Chemistry • Year 12 • Module 7 • Lesson 17
Soaps, Detergents & Saponification
Apply the chemistry of soap and detergents to real data, compare properties, and reason about the cleaning mechanism and hard-water problem.
1. Soap vs synthetic detergent — properties comparison
The table below compares key properties of sodium stearate (a typical bar soap) and sodium dodecylbenzenesulfonate (SDBS, a common anionic synthetic detergent used in many Australian household products). Study the data, then answer the questions. 8 marks
| Property | Sodium stearate (soap) | SDBS (synthetic detergent) |
|---|---|---|
| Head group | Carboxylate (–COO⁻Na⁺) | Sulfonate (–SO3⁻Na⁺) |
| Feedstock / source | Renewable (animal tallow / plant oil) | Petrochemical (non-renewable) |
| Behaviour in hard water (350 mg/L Ca²⁺) | Forms insoluble calcium stearate precipitate (scum); cleaning impaired | Remains soluble; no scum; cleaning unaffected |
| Approximate solution pH | 9.5–10.5 (slightly basic) | 6.5–8.0 (near neutral) |
| Biodegradability | Readily biodegradable (>90% within 28 days) | Modern formulations biodegradable; early 1950s formulations were not |
| Skin compatibility | Generally mild; natural glycerol co-product | Some formulations (e.g. SLS) can irritate skin |
| Cleaning mechanism | Amphipathic; micelle formation; emulsification | Amphipathic; micelle formation; emulsification |
| Used in soft water (0–60 mg/L Ca²⁺) | Effective; no scum | Effective; no scum |
Data: adapted from OECD (2001) SIDS Initial Assessment Report for SDBS; Woollatt (1985) The Manufacture of Soaps, Other Detergents and Glycerin.
1.1 Identify the key structural difference between the head groups of soap and SDBS, and explain how this structural difference accounts for their different behaviours in hard water. 3 marks
1.2 According to the data, in what water condition do soap and SDBS perform comparably? Explain why. 2 marks
1.3 A student concludes from the table: "SDBS is always the better choice." Identify two pieces of evidence from the table that challenge this conclusion. 2 marks
1.4 Explain why the cleaning mechanism row is identical for both soap and SDBS even though they have different head groups. 1 mark
2. Interpret a graph — cleaning efficiency vs water hardness
The graph below shows the cleaning efficiency (% grease removed from a test cotton fabric) for bar soap and a non-ionic synthetic detergent as the hardness of the wash water is increased from 0 to 500 mg/L Ca²⁺. A hardness of ~350 mg/L is representative of bore water in parts of inland NSW (Murray–Darling basin). 9 marks
Model data adapted from: Wortel & Wiechers (1996), International Journal of Cosmetic Science 18(3): 117–127; OECD (2001) SDBS SIDS Report. Murray–Darling hardness reference: MDBA Water Quality Report 2023.
2.1 Describe the trend in cleaning efficiency for bar soap as water hardness increases from 0 to 500 mg/L. Refer to specific values. 2 marks
2.2 Estimate the difference in cleaning efficiency between bar soap and the non-ionic detergent at 350 mg/L (Murray–Darling representative hardness). State this as a percentage point difference. 2 marks
2.3 Using lesson chemistry, explain why bar soap's cleaning efficiency falls sharply as water hardness increases. Write the ionic equation as part of your answer. 3 marks
2.4 Explain at the molecular level why the non-ionic detergent's efficiency is almost unchanged across the hardness range. 2 marks
3. Sequence the steps — soap cleaning mechanism
The six steps below describe how soap removes a grease stain from a cotton shirt in the wash cycle, but they are shuffled. In the “Order” column, write the correct sequence (1–6). 5 marks (award 1 mark per consecutive correct pair)
| Order | Step (shuffled) |
|---|---|
| The negatively charged –COO⁻ heads on the micelle outer surface repel other micelles and repel the cotton fibre surface, preventing the grease from re-depositing. | |
| Mechanical agitation (scrubbing) provides energy to lift the grease droplet away from the cotton fibre; the grease is now enclosed in a soap shell as an emulsified micelle droplet. | |
| Soap molecules are amphipathic: the long hydrophobic tail approaches the non-polar grease deposit, attracted by London dispersion forces. | |
| Rinsing water dilutes and carries the dispersed micelle droplets away from the fabric surface. | |
| Many soap molecules simultaneously insert their hydrophobic tails into the grease while their ionic carboxylate heads (–COO⁻Na⁺) remain pointing into the surrounding water (ion–dipole attractions). | |
| Soap is added to the wash water and dissolves; the solution concentration exceeds the critical micelle concentration (CMC). |
4. Case study — phosphate-free detergent legislation in NSW and the Great Barrier Reef
Read the passage, then answer the questions below. 6 marks
Passage. In 2014, New South Wales implemented regulations under the Protection of the Environment Operations Act 1997 to phase out phosphate-containing laundry detergents. Phosphate builders (polyphosphates, e.g. sodium tripolyphosphate) had been added to many synthetic detergents since the 1950s to bind Ca²⁺ and Mg²⁺ ions in hard water, preventing scum and improving cleaning performance. However, when phosphate-containing wastewater reached Australian waterways — including rivers flowing toward the Great Barrier Reef (GBR) — the phosphate acted as a fertiliser, driving algal blooms (eutrophication). In 2016 the Australian Institute of Marine Science (AIMS) identified nutrient runoff, including detergent-derived phosphate, as a contributor to poor water quality around the GBR. Phosphate-free formulations now use alternative water softeners such as zeolites or citrate complexes that are less bioavailable to algae.
4.1 Explain at the chemical level why phosphate builders were added to early synthetic detergents. Refer to hard water chemistry in your answer. 2 marks
4.2 Explain why natural soaps (sodium carboxylates) would not have required the same phosphate builders to manage the hard water problem if a hard-water soap user switched to using a water softener first. 2 marks
4.3 Identify one advantage and one limitation of soap over synthetic detergent from an environmental perspective, using the passage and lesson content. 2 marks
Q1.1 — Head group difference and hard water behaviour (3 marks)
Soap has a carboxylate head (–COO⁻), while SDBS has a sulfonate head (–SO3⁻). The calcium and magnesium salts of the carboxylate (e.g. calcium stearate, (RCOO)2Ca) are insoluble and precipitate as scum [1]. The calcium and magnesium salts of the sulfonate are soluble, so SDBS remains in solution in hard water [1]. This solubility difference is the key chemical reason soap fails in hard water while SDBS does not [1].
Q1.2 — Comparable in soft water (2 marks)
In soft water (0–60 mg/L Ca²⁺), soap and SDBS perform comparably [1]. Because the Ca²⁺ and Mg²⁺ concentration is very low, the carboxylate groups in soap are not significantly precipitated, so soap remains in solution and both clean by the same amphipathic/micelle mechanism with similar efficiency [1].
Q1.3 — Two pieces of evidence against "SDBS always better" (2 marks)
Any two of: (1) Soap uses a renewable feedstock (animal tallow / plant oil) while SDBS is petrochemical — soap has a sustainability advantage in terms of resource use. (2) Soap is readily biodegradable (>90% within 28 days), while SDBS biodegradability varies (early formulations were problematic). (3) Soap is mild on skin with natural glycerol co-product; some SDBS formulations irritate skin. [1 mark per valid evidence item, max 2]
Q1.4 — Same cleaning mechanism (1 mark)
Both soap and SDBS are amphipathic: they each have a long hydrophobic hydrocarbon tail and a hydrophilic head group. Both clean grease by the same mechanism — hydrophobic tails insert into grease, heads orient into water, forming micelles by emulsification. The type of head group (carboxylate vs sulfonate) does not change this fundamental amphipathic cleaning mechanism. [1 mark]
Q2.1 — Soap trend (2 marks)
As water hardness increases from 0 to 500 mg/L, bar soap's cleaning efficiency falls markedly and progressively [1]. At 0 mg/L it removes approximately 87% of grease; at 350 mg/L (Murray–Darling level) this drops to approximately 47%; at 500 mg/L it falls to approximately 25% [1 for at least two specific values cited].
Q2.2 — Difference at 350 mg/L (2 marks)
At 350 mg/L: soap cleans approximately 47% of grease; non-ionic detergent cleans approximately 88% of grease [1]. The difference is approximately 41 percentage points in favour of the non-ionic detergent [1]. Accept ± 5 percentage points.
Q2.3 — Why soap falls with hardness (3 marks)
As Ca²⁺ concentration rises, more soap molecules are removed from solution by precipitation. Ionic equation: 2RCOO⁻(aq) + Ca²⁺(aq) → (RCOO)2Ca(s)↓ [1 for correct ionic equation]. The precipitated calcium carboxylate is insoluble — it cannot form micelles and is unavailable for cleaning [1]. As more Ca²⁺ is present, more soap is consumed in forming scum, reducing the effective soap concentration available for emulsifying grease [1].
Q2.4 — Why non-ionic detergent is unaffected (2 marks)
A non-ionic detergent has no charged head group — it uses a polyethylene oxide chain (–O–CH2CH2–)n, which is hydrophilic via hydrogen bonding with water [1]. Because the head carries no ionic charge, Ca²⁺ and Mg²⁺ ions cannot precipitate it — there is no ionic interaction between the non-ionic head and the divalent cations. The detergent remains in solution at all hardness levels and continues to form micelles and clean effectively [1].
Q3 — Correct sequence (5 marks)
Correct order: 6 → 3 → 5 → 2 → 1 → 4
- Soap dissolves; concentration exceeds CMC.
- Hydrophobic tails approach grease; London dispersion forces.
- Many molecules insert simultaneously; tails into grease, heads into water.
- Agitation lifts grease off; emulsified micelle forms.
- –COO⁻ heads repel each other and fibre; grease cannot re-deposit.
- Rinsing carries micelles away.
Marking: 1 mark per consecutive correct pair (5–6, 6–3, 3–5 etc.).
Q4.1 — Purpose of phosphate builders (2 marks)
In hard water, Ca²⁺ and Mg²⁺ ions react with the carboxylate (soap) or could also impair ionic detergent performance by competing with surfactant interactions [1]. Polyphosphate builders formed soluble complexes with Ca²⁺ and Mg²⁺ ions (chelation / sequestration), preventing them from precipitating the detergent or reducing cleaning performance — effectively softening the water in situ [1].
Q4.2 — Soap and water softeners (2 marks)
If Ca²⁺ and Mg²⁺ ions are first removed from the water by a water softener (e.g. ion-exchange resin), there are no divalent cations available to precipitate the soap's carboxylate head groups [1]. Without hard water ions, calcium carboxylate scum cannot form and soap performs comparably to detergents — no phosphate builder is needed [1].
Q4.3 — Soap environmental advantage and limitation (2 marks)
Advantage: Soap is readily biodegradable (fatty acid salts, >90% biodegradable within 28 days) from renewable feedstock — less persistent in aquatic ecosystems and lower carbon footprint than petrochemical detergents [1]. Limitation: In hard water (such as rural NSW / Murray–Darling), soap forms insoluble scum and is much less effective — more soap per wash is needed, increasing total environmental load per unit of cleaning; early replacement of soap with synthetic detergents also meant detergent-derived phosphate reached waterways like those feeding the GBR [1].