Laundry agents: soaps, detergents and finishes

22 min

Let's begin

Ask any boarding-school student about their white uniform shirt in the third month of term. It started bright white; now the collar is grey, the underarms are yellowish and the whole shirt looks dull. Hard scrubbing with bar soap has not helped. The shirt is not worn out: it needs the right laundry agents. A detergent that keeps dirt from settling back, an enzyme product for sweat and food stains, and a little bleach or laundry blue for whiteness would bring most of that colour back.

This lesson explains what each laundry agent does, which fabrics it suits, and which fabrics it damages.

Definition

Laundry agent: any substance used in washing, finishing or caring for fabrics: soaps, detergents, bleaches, starch, blue, softeners and water softeners.

Soap: a cleaning agent made from fats or oils and an alkali. Synthetic detergent: a cleaning agent made mainly from petroleum chemicals; unlike soap, it does not form scum in hard water.

Biological detergent: a detergent containing enzymes, proteins that break down stains made of protein (blood, egg, sweat), starch or fat.

Hard water: water containing dissolved calcium and magnesium salts, which react with soap to form a grey scum.

Cleaning agents

Bar soap is cheap and good for hand-washing cotton and linen and for rubbing on collars and cuffs. Laundry soap is alkaline, and in hard water it leaves scum that greys white clothes.
Detergent powder and liquid contain surfactants plus builders (which soften the water), anti-redeposition agents (which stop loosened dirt settling back) and often optical brighteners. They work in hard and cold water.
Biological detergents remove protein, starch and fat stains at low temperatures. Wool and silk are themselves made of protein, so enzymes can weaken them: use a non-biological, mild detergent.
Mild (wool) detergent is close to neutral, for wool, silk and delicate fabrics.

Whitening, finishing and softening agents

Agent	What it does	Use on	Never use on
Chlorine bleach (sodium hypochlorite, sold as Javel)	whitens, removes stains, kills germs	white cotton and linen, always diluted	wool, silk, elastane and coloured fabrics; never mix with ammonia or acids
Oxygen bleach (sodium percarbonate)	gentler whitening and stain removal	whites and most colourfast colours	wool and silk unless the label allows it
Laundry blue	adds a trace of blue that cancels a yellow tinge, so whites look whiter	white cotton and linen, in the last rinse	coloureds; too much leaves blue streaks
Starch (from cassava, maize or rice)	stiffens fabric for a crisp finish and helps it resist dirt	cotton and linen shirts, tablecloths, napkins	underwear and towels, which should stay soft and absorbent
Fabric softener	coats fibres so they feel soft and cling less	clothes and bedding	towels and sportswear: the coating cuts absorbency
Washing soda (sodium carbonate)	softens hard water and helps lift grease	heavily soiled cotton work clothes	wool and silk (strong alkali)

Optical brighteners and laundry blue do not remove dirt. They change how the fabric reflects light, so a white that is still dirty may look clean. Washing well comes first.

American Cleaning Institute: a short animation of how surfactants in detergent lift oily soil off fabric and hold it in the water. Watch what happens to the dirt between the wash and the rinse.

Worked example

Worked example 1: the right dose of detergent. A detergent label says: 40 g of powder per 5 litres of water for normal soiling, and 60 g per 5 litres for heavy soiling. Uwase washes heavily soiled football kit in a basin holding 12 litres. How much powder does she need?

Dose for heavy soiling: 60 g per 5 L.
$\text{powder} = 60\,\mathrm{g} \times \frac{12\,\mathrm{L}}{5\,\mathrm{L}} = 144\,\mathrm{g}$
Answer: 144 g. More powder than this does not wash better; it is hard to rinse out and leaves the kit stiff and irritating to the skin.

Worked example

Worked example 2: matching agents to items. Choose agents for (a) a white cotton school shirt with a blood spot on the cuff; (b) a wool sweater; (c) a set of bath towels.

(a) Soak the cuff in cold water with a biological detergent (enzymes digest the blood protein), wash, and if needed use diluted chlorine bleach, which is safe on white cotton. Laundry blue in the last rinse restores whiteness.
(b) A mild, non-biological wool detergent in lukewarm water. No bleach, no washing soda: wool is a protein fibre damaged by alkalis and enzymes.
(c) Ordinary detergent and no fabric softener, because softener coats the loops of the towel and reduces how much water it can absorb.

Warning

Handle laundry agents safely. Measure, do not guess. Never mix chlorine bleach with ammonia or with acidic products such as vinegar: poisonous gases are released (see Unit 1). Wear gloves for bleach and washing soda, which irritate the skin. Keep powders and liquids, especially brightly coloured liquid capsules, locked away from small children, who may take them for sweets. If detergent gets in the eyes, rinse them at once with plenty of clean running water for 10 to 20 minutes and get medical help.

Tip

Memory hook: protein fibres (wool, silk) hate the three Bs: Bleach, Basic (alkaline) agents and Biological enzymes

✦

Practice

Define a biological detergent and hard water.
Give two advantages of synthetic detergent over bar soap.
What does laundry blue do, and when is it added?
Why should fabric softener not be used on towels?
A label says 30 g of powder per 4 litres. How much powder is needed for a 10-litre basin?
Gatete soaks his mother's silk scarf overnight in washing soda and biological detergent to remove a sauce stain. Predict the result and explain it.
A school laundry must choose one detergent for both cotton bed sheets and wool blankets. Evaluate the choice of a biological detergent and recommend a better plan.

Show answersHide answers

A biological detergent contains enzymes that break down protein, starch or fat stains. Hard water contains dissolved calcium and magnesium salts that react with soap to form scum.
Detergent does not form scum in hard water; it works in cold water; it contains builders and anti-redeposition agents that keep dirt from settling back (any two).
It adds a trace of blue that cancels a yellow tinge, so whites look whiter. It is added to the last rinse.
Softener coats the fibres, so the towel absorbs water less well.
$30 \times \frac{10}{4} = 75$ g.
The silk will be weakened and may lose its shine or even tear: silk is a protein fibre, so it is damaged both by the strong alkali (washing soda) and by enzymes that digest protein.
Not suitable for the wool blankets: enzymes can weaken wool, a protein fibre. Better: biological or ordinary detergent for the cotton sheets, and a mild, non-biological wool detergent for the blankets, washed separately.

Sources and credits

Lesson text, Figure 1, table, worked examples and practice: written by Ganzaa. pH values in Figure 1 are approximate.
Eye first aid: European Resuscitation Council Guidelines 2025: First Aid, Djärv T. et al., Resuscitation 215 (2025), with ERC Guidelines 2021: First aid, Zideman D.A. et al. Summarised in Ganzaa's own words.
Video: How Detergents Works: Surfactants, American Cleaning Institute (cleaninstitute), embedded from YouTube.

1Cleaning agents

2Whitening, finishing and softening agents

Cleaning agents

Whitening, finishing and softening agents