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Carbon and its Compounds - Soaps and detergents

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids) such as stearic acid (C17H35COOHC_{17}H_{35}COOH), palmitic acid (C15H31COOHC_{15}H_{31}COOH), or oleic acid (C17H33COOHC_{17}H_{33}COOH).

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A soap molecule consists of two distinct parts: a long hydrocarbon chain which is hydrophobic (water-repelling) and a short ionic part (−COO−Na+-COO^-Na^+) which is hydrophilic (water-attracting).

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Micelles are spherical aggregates formed when soap is dissolved in water; the hydrophobic tails align towards the center (attaching to dirt/oil) and the hydrophilic heads face outwards towards the water.

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The cleansing action involves the hydrophobic tail attaching to the oily dirt and the hydrophilic head remaining in the water. When water is agitated, the micelle containing the dirt is lifted off the fabric surface.

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Hard water contains Ca2+Ca^{2+} and Mg2+Mg^{2+} ions. Soaps react with these ions to form an insoluble, curd-like precipitate called 'scum', which reduces the effectiveness of the soap.

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Detergents are generally sodium salts of long-chain benzene sulphonic acids or alkyl hydrogen sulphates. Unlike soaps, they do not form scum with Ca2+Ca^{2+} and Mg2+Mg^{2+} ions and are effective in hard water.

📐Formulae

C17H35COONa (Sodium Stearate)C_{17}H_{35}COONa \text{ (Sodium Stearate)}

C15H31COONa (Sodium Palmitate)C_{15}H_{31}COONa \text{ (Sodium Palmitate)}

Fat/Oil+NaOH→ΔSoap+Glycerol (Saponification)\text{Fat/Oil} + NaOH \xrightarrow{\Delta} \text{Soap} + \text{Glycerol (Saponification)}

2C17H35COONa+Ca2+→(C17H35COO)2Ca↓+2Na+2C_{17}H_{35}COONa + Ca^{2+} \rightarrow (C_{17}H_{35}COO)_2Ca \downarrow + 2Na^+

CH3(CH2)11C6H4SO3Na (A common detergent - Sodium n-dodecyl benzene sulphonate)CH_3(CH_2)_{11}C_6H_4SO_3Na \text{ (A common detergent - Sodium n-dodecyl benzene sulphonate)}

💡Examples

Problem 1:

A student tests two samples of water, A and B, with soap solution. Sample A forms lather easily, while Sample B forms a white curdy precipitate. Identify the type of water in both samples and explain the reaction in Sample B.

Solution:

Sample A is soft water. Sample B is hard water. In Sample B, the soap reacts with calcium or magnesium ions to form scum: 2C17H35COONa+MgSO4→(C17H35COO)2Mg↓+Na2SO42C_{17}H_{35}COONa + MgSO_4 \rightarrow (C_{17}H_{35}COO)_2Mg \downarrow + Na_2SO_4.

Explanation:

Soap molecules are consumed by the Mg2+Mg^{2+} or Ca2+Ca^{2+} ions in hard water to form insoluble salts (scum) before they can act on the dirt to form lather.

Problem 2:

Explain the structure of a micelle formed by soap in water.

Solution:

In a micelle, the hydrophobic hydrocarbon tails (represented as zig-zag lines) point towards the center to avoid contact with water, while the hydrophilic heads (−COO−Na+-COO^-Na^+) face the aqueous environment.

Explanation:

This arrangement is energetically favorable because the hydrophobic parts are sequestered away from the water molecules, while the ionic heads interact with the polar water molecules through ion-dipole interactions.

Soaps and detergents - Revision Notes & Key Formulas | CBSE Class 10 Science