Sodium Hydroxide

Lye — the essential strong base for redox reactions, precipitation, and soap making

■High hazardhydroxidebase

Sodium Hydroxide (NaOH)

Formula: NaOH — Lye, caustic soda
Molar mass: 40.00 g/mol
Appearance: White pellets or flakes; highly hygroscopic
Hazard: Corrosive · Severe burns · Strong base

Summary

Strong base that fully dissociates in water, releasing Na⁺ and OH⁻ ions. Dissolution is strongly exothermic (~44 kJ/mol — the solution heats noticeably). Absorbs both moisture and CO₂ from air, gradually converting to sodium carbonate; store in sealed containers. Saponifies fats and oils (soap making). Precipitates most metal hydroxides when added to their salt solutions: blue Cu(OH)₂, green Fe(OH)₂, red-brown Fe(OH)₃. A 10 g solution in 100 mL water reaches approximately pH 13.

History

Sodium hydroxide’s story is inseparable from soap. For millennia, people made lye by leaching wood ash (potassium carbonate, K₂CO₃) through water, then boiling the caustic liquid with animal fat. The triglyceride fat molecules are hydrolyzed to glycerol and fatty acid salts — soap. Pure sodium hydroxide was not isolated until Humphry Davy¹ electrolyzed molten NaOH in 1807, obtaining metallic sodium.

Industrial production came first from causticizing soda ash with slaked lime (Na₂CO₃ + Ca(OH)₂ → 2 NaOH + CaCO₃), and later from the chlor-alkali process, which electrolyzes brine (NaCl solution) and co-produces NaOH and chlorine. The chlor-alkali industry is still one of the world’s largest chemical processes — NaOH is essential to paper making (wood pulp digestion), textile manufacture, water treatment, oil refining, and food processing.

In food, sodium hydroxide (food-grade lye) is used to cure olives (removing bitter glucosides), make Chinese century eggs, give pretzels and bagels their characteristic dark, glossy crust (lye treatment enables fast Maillard² browning at baking temperature), and process cocoa (Dutch-process chocolate).

Experiments

Blue Bottle Reaction: Dissolve glucose, methylene blue indicator, and NaOH in water. Shake to turn blue, then let stand to decolorize — a reversible redox cycle driven by dissolved oxygen. The alkaline medium is essential for enabling glucose to reduce the indicator.

Traffic Light Reaction: NaOH dissolved with glucose and indigo carmine indicator cycles through three colors (green → yellow → red) as glucose reduces the dye in stages, with shaking to re-oxidize.

Metal Hydroxide Precipitation: Add dilute NaOH solution to copper sulfate (blue Cu(OH)₂ precipitate), ferrous sulfate (green Fe(OH)₂), or ferric chloride (red-brown Fe(OH)₃). Each produces a distinctively colored precipitate — the same colors explored in The Many Colors of Iron. (These are simple metal hydroxides — not Prussian blue, which needs a hexacyanoferrate source.)

Experiments using this chemical:

• Blue Bottle Reaction — Alkaline medium for reversible methylene blue redox
• Traffic Light Reaction — Alkaline medium for multi-color redox cycling
• Indigo Vat Dyeing — Alkaline reducing vat
• The Many Colors of Iron — Precipitation of colored metal hydroxides
• Making Sodium Metal Without Electrolysis — Reduced by magnesium to elemental sodium (advanced)

How to get it

Sodium hydroxide (lye) is available from several sources:

• Soap making suppliers: the most common source online; sold as food-grade lye (NaOH) in 1–2 lb bags of white flakes or pellets. Essential for saponification and widely available.
• Drain cleaners: products like Red Devil Lye (now hard to find) or pure NaOH drain cleaners exist, but verify the product is 100% NaOH — many contain additives. Check the label carefully.
• Online: search “sodium hydroxide food grade lye” for reliable lab-quality product.

Store in a tightly sealed container — it absorbs moisture and CO₂ from air, gradually converting to sodium carbonate.

Buy online:

Amazon: Sodium Hydroxide Lye for Soap Making — Pure Food Grade, 2 lbs affiliate

Safety

High hazard — severely corrosive; causes immediate tissue damage on contact.

Wear gloves and eye protection at all times. Dissolve pellets slowly with stirring — the solution heats significantly and can splash. Never seal a hot solution.

First aid: skin or eye contact — flush immediately with large amounts of running water for at least 15 minutes; for eyes, or any significant burn, seek medical attention while flushing. Do not try to neutralize a burn on skin with acid — keep rinsing with water. If swallowed, do not induce vomiting; rinse the mouth, drink water, and get medical help.

Incompatible with: Strong acids (very exothermic neutralisation); aluminium, zinc, and tin (hydrogen gas evolved — reaction can be vigorous); ammonium salts (releases ammonia gas on heating); moisture in containers (heat and potential spattering if added carelessly); high concentrations around organic materials (risk of charring)

Disposal: neutralize spills and spent solutions with dilute acid or plenty of vinegar to near-neutral pH, then flush down the drain with lots of water. Keep solid lye sealed for reuse rather than discarding.

Footnotes

1. Humphry Davy — British chemist (1778–1829) who used electrolysis to isolate sodium, potassium, and other elements.

2. Louis-Camille Maillard — French chemist (1878–1936) who described the browning reaction between sugars and amino acids.