Chemistry 11–12 · Year 12

Making soap from castor oil and sodium hydroxide, and comparing soap and a synthetic detergent in hard water

Module 7: Organic Chemistry

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Alkaline hydrolysis of a triglyceride gives glycerol and the sodium salts of fatty acids, whose ionic head and hydrocarbon tail let them lift grease, while calcium ions in hard water precipitate soap but not detergent.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

5 mol/L sodium hydroxide is corrosive and the hot mixture spits
ethanol flammable
soap curd is alkaline

Controls

tongs and gloves; the beaker is stirred in the water bath, never over a flame
eye protection throughout
do not touch the curd; do not take it home
wash hands

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Castor oil 2 mL, ethanol 5 mL, 5 mol/L sodium hydroxide 10 mL
100 mL beaker, glass rod, water bath near boiling (kettle water in a 400 mL beaker), tongs
Saturated sodium chloride solution 10 mL, ice bath, filter funnel and paper
Commercial liquid detergent 1 mL, 0.1 mol/L calcium chloride 10 mL, deionised water, test tubes, oil for the emulsion test

How to do it

Mix the castor oil, ethanol and sodium hydroxide in the beaker; stand it in the hot water bath and stir for five minutes until the mixture is uniform and the ethanol has mostly evaporated.
Add the saturated salt solution, stir, and cool in the ice bath; filter off the soap curd and press it dry between filter papers.
Dissolve a pea-sized piece of soap in 5 mL deionised water; shake and record the lather height; repeat with the same amount of detergent.
Add 2 mL of calcium chloride solution to each and shake; record the lather and any scum.
Add one drop of oil to soap solution and to plain water; shake both and record how long the cloudiness lasts.
Write the equation for saponification of a triglyceride, draw the soap anion showing the ionic head and hydrocarbon tail, and explain the hard-water and emulsion results.

What you should see

The mixture thickens into a pale paste and, after salting out and cooling, gives a soft off-white curd of sodium ricinoleate soap. In deionised water the soap lathers on shaking; adding calcium chloride kills the lather and forms a white scum (calcium ricinoleate). The detergent lathers in both and forms no scum. The oil drop stays dispersed as a cloudy emulsion in soap solution for minutes but separates within seconds in plain water. The curd still contains sodium hydroxide and must not touch skin.

What changes

What you change: cleaning agent (soap or synthetic detergent) and water hardness
What you measure: lather height and scum formation, emulsion persistence
What you keep the same: mass of agent
water volume
shaking method and time
calcium chloride volume

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Soap is a strong base (the soap solution is mildly alkaline; the corrosive base is the sodium hydroxide reagent that must be removed).
Detergents and soaps are the same thing (both have an ionic head and hydrocarbon tail, but detergents' sulfonate heads do not precipitate with calcium).
Salt is added to make the soap saltier (it lowers the soap's solubility so it separates from the glycerol and water).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-14 CH11/12-5
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 7, which it allows in any moduleCH11/12-3
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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