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Water alone cannot lift oil off a plate, but a little soap can. And in a bucket of hand-pump water, the same soap gives little lather and leaves a white scum. The last section of the Class 10 chapter on carbon compounds explains both with one idea: a molecule with two different ends. This post explains micelles, works out how much soap hard water wastes, and walks through the activities and mistakes.
Where is this in the syllabus?
The CBSE Class X Science curriculum for 2026-27 lists "soaps and detergents" under Carbon and its Compounds. Its practical list includes "Study of the comparative cleaning capacity of a sample of soap in soft and hard water". The NCERT Class 10 Science chapter Carbon and its Compounds (reprint 2026-27) covers it in Section 4.5, with Activities 4.10 to 4.12. Check your own year's curriculum.
What is a soap molecule?
NCERT defines soaps as "sodium or potassium salts of long-chain carboxylic acids". They are made by saponification: an ester treated with sodium hydroxide gives back an alcohol and the sodium salt of a carboxylic acid. A typical example is sodium stearate, C₁₇H₃₅COONa. PubChem lists its formula as C₁₈H₃₅NaO₂, its molecular weight as 306.5, and its cosmetic functions as including surfactant and cleansing.
Each molecule has two parts:
an ionic head, –COO⁻ Na⁺, which is hydrophilic: it interacts with water;
a long hydrocarbon tail, which is hydrophobic: it interacts with oil and hydrocarbons.
How does a micelle form?
At the surface of water, NCERT explains, soap molecules line up with the ionic end in the water and the tail sticking out. Inside the water, they form clusters with the tails in the interior and the ionic ends on the surface. Each cluster is a micelle.
Oily dirt collects in the centre of the micelle, surrounded by tails, while the heads face the water. NCERT says the micelles stay in solution as a colloid and do not come together to precipitate, because of ion-ion repulsion, so the dirt is rinsed away. The micelles are large enough to scatter light, which is why soap solution looks cloudy.
NCERT asks what micelle forms if you dissolve soap in a hydrocarbon. Turn the picture inside out: now the tails face the hydrocarbon outside, and the ionic heads cluster in the middle.
Why does hard water waste soap?
Hard water contains calcium and magnesium salts. NCERT explains that soap reacts with them to form an insoluble substance, scum, so you need more soap before you get lather. With calcium:
2C₁₇H₃₅COONa + Ca²⁺ → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺
Detergents solve this. NCERT describes them as generally sodium salts of sulphonic acids, or ammonium salts with chloride or bromide ions. Their charged ends do not form insoluble precipitates with calcium and magnesium ions, so they keep working in hard water. They are used in shampoos and laundry products.
Worked example: how much soap does hard water waste?
Use rounded atomic masses: C = 12 u, H = 1 u, O = 16 u, Na = 23 u, Ca = 40 u (the CIAAW standard atomic weights round to these).
Mass of sodium stearate, C₁₈H₃₅NaO₂: 18 × 12 + 35 × 1 + 2 × 16 + 23 = 216 + 35 + 32 + 23 = 306 u.
Ratio from the equation: two soap molecules for each calcium ion, so 612 u of soap for every 40 u of calcium.
Soap per gram of calcium: 612 ÷ 40 = 15.3 g.
A bucket (sample numbers): 10 litres of hand-pump water with 60 mg of calcium ions per litre holds 600 mg, or 0.6 g, of calcium. That turns about 0.6 × 15.3 ≈ 9.2 g of soap into scum before any lather forms.
With magnesium (24 u) the figure is 612 ÷ 24 = 25.5 g of soap per gram, because each magnesium ion is lighter. With unrounded atomic weights the calcium figure is 15.29 g, so the rounding makes no real difference.
How do the NCERT activities work?
Activity 4.10: oil in water with and without soap, shaken. The oil layer separates out sooner in the tube without soap.
Activity 4.11: soap in distilled or rain water and in hard water. You get more foam in the soft water and a white curdy precipitate in the hard water.
Activity 4.12: soap and detergent in hard water. The detergent foams; the soap forms the curdy solid.
Make each one a fair test: same volume of water, same number of drops, same shaking time.
Which mistakes cost marks?
Drawing the tails pointing outwards in water. In water, tails point in and heads point out.
Saying soap dissolves oil. It traps oil inside micelles so it can be rinsed away.
Calling detergents salts of carboxylic acids. That describes soaps.
Mixing up saponification and esterification. Esterification makes an ester; saponification breaks one with an alkali.
A practice routine
Draw a micelle in water and in a hydrocarbon, labelling head, tail and dirt.
Write the saponification word equation and the calcium scum equation from memory.
Redo the worked example for 5 litres of water with 80 mg of calcium per litre. (Answer: 0.4 g of calcium, about 6.1 g of soap.)
Try Activity 4.11 at home with tap water and rain water, and note which gives more foam.
For naming and the reactions earlier in the chapter, see our guide to carbon compounds in Class 10. The chemist who worked out what fats and soaps are made of is in our post on Michel Chevreul.
If an idea like hydrophobic or colloid is still unclear, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson.
Tonight, draw a micelle round an oil drop from memory, then check it against NCERT's Figure 4.12.
