Make Soap — Saponification

React vegetable oil with sodium hydroxide to produce real soap and glycerol
Intermediate🕐180 min▲Moderate hazardorganic

Summary

You’ll turn oil and lye into real soap by saponification1 — the alkaline hydrolysis of fat’s ester bonds — plus glycerol. You’ll be able to explain why different oils need different amounts of lye, how an amphiphilic soap molecule lifts grease, and why hard water makes scum.

History

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.

Hazards & preparation

Warning

Sodium hydroxide is severely caustic — this is the main hazard.

  • PPE: gloves and eye protection throughout, plus long sleeves.
  • Sodium hydroxide (NaOH) dissolving in water heats up violently — always add the solid to the water, never water to the solid, and work in a ventilated area. If it contacts skin, flush with running water for 15 minutes.
  • The fresh soap batter is strongly alkaline (pH ~12) until cured — keep it off skin.

Disposal: neutralize leftover lye (add dilute acid/vinegar toward neutral) before draining. Cured soap is harmless. See the Safety page.

Materials

  • Vegetable oil — 200 g (coconut oil gives a harder, lathering soap; olive oil gives a softer, conditioning soap)
  • Sodium hydroxide (NaOH) — 38 g (for coconut oil) or 27 g (for olive oil), matching each oil’s SAP value below (reduce ~5% for a gentler, superfatted bar — see Going further)
  • 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)

Procedure

Part 1 — Preparing the lye solution (20 min)

  1. Measure 75 mL cold water into a heatproof glass container.
  2. Weigh 38 g NaOH (for coconut oil; use 27 g for olive oil).
  3. In a ventilated area, slowly add the NaOH to the water while stirring.
  4. The solution heats rapidly to 80–90°C — do not touch the container.
  5. Stir until fully dissolved (clear solution).
  6. Set aside to cool to ~40–45°C.

Part 2 — Preparing the oil (10 min)

  1. Melt coconut oil if solid (gently, to ~40°C).
  2. Bring the oil to 40–45°C so oil and lye are at similar temperatures.

Part 3 — Saponification (30 min)

  1. Slowly pour the lye 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 leaves a visible trail on the surface before sinking.
  4. At trace, optionally add fragrance or colourant.
  5. Pour into the mould.

Part 4 — Curing (48 h + 4 weeks)

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

Part 5 — Testing the soap (after curing)

  1. Dissolve a shaving of cured soap in warm water (~1% solution) and test the pH.
  2. Rub a piece on greasy skin or a greasy plate to see it lather.
  3. Dissolve soap in hard vs. soft water and compare soap-scum formation.

What you should see

As you blend, the thin oily mix slowly thickens and turns opaque and creamy, until a drizzle sits on the surface as a “trace” before sinking — the moment to pour. Wrapped up, it heats and gels, then sets to a firm block you can cut. Cured bars lather and, in hard water, throw a cloudy scum that soft water doesn’t.

Symptom Likely cause Fix
Never reaches trace Oil/lye too cool, or under-mixed Warm both to ~40°C; blend longer (a stick blender is far faster)
Oily layer separates Lye and oil didn’t emulsify / bad measurement Re-blend; double-check the NaOH weight for your oil (SAP value)
Crumbly, cracked, or lye-heavy soap Too much NaOH Reweigh carefully next time; superfat (5–8% less lye) for safety
Soft, won’t harden Too much soft oil (e.g. olive) or too much water Cure longer; use more hard oil (coconut) next batch

The reactions

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, \(R\) is a long hydrocarbon chain (C₁₂–C₁₈), \(RCOONa\) is sodium soap, and \(C_3H_5(OH)_3\) is glycerol — the sweet, syrupy by-product left in every batch of soap.

fat + lye → soap + glycerol.

For coconut oil, the dominant fatty acid is lauric acid (C₁₂), giving 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 gram 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: the carboxylate head (-COO⁻) is polar and dissolves in water; the hydrocarbon tail is non-polar and dissolves in oil. In water, soap molecules arrange into micelles — spheres 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, forming insoluble calcium soaps:

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

The white precipitate is soap scum — which is why synthetic detergents replaced soap in laundry: their surfactants don’t form insoluble calcium salts.

Hot vs. cold process

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

Questions to Explore

  1. Why does coconut oil give a harder, more lathering soap than olive oil? Coconut is lauric (C₁₂), olive is oleic (C₁₈). How does chain length change the soap?

    Hint / answer

    Shorter, more saturated chains pack tightly and dissolve more readily in water, giving a hard bar that lathers freely. Olive oil’s long, kinked unsaturated chains pack poorly and are less water-soluble, so the soap is softer and more conditioning.

  2. What happens chemically at “trace”? The mix thickens to custard. Why, and why does a stick blender reach it faster?

    Hint / answer

    Trace marks the point where enough oil and lye have emulsified and begun reacting into soap that the mixture thickens and won’t separate again. A stick blender emulsifies the two liquids into tiny droplets with huge contact area, so they react (and thicken) far faster than slow hand-stirring.

  3. Why is the SAP value different for different oils? Shorter-chain oils need more NaOH per gram — why, thinking in moles?

    Hint / answer

    NaOH reacts mole-for-mole with fatty acid groups. A shorter-chain fat weighs less per molecule, so a gram of it contains more molecules (more ester bonds) than a gram of a long-chain fat — hence more NaOH needed per gram.

  4. How does a micelle remove grease? Why doesn’t it fall apart and release the grease in water?

    Hint / answer

    The tails hide from water by burying in the grease core while the heads face out into the water — the lowest-energy arrangement, so it’s stable. It stays assembled because pulling a tail out into water would be energetically unfavourable, keeping the grease trapped until it’s rinsed away.

  5. Why did synthetic detergents replace soap for laundry? What structural feature avoids reacting with calcium?

    Hint / answer

    Many synthetic surfactants use head groups (like sulfonates) whose calcium salts stay soluble, so they don’t precipitate scum in hard water. Soap’s carboxylate head forms insoluble calcium soap, so detergents win wherever water is hard.

Going further

  • Superfat it. Reduce the lye by 5–8% to leave a little unreacted oil, making a gentler, more moisturising bar with no risk of excess lye.
  • Make liquid soap. Substitute KOH for NaOH to get a soft, water-soluble potassium soap you can dissolve into liquid soap.
  • Measure a molecule of the same family: oleic acid is one of these fatty acids — estimate its length in the Oil Film experiment.

Related experiments and chemicals:

Footnotes

  1. Saponification — The reaction of a fat or oil with a strong base to produce soap and glycerol.↩︎