Soaps vs Detergents: Scum and Hard Water | Class 10 Science

Quick answer: Soaps are sodium or potassium salts of long-chain fatty acids (head group $-\mathrm{COO^-}$). In hard water, they react with calcium and magnesium ions to form an insoluble, curdy precipitate called scum, so they don't lather well. Synthetic detergents are usually sodium salts of sulphonic acids (head group $-\mathrm{SO_3^-}$) or alkyl hydrogen sulphates. Their calcium and magnesium salts are soluble, so they work well even in hard water. Soaps are biodegradable, while detergents with branched chains are not easily broken down by bacteria.

Water alone cannot wash oily dirt off clothes, because oil and water do not mix. We use two kinds of cleaning agents to solve this problem: soaps, which people have made for centuries from animal fats and vegetable oils, and synthetic detergents, which are made from petroleum products and work even in hard water.

Early detergents also caused a pollution problem: because bacteria could not break them down, they built up as thick foam in rivers and sewage systems.

In CBSE Class 10 Science, Chapter 4 (Carbon and its Compounds), the difference between soaps and detergents and why soap forms scum in hard water are frequently asked board exam questions.

What You Will Learn

• The structure of soap molecules: sodium or potassium salts of long-chain fatty acids
• The structure of synthetic detergent molecules: sulphonate or sulphate head groups
• What hard water is and how it reacts with soap to form scum
• Why detergents lather easily in hard water
• Biodegradability of soaps and detergents
• A full comparison table of soaps and detergents

1. Structure of Soaps and Detergents

Both soap and detergent molecules have two ends:

• A long hydrocarbon tail that is hydrophobic (repels water but dissolves in oil and grease).
• An ionic head that is hydrophilic (attracted to water).

The difference is in the head group:

Definitions

Soap: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids), such as sodium stearate ($\mathrm{C_{17}H_{35}COONa}$) and sodium palmitate ($\mathrm{C_{15}H_{31}COONa}$).

Synthetic detergent: Synthetic detergents are generally sodium salts of long-chain sulphonic acids or alkyl hydrogen sulphates, such as sodium lauryl sulphate and sodium dodecylbenzene sulphonate.

2. Soaps and Detergents in Hard Water

Hard water contains dissolved salts of calcium ($\mathrm{Ca^{2+}}$) and magnesium ($\mathrm{Mg^{2+}}$), such as their hydrogen carbonates, chlorides and sulphates.

Why Soap Forms Scum in Hard Water

When soap is added to hard water, the calcium and magnesium ions react with the soap to form insoluble salts:

$2\mathrm{C_{17}H_{35}COONa} + \mathrm{Ca^{2+}} \longrightarrow (\mathrm{C_{17}H_{35}COO})_2\mathrm{Ca} \downarrow + 2\mathrm{Na^+}$

$2\mathrm{C_{17}H_{35}COONa} + \mathrm{Mg^{2+}} \longrightarrow (\mathrm{C_{17}H_{35}COO})_2\mathrm{Mg} \downarrow + 2\mathrm{Na^+}$

Scum is the insoluble, curdy white precipitate of calcium and magnesium salts of fatty acids that forms when soap is used in hard water.

Effects of scum:

• A lot of soap is wasted reacting with the calcium and magnesium ions before any lather forms.
• The scum sticks to clothes, making them look dull and feel stiff.

Why Detergents Work in Hard Water

The calcium and magnesium salts of detergents are soluble in water. So detergents do not form scum in hard water, and they lather and clean well in both soft and hard water.

Key point: Soap forms insoluble scum with calcium and magnesium ions in hard water. Detergents form soluble salts, so they are effective in hard water.

3. Biodegradability of Soaps and Detergents

• Bacteria can easily break down the straight hydrocarbon chains of soap, so soap is biodegradable.
• Many early detergents had branched chains that bacteria could not break down easily. These detergents stayed in the water and produced thick foam in rivers, lakes and sewage systems, harming aquatic life.
• Modern detergents are made with straight (linear) chains (for example, linear alkylbenzene sulphonates), which makes them much more biodegradable.

Syllabus note: Biodegradability of detergents goes a little beyond the current NCERT Class 10 textbook, but it is useful background for understanding why detergents can pollute water.

4. Soaps vs Synthetic Detergents: Comparison Table

5. Exam Tips and Common Mistakes

Exam Tip: Can you use a detergent to check whether water is hard? No. Detergents lather in both soft and hard water, so they cannot tell the two apart. Use soap instead: it lathers in soft water but forms scum in hard water.

Exam Tip: When asked why detergents are better in hard water, say that the calcium and magnesium salts of detergents are soluble, so no scum forms.

Common Mistake: Mixing up the head groups. Soap has a $-\mathrm{COO^-}$ (carboxylate) head; detergents have a $-\mathrm{SO_3^-}$ (sulphonate) or $-\mathrm{OSO_3^-}$ (sulphate) head.

Frequently Asked Questions

What is the difference between soaps and detergents?

Soaps are sodium or potassium salts of long-chain fatty acids and form scum in hard water. Detergents are sodium salts of sulphonic acids or alkyl hydrogen sulphates and work well in hard water.

Why does soap form scum in hard water?

Hard water contains calcium and magnesium ions, which react with soap to form insoluble calcium and magnesium salts of fatty acids. This insoluble precipitate is called scum.

Why do detergents work in hard water?

The calcium and magnesium salts of detergents are soluble in water, so no scum forms and the detergent can lather and clean.

Can detergents be used to test hard water?

No. Detergents lather in both soft and hard water, so only soap can show whether water is hard.

Are soaps biodegradable?

Yes. Soaps have straight hydrocarbon chains that bacteria can break down easily.

Why are some detergents harmful to the environment?

Detergents with branched hydrocarbon chains are not easily broken down by bacteria, so they can stay in water bodies and form foam that harms aquatic life.