Make Soap — Saponification

React vegetable oil with sodium hydroxide to produce real soap and glycerol

Difficulty: Intermediate | Time: 2–3 hours (plus 24–48 h curing) | Visual Impact: High

Historical Context

Soap is one of the oldest manufactured chemicals. Babylonian clay tablets from around 2800 BCE describe boiling animal fat with ashes. The Egyptians mixed animal and vegetable oils with alkaline salts. Ancient Roman soap-making sites have been found at Pompeii.

For most of history, soap was made at home from wood-ash lye (potassium carbonate leached from ashes) and animal fat. The process required weeks of rendering fat, leaching ash, and testing lye strength. The chemistry was completely unknown — what mattered was the empirical rule that the lye be strong enough to float an egg. Good soap was a prized household product, and soapmakers were skilled craftspeople.

The understanding of saponification — the reaction between esters and alkalis — came with the development of organic chemistry in the 19th century. Michel Eugène Chevreul established the modern understanding between 1813 and 1823, showing that fats are esters of glycerol and fatty acids. Modern industrial soap is made by the same reaction, now precisely controlled.

Materials

  • Vegetable oil — 200 g (coconut oil gives a harder, lathering soap; olive oil gives a softer, conditioning soap)
  • Sodium hydroxide (NaOH) — 28 g (for coconut oil) or 27 g (for olive oil)
  • Distilled or deionised water — 75 mL
  • Thermometer (0–100°C)
  • Heatproof containers × 2 (glass or stainless steel — not aluminium)
  • Rubber spatula or stick blender
  • Silicone soap mould, lined box, or plastic container
  • Gloves and eye protection (essential)
Warning

Sodium hydroxide is severely caustic. Wear gloves and eye protection throughout. Dissolving NaOH in water produces significant heat — add the solid to the water, never water to the solid. If skin contact occurs, flush with running water for 15 minutes.

Procedure

Part 1 — Preparing the Lye Solution (20 min)

  1. Measure 75 mL of cold water into a heatproof glass container
  2. Weigh 28 g NaOH
  3. Working in a ventilated area, slowly add the NaOH to the water while stirring
  4. The solution will heat rapidly to 80–90°C — do not touch the container
  5. Stir until fully dissolved (solution will be clear)
  6. Set aside to cool to approximately 40–45°C

Part 2 — Preparing the Oil (10 min)

  1. Melt coconut oil if solid (heat gently to ~40°C)
  2. Cool oil to 40–45°C if it was heated — oil and lye should be at similar temperatures

Part 3 — Saponification (30 min)

  1. Slowly pour the lye solution into the oil (not the other way around)
  2. Stir continuously — by hand (vigorous, 15–20 min) or with a stick blender (3–5 min)
  3. Continue until the mixture reaches trace — a thick, custard-like consistency where a drizzle from the spatula leaves a visible trail on the surface before sinking
  4. At trace, optionally add fragrance (a few drops of essential oil) or colourants
  5. Pour into the mould

Part 4 — Curing (48 h + 4 weeks)

  1. Cover and insulate the mould (wrap in towels — the soap will gel as heat continues the reaction)
  2. After 24 hours, the soap should be firm; unmould after 48 hours
  3. Cut into bars and leave to cure in a ventilated place for 4 weeks minimum
  4. The pH will drop from ~12 at pouring to ~9–10 when cured

Part 5 — Testing the Soap (after curing)

  1. Dissolve a small shaving of cured soap in warm water to make a ~1% solution
  2. Test the pH with indicator paper
  3. Rub a piece on greasy skin or a greasy plate — observe lathering
  4. Dissolve soap in hard water (if available) vs. soft water — observe soap scum formation

The Reaction

Saponification is the alkaline hydrolysis of an ester. A fat (triglyceride) has three ester bonds; NaOH cleaves all three:

\[\ce{(RCOO)3C3H5 + 3 NaOH -> 3 RCOONa + C3H5(OH)3}\]

Where: - \((RCOO)_3C_3H_5\) is the triglyceride (fat or oil) - \(R\) is a long hydrocarbon chain (C₁₂–C₁₈ for common fats) - \(RCOONa\) is sodium soap (e.g. sodium laurate from coconut oil) - \(C_3H_5(OH)_3\) is glycerol

For coconut oil, the dominant fatty acid is lauric acid (C₁₂), giving sodium laurate:

\[\ce{CH3(CH2)10COO^- + Na+ -> sodium\ laurate}\]

The Science

Saponification Number

Different oils have different fatty acid compositions and require different amounts of NaOH. The saponification value (SAP value) is the grams of NaOH needed per 100 g of fat:

Oil | Dominant fatty acid | SAP value |

|—–|——————–|———–|| | Coconut oil | Lauric acid (C₁₂) | 0.190 | | Palm kernel oil | Lauric acid (C₁₂) | 0.156 | | Olive oil | Oleic acid (C₁₈:1) | 0.134 | | Castor oil | Ricinoleic acid (C₁₈:1-OH) | 0.128 |

Shorter carbon chains and more saturated fatty acids give harder, more water-soluble soaps that lather easily. Longer, unsaturated chains give softer, conditioning soaps (castile soap from olive oil).

Why Soap Cleans

Each soap molecule is amphiphilic: one end (the carboxylate head, -COO⁻) is polar and dissolves in water; the other end (the hydrocarbon tail) is non-polar and dissolves in oil. In water, soap molecules arrange into micelles — spherical structures with tails pointing inward around trapped grease. The micelle is water-soluble and washes away.

Why Soap Scum Forms

In hard water, calcium and magnesium ions replace sodium in the soap molecule, forming insoluble calcium soaps:

\[\ce{2 RCOONa + Ca^{2+} -> (RCOO)2Ca v + 2 Na+}\]

The white precipitate is soap scum. This is why modern detergents replaced soap in laundry — synthetic surfactants do not form insoluble salts with hard water.

Hot vs. Cold Process

The cold process (this experiment) uses the heat of saponification itself to complete the reaction during curing. The hot process (soap making in a slow cooker) accelerates the reaction to completion in a few hours — the soap is ready to use sooner but has a rougher texture.

Explore Further

  1. Superfatting: Reduce the lye by 5–8% (this leaves excess unreacted oil in the final soap, making it more moisturising and reducing risk from excess NaOH)
  2. Potassium soap: Substitute KOH for NaOH — the product is a softer, more water-soluble soap that can be dissolved into liquid soap
  3. Soap scum test: Make soap, then dissolve a small piece in hard tap water vs. deionised water — observe and compare the cloudiness
  4. Glycerol test: Commercial soap manufacturers remove the glycerol for use in cosmetics. Home soap retains it, making it more moisturising — test home soap vs. commercial bar soap on skin

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