8 min readConcept sheet3 or 5-mark question on micelles; 1-mark MCQ and assertion-reason on hard water; sometimes a case-based question

Soaps and detergents

A soap molecule has a water-loving ionic head and an oil-loving carbon tail. In water, the molecules form micelles that trap oily dirt so it can be rinsed away.

SoapSodium or potassium salt of a long-chain carboxylic acid

The two endsIonic head is hydrophilic (water-loving); carbon tail is hydrophobic (water-hating, so it dissolves in oil)

Hard waterContains calcium and magnesium salts; forms scum with soap

DetergentUsually a sodium salt of a sulphonic acid, or an ammonium salt; works in hard water

The soap molecule

Soap is the sodium or potassium salt of a long-chain carboxylic acid. One end is ionic (-COO⁻Na⁺). It dissolves in water, so it is called hydrophilic (water-loving). The other end is a long hydrocarbon chain. It dissolves in oil, so it is called hydrophobic (water-hating).

How soap cleans: micelles

1
In water, soap molecules gather with their ionic heads in the water and their tails pointing inward.
2★
This cluster of molecules is called a micelle.
3
The tails dissolve in the oily dirt, which collects in the centre of the micelle.
4
The micelles stay apart in the water, because their charged heads push each other away.
So the dirt stays in the water and does not settle back on the cloth
5
Scrubbing or beating the cloth helps, and rinsing with water washes the micelles and the dirt away.
A soap solution looks cloudy, because micelles are large enough to scatter light

★ key step

How to draw a micelle

Draw a small circle in the centre for the oily dirt.
Around it, draw soap molecules like tadpoles: a zig-zag tail with a round head.
Point all the tails inward, into the dirt, and all the heads outward, into the water.
Write the charge (-COO⁻) on the heads. Label: micelle, oily dirt, hydrophobic tail, ionic (hydrophilic) head, water.

Activity: soap in soft and hard water

1
Take 10 mL of distilled water (soft) in one test tube and 10 mL of hard water (from a tube well or hand pump) in another.
2
Add the same number of drops of soap solution to each.
3
Shake both tubes hard for the same length of time.
4★
Compare: distilled water gives a lot of foam; hard water gives little foam and a white, curdy scum.
Calcium and magnesium salts in hard water react with soap and form an insoluble scum
5
Repeat with a detergent solution.
The detergent foams well in both, because it does not form scum with hard water

★ key step

Soaps vs detergents

SoapsSodium or potassium salts of long-chain carboxylic acids. They form scum in hard water, so a lot of soap is wasted.

DetergentsUsually sodium salts of sulphonic acids, or ammonium salts with chloride or bromide ions. Their charged ends do not form insoluble compounds with calcium and magnesium, so they work in hard water. Used in shampoos and washing powders.

Hard water has calcium and magnesium salts dissolved in it. That is why washing clothes in hard water needs more soap.

In real life

Where borewell water is hard, soap forms scum and clothes stay dull, so people use detergent powders. Washing your hands well with soap and water removes oily dirt and germs, which helps protect you from illness.

Exam angle

1 marks: “In a soap micelle, which end of each soap molecule faces the water?” The ionic (hydrophilic) end. The hydrocarbon tails point inward, into the oily dirt.

2 marks: “Why does soap not work well in hard water? How do detergents solve this problem?” Calcium and magnesium salts in hard water react with soap to form an insoluble scum. Detergents do not form insoluble compounds with these ions.

3 marks: “Oily stains on a shirt do not wash out with water alone, but they come out with soap. Explain why, with a labelled micelle diagram.” Ionic hydrophilic head, hydrocarbon hydrophobic tail. In water, the tails dissolve in oil and form a micelle with the dirt in the centre and the heads facing the water. Rinsing removes the micelles. Draw a labelled micelle.

Remember it

Head in water, tail in oil, dirt in the middle: that is a micelle. Hard water + soap = scum; a detergent gives no scum.