing to the delivery tube. The generator must be gas-tight except for the exit tube — any leak releases toxic chlorine into your workspace.

Connect the delivery tube from the generator into a second flask (the wash bottle) containing 100 ml of distilled water. The tube must dip below the water surface so chlorine gas bubbles through. This wash removes hydrochloric acid vapour that would otherwise contaminate the bleaching powder with excess acidity. A second delivery tube exits above the water level and continues to the lime chamber. The wash water will gradually turn pale yellow-green as it absorbs some chlorine — this is dilute chlorine water, itself a bleaching solution.

Spread 30 g of slaked lime (calcium hydroxide, Ca(OH)₂) in an even layer in a porcelain evaporating dish. The layer should be no more than 5 mm deep — a thin layer ensures the chlorine gas can penetrate and react with all the lime. Cover the dish loosely with a watch glass, leaving a gap on one side for the gas inlet tube. Position the dish at the end of the delivery system so the washed chlorine gas flows over the lime bed. The exit gas should be vented safely downwind or into a second wash bottle containing sodium hydroxide solution to neutralise excess chlorine.

Carefully pour 50 ml of concentrated hydrochloric acid (37% HCl) through the dropping funnel into the flask containing the manganese dioxide. The reaction begins immediately — you may see a faint yellow-green colour appearing in the flask: MnO₂ + 4HCl → MnCl₂ + Cl₂↑ + 2H₂O. The manganese dioxide oxidises the chloride ions from HCl to elemental chlorine gas. Add the acid slowly — rapid addition produces a surge of chlorine that could overwhelm the delivery system.

Warm the generator flask gently with a spirit lamp. At room temperature, the reaction is slow — gentle heating to 40–50 °C accelerates chlorine evolution significantly. Bubbles of yellow-green chlorine gas appear in the wash bottle as the gas passes through the water. The characteristic sharp, acrid smell of chlorine (the 'swimming pool' odour) will be noticeable even through a respirator if the gas escapes — if you detect this, check all connections and move upwind. The reaction proceeds steadily for 30–60 minutes as long as excess HCl remains.

Watch the lime bed as chlorine flows over it. The white powder gradually changes to a faint cream or pale yellow colour as calcium hypochlorite forms — Ca(OH)₂ + Cl₂ → CaCl(OCl) + H₂O. This colour change is subtle but definite. If you lift the watch glass briefly (carefully, upwind), the lime may have developed a pungent, chlorine-like smell distinctly different from fresh slaked lime. Stir the lime bed gently with a glass rod every 10 minutes to expose fresh surfaces to the gas flow.

Continue the chlorine generation for 30–60 minutes until either the manganese dioxide is consumed (the flask contents turn from black to a pale brown solution of manganese chloride) or the lime bed has turned uniformly pale yellow and no longer absorbs chlorine readily (gas begins to pass through without reacting). A well-saturated bleaching powder contains approximately 30–35% available chlorine by weight. The industrial product was shipped at this concentration in wooden casks lined with lead.

Remove the heat source first to stop generating new chlorine. Wait 5 minutes for residual gas to clear through the system. Then disconnect the tubing from the wash bottle end FIRST (not the generator end — disconnecting the generator first would vent raw chlorine). Allow any remaining chlorine in the wash bottle to be absorbed. The generator flask contains a solution of manganese chloride (MnCl₂) in excess HCl — this can be neutralised with sodium carbonate and disposed of safely.

The porcelain dish now contains bleaching powder — a pale cream or yellowish-white powder with a strong chlorine odour. To test: dissolve 1 g in 50 ml of water and stir. The solution should be cloudy (calcium carbonate precipitates from CO₂ absorption) but distinctly chlorine-scented. Dip a strip of coloured cotton fabric into the solution — the colour should fade visibly within minutes, demonstrating the oxidising power of the hypochlorite ion. Before Tennant's powder, this bleaching action required months of sunlight exposure.

Transfer the bleaching powder to a glass jar with a tight-fitting lid. Label: BLEACHING POWDER (Calcium Hypochlorite), OXIDISER, CORROSIVE, date. Store in a cool, dark, dry place — bleaching powder slowly decomposes, losing available chlorine. Heat and moisture accelerate this decomposition. Expected yield from 30 g slaked lime: approximately 30–35 g of bleaching powder (the mass increases as chlorine is absorbed). Never store near acids — mixing bleaching powder with any acid releases chlorine gas immediately. Charles Tennant's 1799 patent for this simple powder transformed the textile industry overnight and made Glasgow the bleaching capital of the world.