Bonding In Carbon – The Covalent Bond

In Chapter 3, we learned about ionic compounds, which have high melting/boiling points and conduct electricity when molten or in solution due to the formation of ions (transfer of electrons). However, many carbon compounds, such as acetic acid, chloroform, ethanol, and methane, have relatively low melting and boiling points and are poor conductors of electricity.

Melting and boiling points of some carbon compounds:

These low melting/boiling points suggest that the forces of attraction between the molecules of these carbon compounds are weak. Their poor electrical conductivity indicates the absence of mobile charged particles (ions).

This behaviour can be explained by the nature of bonding in carbon compounds. Carbon has an atomic number of 6, so its electronic configuration is 2, 4. It has **four valence electrons** in its outermost shell. To attain a stable noble gas configuration (like Helium with 2 electrons in K shell or Neon with 8 electrons in L shell), carbon needs to either gain four electrons or lose four electrons.

• Gaining four electrons to form C$^{4-}$ anion: This would be difficult for the nucleus with only six protons to hold onto ten electrons (four extra).
• Losing four electrons to form C$^{4+}$ cation: This would require a very large amount of energy to remove the four valence electrons.

Instead of gaining or losing electrons, carbon solves this problem by **sharing its valence electrons** with other atoms (carbon or other elements). This sharing of electrons leads to the formation of **covalent bonds**. The shared electron pairs are considered to belong to the outermost shells of both participating atoms, allowing them to achieve stable, filled outer shells (noble gas configuration).

Let's look at examples of covalent bond formation:

• **Hydrogen molecule (H$_2$):** Hydrogen (Atomic number 1) has 1 electron in its K shell and needs 1 more to fill it (like Helium). Two hydrogen atoms share their single electrons, forming one shared pair and a single covalent bond.

  H $\cdot$ + $\cdot$ H $\to$ H : H

  A single covalent bond is represented by a line: H—H.
• **Chlorine molecule (Cl$_2$):** Chlorine (Atomic number 17) has electronic configuration 2, 8, 7. It needs 1 electron to complete its octet. Two chlorine atoms share one pair of electrons, forming a single covalent bond.

  $: \ddot{\text{Cl}} \cdot$ + $\cdot \ddot{\text{Cl}} :$ $\to$ $: \ddot{\text{Cl}} : \ddot{\text{Cl}} :$

  Cl—Cl
• **Oxygen molecule (O$_2$):** Oxygen (Atomic number 8) has electronic configuration 2, 6. It needs 2 electrons to complete its octet. Two oxygen atoms share two pairs of electrons, forming a **double covalent bond**.

  $: \ddot{\text{O}} :$ + $: \ddot{\text{O}} :$ $\to$ $: \ddot{\text{O}} :: \ddot{\text{O}} :$

  O=O
• **Nitrogen molecule (N$_2$):** Nitrogen (Atomic number 7) has electronic configuration 2, 5. It needs 3 electrons to complete its octet. Two nitrogen atoms share three pairs of electrons, forming a **triple covalent bond**.

  $\cdot \ddot{\text{N}} :$ + $: \ddot{\text{N}} \cdot$ $\to$ $: \text{N} ::: \text{N} :$

  N$\equiv$N
• **Methane molecule (CH$_4$):** Carbon (4 valence electrons) shares each of its four valence electrons with one electron from four different hydrogen atoms (1 valence electron each). This forms four single covalent bonds between carbon and hydrogen, satisfying the valency of both carbon (octet) and hydrogen (duplet like Helium).

  $$ \begin{array}{c} \text{H} \\ | \\ \text{H}—\underset{|}{\text{C}}—\text{H} \\ | \\ \text{H} \end{array} $$

Covalent bonds are formed by the mutual sharing of electron pairs. Covalently bonded molecules have strong bonds within the molecule, but the intermolecular forces (forces between different molecules) are relatively weak. This explains their low melting and boiling points. Since covalent compounds are formed by sharing and do not produce free ions, they are generally poor conductors of electricity.

Allotropes of Carbon:

Carbon exists in different forms in nature with different physical properties but the same chemical properties. These forms are called **allotropes**. The difference lies in the arrangement of carbon atoms.

• **Diamond:** Each carbon atom is bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral structure. This strong network makes diamond the hardest known natural substance and gives it a very high melting point. It is a poor conductor of electricity.
• **Graphite:** Each carbon atom is bonded to three other carbon atoms in the same plane, forming hexagonal layers. These layers are held together by weak intermolecular forces. One bond in each hexagonal ring is a double bond. Graphite is relatively soft and slippery because the layers can slide over each other. It is a good conductor of electricity due to delocalized electrons.
• **Fullerenes:** Another class of carbon allotropes, often spherical or cage-like structures. The most famous is C-60, with carbon atoms arranged like a football (Buckminsterfullerene).

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Sulphur (S) has 6 valence electrons (like oxygen). In an S$_8$ ring molecule, each sulphur atom is bonded to two other sulphur atoms. Each sulphur atom needs 2 electrons to complete its octet.

Each sulphur atom forms single covalent bonds with its two neighbours. To satisfy the octet rule, each sulphur atom shares 1 electron with each neighbour, forming two single bonds. Each sulphur atom will also have 2 lone pairs of electrons.

Electron dot structure of S$_8$ ring:

Each line represents a shared pair of electrons (single bond). Each pair of dots represents a lone pair of electrons.

Versatile Nature Of Carbon

The ability of carbon to form a vast number of compounds is attributed to two main factors:

1. **Catenation:** Carbon atoms have the unique ability to form bonds with other carbon atoms to create long chains, branched chains, or rings. This property is called catenation. Carbon-carbon bonds can be single (—C—C—), double (—C=C—), or triple (—C$\equiv$C—).
   • Compounds linked by only single bonds between carbon atoms are called **saturated carbon compounds**.
   • Compounds having double or triple bonds between carbon atoms are called **unsaturated carbon compounds**.

   No other element shows catenation to the extent carbon does. Although silicon forms chains with hydrogen, they are limited to about 7-8 atoms and are highly reactive. The carbon-carbon bond is very strong and stable, allowing for the formation of large and diverse molecules.

2. **Tetravalency:** Carbon has a valency of four, meaning it can form four bonds with other atoms. It can bond with up to four other carbon atoms or atoms of other elements like hydrogen, oxygen, nitrogen, sulphur, halogens, etc.

The bonds carbon forms with most other elements are strong due to carbon's small size, which allows its nucleus to hold the shared electrons strongly. This stability contributes to the vast number of carbon compounds.

These compounds, initially thought to originate only from living systems (vital force theory, disproved by Wöhler's synthesis of urea), are studied under **organic chemistry**, excluding carbides, oxides of carbon, and carbonate/hydrogencarbonate salts.

Saturated And Unsaturated Carbon Compounds

**Saturated hydrocarbons** are compounds containing only carbon and hydrogen where all carbon-carbon bonds are **single bonds**. Examples include methane (CH$_4$), ethane (C$_2$H$_6$), propane (C$_3$H$_8$), butane (C$_4$H$_{10}$), etc. Their structures can be drawn by linking carbon atoms with single bonds and then attaching hydrogen atoms to satisfy the remaining valencies (4 for carbon, 1 for hydrogen).

Saturated hydrocarbons are generally less reactive. The general formula for saturated hydrocarbons (alkanes) is **C$_n$H$_{2n+2}$**, where $n$ is the number of carbon atoms ($n \ge 1$).

**Unsaturated hydrocarbons** contain at least one **double or triple bond** between carbon atoms. They are more reactive than saturated hydrocarbons.

• **Alkenes:** Contain at least one carbon-carbon **double bond** (—C=C—). Example: Ethene (C$_2$H$_4$). Carbon atoms linked by a double bond, then hydrogens added to satisfy remaining valencies.
  The general formula for alkenes with one double bond is **C$_n$H$_{2n}$**, where $n \ge 2$.
• **Alkynes:** Contain at least one carbon-carbon **triple bond** (—C$\equiv$C—). Example: Ethyne (C$_2$H$_2$). Carbon atoms linked by a triple bond, then hydrogens added. Ethyne has a triple bond between the two carbons.

  H—C$\equiv$C—H

  The general formula for alkynes with one triple bond is **C$_n$H$_{2n-2}$**, where $n \ge 2$.

Chains, Branches And Rings

Carbon compounds can exist as **straight chains**, **branched chains**, or **cyclic structures (rings)**.

• **Straight chains:** Carbon atoms linked sequentially, one after another. Example: Butane (C$_4$H$_{10}$) can exist as a straight chain of four carbons.

C—C—C—C

• **Branched chains:** Carbon atoms forming side branches off a main carbon chain. Example: Butane (C$_4$H$_{10}$) can also exist as isobutane, where a carbon atom is branched off a three-carbon chain.

$$ \begin{array}{c} \text{C} \\ | \\ \text{C}—\text{C}—\text{C} \end{array} $$

Compounds with the same molecular formula but different structures (like butane and isobutane, both C$_4$H$_{10}$) are called **structural isomers**.

• **Cyclic structures (Rings):** Carbon atoms arranged in a ring formation. These can be saturated or unsaturated.
  • Saturated cyclic hydrocarbon: Cyclohexane (C$_6$H$_{12}$). Six carbon atoms form a ring, each bonded to two hydrogens and single bonds between carbons.
  • Unsaturated cyclic hydrocarbon: Benzene (C$_6$H$_6$). Six carbon atoms form a ring with alternating single and double bonds.

Hydrocarbons are compounds containing only carbon and hydrogen. Alkanes are saturated hydrocarbons. Alkenes and alkynes are unsaturated hydrocarbons.

Will You Be My Friend? (Heteroatoms and Functional Groups)

Carbon is not limited to bonding with only hydrogen. It forms bonds with many other elements like oxygen, nitrogen, sulphur, and halogens (chlorine, bromine, iodine). These elements can replace one or more hydrogen atoms in a hydrocarbon chain, while maintaining carbon's valency.

Elements that replace hydrogen in hydrocarbon chains are called **heteroatoms**. These heteroatoms, or groups of atoms containing them, give specific properties to the carbon compound, regardless of the length or structure of the carbon chain. These specific groups are called **functional groups**.

Examples of functional groups:

Functional groups are attached to the carbon chain via a single bond (indicated by the single line), replacing a hydrogen atom or atoms.

Homologous Series

A **homologous series** is a series of carbon compounds that have the same functional group substituting for hydrogen in a carbon chain, but differ from each other by a repeating unit, typically a **—CH$_2$— unit**. The members of a homologous series have similar chemical properties because they have the same functional group.

Example: Alkanes (saturated hydrocarbons).

• Methane (CH$_4$)
• Ethane (C$_2$H$_6$) - differ from CH$_4$ by —CH$_2$—
• Propane (C$_3$H$_8$) - differ from C$_2$H$_6$ by —CH$_2$—
• Butane (C$_4$H$_{10}$) - differ from C$_3$H$_8$ by —CH$_2$—

The molecular mass difference between consecutive members in a homologous series is also constant (atomic mass of C = 12 u, H = 1 u, so —CH$_2$— unit has a molecular mass of $12 + 2 \times 1 = 14$ u).

Physical properties within a homologous series show a gradual change (gradation) as the molecular mass increases. For example, melting and boiling points generally increase with increasing molecular mass. Solubility in a given solvent may also change gradually. However, the chemical properties, which are primarily determined by the functional group, remain similar across the series.

General formulas for homologous series of hydrocarbons (with one double/triple bond):

• Alkanes (single bonds only): C$_n$H$_{2n+2}$ ($n \ge 1$)
• Alkenes (one double bond): C$_n$H$_{2n}$ ($n \ge 2$)
• Alkynes (one triple bond): C$_n$H$_{2n-2}$ ($n \ge 2$)

Nomenclature Of Carbon Compounds

A systematic method is used to name carbon compounds, especially those in a homologous series. Names are based on the structure of the carbon chain and the functional group present. The system is governed by IUPAC (International Union of Pure and Applied Chemistry).

Basic rules for naming carbon compounds:

1. Identify the parent hydrocarbon chain based on the number of carbon atoms (e.g., 1C-meth-, 2C-eth-, 3C-prop-, 4C-but-, 5C-pent-, 6C-hex-). The base name ends in '-ane' for saturated chains.
2. Identify the functional group(s) present.
3. The presence of a functional group is indicated by a prefix (placed before the parent name) or a suffix (placed after the parent name).
4. If using a suffix for the functional group, and the suffix begins with a vowel (a, e, i, o, u), the final '-e' from the parent alkane name is removed, and the suffix is added (e.g., Propane - 'e' + 'ol' = Propanol for an alcohol with 3 carbons).
5. If the carbon chain is unsaturated (double or triple bonds), the '-ane' suffix of the parent name is replaced with '-ene' for double bonds or '-yne' for triple bonds. (e.g., Propane $\to$ Propene for a 3C chain with a double bond, Propane $\to$ Propyne for a 3C chain with a triple bond).
6. Halogen functional groups (like chloro, bromo) are indicated as prefixes.

Summary of nomenclature suffixes/prefixes for common functional groups:

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Cyclopentane is a cyclic saturated hydrocarbon with 5 carbon atoms. It forms a 5-membered ring, with single bonds between the carbon atoms. Each carbon atom forms 4 bonds. Since it's a ring, each carbon atom is bonded to two other carbon atoms in the ring, leaving 2 valencies free to bond with hydrogen atoms.

Number of carbon atoms, n = 5.

Number of hydrogen atoms = 2 hydrogens per carbon = $2 \times 5 = 10$.

The formula of cyclopentane is **C$_5$H$_{10}$**.

Electron dot structure:

Each dash represents a shared pair of electrons (a single covalent bond).

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Chemical Properties Of Carbon Compounds

Carbon compounds, particularly hydrocarbons, are used extensively as fuels due to their energy-releasing chemical reactions. Let's examine some key chemical properties.

Combustion

**Combustion** is the process of burning in oxygen. Carbon and most carbon compounds release a large amount of heat and light when they burn in sufficient oxygen. This makes them excellent fuels. Combustion is a form of **oxidation**.

Examples:

• Burning of carbon:

  C(s) + O$_2$(g) $\to$ CO$_2$(g) + heat + light

• Burning of methane (natural gas):

  CH$_4$(g) + 2O$_2$(g) $\to$ CO$_2$(g) + 2H$_2$O(g) + heat + light

• Burning of ethanol (alcohol):

  CH$_3$CH$_2$OH(l) + 3O$_2$(g) $\to$ 2CO$_2$(g) + 3H$_2$O(g) + heat + light

The type of flame produced during combustion depends on whether the fuel is saturated or unsaturated and the amount of oxygen available.

• **Saturated hydrocarbons** generally burn with a **clean, blue flame** in sufficient oxygen (complete combustion).
• **Unsaturated carbon compounds** generally burn with a **yellow, sooty flame** (luminous flame) due to incomplete combustion. This produces unburnt carbon particles (soot).
• Limiting the supply of air or oxygen to even saturated hydrocarbons can also lead to **incomplete combustion**, producing soot and a yellow flame, and potentially carbon monoxide (CO), a toxic gas.

Gas stoves at home have air inlets to ensure enough oxygen for complete combustion, producing a clean blue flame. Blackened cooking vessels indicate incomplete combustion due to insufficient air (e.g., blocked air holes).

Fossil fuels like coal and petroleum are formed from the biomass of ancient organisms under high pressure and temperature over millions of years. They contain nitrogen and sulphur compounds, which produce polluting oxides of nitrogen and sulphur upon combustion.

Oxidation

Besides combustion, carbon compounds can undergo other types of oxidation. Some substances called **oxidising agents** can add oxygen to other substances or remove hydrogen.

Example: Oxidation of alcohols to carboxylic acids.

Alcohols like ethanol can be oxidised to carboxylic acids like ethanoic acid using oxidising agents. Alkaline potassium permanganate or acidified potassium dichromate are common oxidising agents. They provide oxygen for the oxidation reaction.

Reaction: Ethanol is oxidised to ethanoic acid.

CH$_3$CH$_2$OH $\xrightarrow{\text{Alkaline KMnO}_4 \text{ or Acidified K}_2\text{Cr}_2\text{O}_7}$ CH$_3$COOH

In this reaction, oxygen is added to ethanol to form ethanoic acid. The colour of the oxidising agent (e.g., purple KMnO$_4$) changes during the reaction as it gets reduced.

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Addition Reaction

**Addition reactions** are characteristic of **unsaturated hydrocarbons** (alkenes and alkynes). In these reactions, atoms (like hydrogen, halogens) are added across the double or triple bond, converting the unsaturated hydrocarbon into a saturated one (or a less unsaturated one).

Example: Hydrogenation of unsaturated hydrocarbons.

Unsaturated hydrocarbons add hydrogen gas in the presence of catalysts like **palladium (Pd)** or **nickel (Ni)**, converting the double or triple bonds into single bonds.

$$ \text{—C=C—} + \text{H}_2 \xrightarrow{\text{Ni or Pd catalyst}} \text{—C—C—} $$

$$ \text{—C}\equiv\text{C—} + \text{2H}_2 \xrightarrow{\text{Ni or Pd catalyst}} \text{—C—C—} $$

This reaction is widely used industrially in the **hydrogenation of vegetable oils**. Vegetable oils contain unsaturated fatty acid chains with double bonds. Hydrogenation converts these double bonds into single bonds, making the oils saturated (e.g., converting liquid vegetable oil into solid or semi-solid 'vegetable ghee' or margarine). Animal fats typically contain saturated fatty acid chains.

Substitution Reaction

**Substitution reactions** are characteristic of **saturated hydrocarbons** (alkanes), which are generally unreactive. In a substitution reaction, one or more hydrogen atoms of a saturated hydrocarbon are replaced by other atoms or groups.

Example: Reaction of methane with chlorine in the presence of sunlight.

Methane (CH$_4$) reacts with chlorine (Cl$_2$) in the presence of sunlight. Hydrogen atoms are substituted by chlorine atoms, one by one. This is a fast reaction.

CH$_4$ + Cl$_2$ $\xrightarrow{\text{Sunlight}}$ CH$_3$Cl + HCl

(Methane) (Chlorine) (Chloromethane) (Hydrogen chloride)

Further substitution can occur:

CH$_3$Cl + Cl$_2$ $\xrightarrow{\text{Sunlight}}$ CH$_2$Cl$_2$ + HCl (Dichloromethane)

CH$_2$Cl$_2$ + Cl$_2$ $\xrightarrow{\text{Sunlight}}$ CHCl$_3$ + HCl (Chloroform)

CHCl$_3$ + Cl$_2$ $\xrightarrow{\text{Sunlight}}$ CCl$_4$ + HCl (Carbon tetrachloride)

This is called a substitution reaction because one atom (Cl) or group replaces (substitutes) another atom (H).

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Some Important Carbon Compounds – Ethanol And Ethanoic Acid

Among the vast number of carbon compounds, ethanol and ethanoic acid are two commercially significant examples with various uses.

Properties Of Ethanol

**Ethanol (CH$_3$CH$_2$OH)** is a primary alcohol with two carbon atoms. It is a liquid at room temperature. It is the active component in alcoholic drinks and is commonly referred to as **alcohol**. Ethanol is a good solvent and is used in medicines (like tincture iodine, cough syrups, tonics) and as an industrial solvent. It is soluble in water in all proportions.

Pure ethanol is called **absolute alcohol**. Consumption of pure ethanol, even in small quantities, can be lethal. Consumption of alcoholic drinks (dilute ethanol) causes drunkenness and can lead to serious health problems with long-term use.

Chemical properties of ethanol:

• **Reaction with Sodium:** Ethanol reacts with active metals like sodium to produce sodium ethoxide and hydrogen gas.

  2Na(s) + 2CH$_3$CH$_2$OH(l) $\to$ 2CH$_3$CH$_2$O$^-$Na$^+$(aq) + H$_2$(g)

  (Sodium ethoxide) (Hydrogen gas) Hydrogen gas can be tested with a pop sound. Other substances that produce hydrogen with metals are acids. This reaction shows that alcohols can behave like weak acids in reacting with reactive metals.
• **Reaction to give Unsaturated Hydrocarbon (Dehydration):** Heating ethanol at 443 K (170°C) with excess concentrated sulphuric acid causes the removal of a water molecule from the ethanol molecule, converting it into an alkene (ethene).

  CH$_3$CH$_2$OH $\xrightarrow{\text{Hot Conc. H}_2\text{SO}_4}$ CH$_2$=CH$_2$ + H$_2$O

  (Ethanol) (Ethene) (Water) Concentrated sulphuric acid acts as a **dehydrating agent** in this reaction, removing water.
• **Combustion:** Ethanol burns readily in air, releasing CO$_2$, H$_2$O, heat, and light, making it a fuel.

  CH$_3$CH$_2$OH + 3O$_2$ $\to$ 2CO$_2$ + 3H$_2$O

• **Oxidation:** Ethanol can be oxidised to ethanoic acid using oxidising agents (as discussed in the previous section).

**Denatured alcohol** is ethanol made unfit for drinking by adding poisonous substances (like methanol) and dyes, to prevent its misuse for industrial purposes.

Methanol is highly toxic; even small quantities can cause death or blindness. It is oxidised to methanal in the liver, which damages cells.

Properties Of Ethanoic Acid

**Ethanoic acid (CH$_3$COOH)** is a carboxylic acid with two carbon atoms. It is commonly known as **acetic acid**. A 5-8% solution of acetic acid in water is called **vinegar** and is used as a food preservative (e.g., in pickles). Pure ethanoic acid has a melting point of 290 K (17°C) and often freezes in cold climates, hence it is sometimes called **glacial acetic acid**. Carboxylic acids are characterized by the presence of the -COOH functional group and are acidic in nature. They are generally weak acids compared to mineral acids like HCl or H$_2$SO$_4$, which are strong acids and dissociate completely in water.

Chemical properties of ethanoic acid:

• **Esterification Reaction:** Ethanoic acid reacts with alcohols (like ethanol) in the presence of an acid catalyst (like concentrated H$_2$SO$_4$) to form **esters**. Esters are often sweet-smelling substances used in perfumes and flavouring agents.

  CH$_3$COOH + CH$_3$CH$_2$OH $\xrightarrow{\text{Acid}}$ CH$_3$COOCH$_2$CH$_3$ + H$_2$O

  (Ethanoic acid) (Ethanol) (Ethyl ethanoate - an ester) (Water) The reverse reaction, hydrolysis of an ester with a base (like NaOH), breaks the ester bond and is called **saponification**, used in soap preparation (soap is a sodium or potassium salt of a long-chain carboxylic acid).

  CH$_3$COOCH$_2$CH$_3$ + NaOH $\to$ CH$_3$CH$_2$OH + CH$_3$COONa

  (Ester) (Base) (Alcohol) (Sodium ethanoate - a soap if the acid chain is long)
• **Reaction with a Base:** Ethanoic acid reacts with bases (like NaOH) to form a salt (sodium ethanoate) and water, just like mineral acids do. This is a neutralisation reaction.

  NaOH(aq) + CH$_3$COOH(aq) $\to$ CH$_3$COONa(aq) + H$_2$O(l)

  (Sodium hydroxide) (Ethanoic acid) (Sodium ethanoate) (Water)
• **Reaction with Carbonates and Hydrogencarbonates:** Ethanoic acid reacts with metal carbonates and hydrogencarbonates to produce a salt (sodium ethanoate), carbon dioxide gas, and water, just like mineral acids do.

  2CH$_3$COOH(aq) + Na$_2$CO$_3$(s) $\to$ 2CH$_3$COONa(aq) + H$_2$O(l) + CO$_2$(g)

  CH$_3$COOH(aq) + NaHCO$_3$(s) $\to$ CH$_3$COONa(aq) + H$_2$O(l) + CO$_2$(g)

  The evolved CO$_2$ can be tested with lime water. This reaction is used to distinguish carboxylic acids from neutral compounds.

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Soaps And Detergents

**Soaps** and **detergents** are cleaning agents. Most dirt, including oil and grease, is non-polar and does not dissolve in polar water. Soaps and detergents help to remove this oily dirt by a special mechanism.

Soaps are typically **sodium or potassium salts of long-chain carboxylic acids**. A soap molecule has two parts:

• A long hydrocarbon chain (non-polar), which is **hydrophobic** (water-repelling) and interacts with oil/grease.
• An ionic end (polar, e.g., -COO$^-$Na$^+$), which is **hydrophilic** (water-attracting) and interacts with water.

When soap is added to water, the hydrophobic tails avoid water and project outwards at the surface. Inside the water, the molecules arrange themselves into spherical structures called **micelles**. In a micelle, the hydrophobic tails are clustered in the interior (interacting with oily dirt), while the hydrophilic ionic ends face outwards towards the water.

The micelle traps the oily dirt in its core. The outer ionic layer of the micelle is charged, preventing the micelles from aggregating and causing them to stay dispersed as a colloid in water. This **emulsifies** the oily dirt in water. When the clothes are agitated (beating, scrubbing, or in a washing machine), the micelles carrying the dirt are dispersed and easily rinsed away with water, cleaning the clothes.

Problem with soaps: Soaps react with calcium and magnesium salts present in **hard water** to form an insoluble, curdy white precipitate called **scum**. This scum does not dissolve in water, sticks to clothes, and makes the soap less effective. This also means a larger amount of soap is needed in hard water.

Reaction of soap (represented by R-COONa, where R is the hydrocarbon chain) with calcium ions in hard water:

2R-COONa(aq) + Ca$^{2+}$(aq) $\to$ (R-COO)$_2$Ca(s) + 2Na$^+$(aq)

(Soluble soap) (Calcium ion from hard water) (Insoluble calcium soap - scum)

**Detergents** are synthetic cleansing agents. They are generally sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions. Like soaps, they have long hydrocarbon chains and charged hydrophilic ends.

Advantage of detergents: Unlike soaps, the charged ends of detergents **do not form insoluble precipitates** with calcium and magnesium ions in hard water. Therefore, detergents remain effective cleansing agents in hard water. Detergents are commonly used in shampoos and laundry products.

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