1. Introduction – Carbon: The Backbone of Living Things
Every living cell you are made of is built around a single element: carbon. Carbohydrates, fats, proteins, the DNA in your genes and even the air you breathe out as carbon dioxide all contain carbon. Carbon also makes coal, petroleum, graphite, diamond, plastics, medicines, dyes, soaps and synthetic fibres.
What makes carbon so special is that it forms strong bonds with itself and with hydrogen, oxygen, nitrogen and halogens. With just carbon and hydrogen over a lakh compounds are already known, and the total number of carbon compounds runs into millions. No other element comes close.
Carbon is a non-metal of atomic number 6 and electronic configuration 2, 4. It has four electrons in its outermost shell and forms covalent compounds by sharing these electrons with other atoms.
Methane (natural gas), ethanol (in sanitisers), ethanoic acid (vinegar), urea (in fertilisers), glucose, benzene, LPG, kerosene, sugar, cotton and nylon are all carbon compounds. Air, rocks and sea water too contain carbon — mainly as CO₂ and carbonates.
Why argue about carbon before anything else in this chapter?
Because nearly every other idea in organic chemistry — covalent bonding, saturated and unsaturated chains, functional groups, homologous series, esters, soaps — is just a different way of asking: how do carbon atoms hold on to each other and to their neighbours? Once you see that picture clearly, the whole chapter becomes one connected story.
2. Covalent Bonding in Carbon Compounds
Carbon has 4 electrons in its outermost shell (configuration 2, 4). A stable noble-gas configuration needs 8 outer electrons. Can carbon simply lose 4 electrons or gain 4 electrons like ionic compounds do?
A C4+ cation would need enormous energy to pull all four electrons away from the small nucleus. A C4- anion would need the tiny carbon nucleus to hold ten electrons — which is just as difficult. Carbon therefore cannot lose or gain four electrons easily.
So carbon chooses a third way: it shares its four electrons with other atoms. Sharing lets both atoms complete their octets, and the electron pair that is shared belongs to both atoms together.
A covalent bond is a bond formed between two atoms by the sharing of one or more pairs of electrons, so that each atom reaches a noble-gas configuration.
A single bond shares one pair of electrons (e.g. H–H, Cl–Cl). A double bond shares two pairs (e.g. oxygen O=O). A triple bond shares three pairs (e.g. nitrogen N≡N). Carbon forms all three types.
Electron-dot (Lewis) structures of simple molecules
An electron-dot structure shows the valence electrons of each atom as dots around the element symbol. A shared pair is written between the two atoms as a pair of dots (or a colon). Here each colon stands for one shared electron pair.
Methane, ethane, ethene and ethyne — the carbon family in one glance
These four molecules are the heart of this chapter’s diagrams. In methane one carbon shares with four hydrogens; in ethane two carbons share a single bond; in ethene they share a double bond; in ethyne a triple bond.
Properties that follow from covalent bonding
Covalent bonds are strong, but the forces between covalent molecules are weak. That single fact explains most physical properties of carbon compounds.
| Property | Observation | Why |
|---|---|---|
| Melting point | Generally low (e.g. sugar melts around 186°C) | Weak intermolecular forces need little energy to break |
| Boiling point | Generally low; many carbon compounds are gases or liquids | Weak attraction between molecules |
| Electrical conductivity | Generally poor conductors in solid/liquid state | No free electrons or ions to carry current |
| Solubility | Usually insoluble in water, soluble in organic solvents | Non-polar molecules do not mix well with polar water |
Contrast is easy marks in the board exam: table salt (NaCl, ionic) has a high melting point and conducts electricity when molten or in solution, while sugar or naphthalene (covalent) melts easily and never conducts. When a question asks "why do covalent compounds have low melting points?", say weak intermolecular forces and quote no free ions.
3. Versatile Nature of Carbon – Why Carbon Makes So Many Compounds
Carbon appears in millions of different compounds. Two properties make this possible: catenation and tetravalency.
Tetravalency is the property of an element to make four covalent bonds by sharing all four of its valence electrons. Carbon is tetravalent; it always tries to complete four bonds.
Catenation is the property of an element to form bonds with atoms of the same element, making long chains. Carbon shows catenation to the greatest extent because the C–C bond is strong and stable. That is why carbon forms so many compounds.
Homoatomic compounds contain only one kind of atom joined together (e.g. a C–C chain). Heteroatomic compounds contain other atoms such as hydrogen, oxygen, nitrogen or chlorine attached to the chain. Carbon chains may also close into rings.
The three shapes carbon chains take
Because carbon can join to itself in rows, with side branches, or in loops, its chains come in three general shapes:
- Straight chains — a single line of carbons, e.g. CH₃–CH₂–CH₂–CH₃ (butane).
- Branched chains — a main chain with side chains, e.g. isobutane, an isomer of C₄H₁₀.
- Cyclic (ring) chains — carbons joined into a closed ring, e.g. cyclohexane (C₆H₁₂) and benzene (C₆H₆).
| Chain type | Example | Formula | Notes |
|---|---|---|---|
| Straight chain | n-butane | C₄H₁₀ | All 4 carbons in one line |
| Branched chain | isobutane (2-methylpropane) | C₄H₁₀ | Same molecule formula, different structure (isomer) |
| Cyclic | cyclohexane | C₆H₁₂ | Ring of 6 carbons; loses the 2 end H-atoms |
| Cyclic with double bonds | benzene | C₆H₆ | Ring with alternating double bonds |
Butane and isobutane share the same formula C₄H₁₀ but different structures. Such pairs are called structural isomers — a favourite one-mark definition.
4. Saturated and Unsaturated Carbon Compounds
Look at the middle of a carbon chain. How many ways can two carbons hold together? They can share a single electron pair, a double pair or a triple pair. This difference decides the whole character of the compound.
Saturated compounds contain only carbon–carbon single bonds (–C–C–). Since all four valencies of every carbon are already used up in single bonds, they cannot add more atoms ... they are "saturated". Alkanes such as methane and ethane are saturated.
Unsaturated compounds contain carbon–carbon double or triple bonds (C=C or C≡C). The double/triple bond can open up and add more atoms, so these compounds are "unsaturated". Alkenes and alkynes are unsaturated.
Comparing saturated and unsaturated hydrocarbons
| Feature | Saturated (alkanes) | Unsaturated (alkenes / alkynes) |
|---|---|---|
| Bonding between C atoms | Only single bonds, –CH₂–CH₂– | At least one double bond (C=C) or triple bond (C≡C) |
| General formula | CnH2n+2 | Alkenes CnH2n; alkynes CnH2n−2 |
| Hydrogen content | Maximum possible for the carbon count | Fewer hydrogens than the saturated case |
| Addition reactions | Do not add hydrogen or halogens easily | Readily take up H₂ or Br₂ across the multiple bond |
| Examples | CH₄, C₂H₆, C₃H₈ | Ethene C₂H₄; ethyne C₂H₂ |
The general formula for alkanes is CnH2n+2 only for straight and branched chains. Cyclic alkanes follow CnH2n. And benzene, though it has double bonds in its ring, does magnetic-style behaviour all of its own — a Class 11 story.
Hydrocarbons – compounds of carbon and hydrogen only
A hydrocarbon is a compound made of carbon and hydrogen only. Hydrocarbons are the simplest carbon compounds and are divided into alkanes (saturated) and alkenes/alkynes (unsaturated).
Alkanes – the saturated family
| Alkane | Formula | Structure | Use |
|---|---|---|---|
| Methane | CH₄ | One C bonded to 4 H | Natural gas / cooking gas (CNG) |
| Ethane | C₂H₆ | CH₃–CH₃ | LPG component, chemical raw material |
| Propane | C₃H₈ | CH₃–CH₂–CH₃ | LPG, camping gas |
| Butane | C₄H₁₀ | CH₃–CH₂–CH₂–CH₃ | Lighter fluid, LPG |
Alkenes and alkynes – the unsaturated families
| Family | Name | Formula | Structure | Distinctive bond |
|---|---|---|---|---|
| Alkene | Ethene | C₂H₄ | CH₂=CH₂ | C=C double bond |
| Alkene | Propene | C₃H₆ | CH₃–CH=CH₂ | C=C double bond |
| Alkyne | Ethyne (acetylene) | C₂H₂ | CH≡CH | C≡C triple bond |
| Alkyne | Propyne | C₃H₄ | CH₃–C≡CH | C≡C triple bond |
Memorise the formula jumps: alkanes CnH2n+2 → alkenes CnH2n → alkynes CnH2n−2. Each step lowers the hydrogen count by two — which is exactly the two electrons tied up in each extra bond.
5. Homologous Series
Carbon compounds are not a random zoo. They line up in families, and within one family every member is built by adding one CH₂ unit to the previous member. Such a family is a homologous series.
A homologous series is a series of carbon compounds in which each successive member differs from the previous one by a –CH₂– group and shows the same general formula and similar chemical properties.
The alkane family – first six members
| Name | Formula | General formula check (n) |
|---|---|---|
| Methane | CH₄ | n = 1 |
| Ethane | C₂H₆ | n = 2 |
| Propane | C₃H₈ | n = 3 |
| Butane | C₄H₁₀ | n = 4 |
| Pentane | C₅H₁₂ | n = 5 |
| Hexane | C₆H₁₄ | n = 6 |
Check the jump: methane CH₄ gains one CH₂ to give ethane C₂H₆. Ethane gains CH₂ to give propane C₃H₈. Watch the steady pattern: CH₄ → C₂H₆ → C₃H₈ → C₄H₁₀, each time adding CH₂.
Characteristics of a homologous series
- Successive members differ by one CH₂ group (added mass 14 u).
- All members can be written with a single general formula (e.g. CnH2n+2 for alkanes).
- All members share the same functional group, so they have similar chemical properties.
- Physical properties change gradually with increasing molecular mass, e.g. boiling point rises as the chain grows.
- The difference in boiling point between neighbours is roughly regular.
If you are asked to "give the next member", add CH₂: propanol C₃H₇OH → butanol C₄H₉OH. If you are asked the molecular mass of the CH₂ unit, say 14 u. This is the highest-frequency 1-mark/2-mark question in this chapter.
Homologous series examples beyond alkanes
| Series | Members | Functional group |
|---|---|---|
| Alcohols | CH₃–OH (methanol), C₂H₅–OH (ethanol) | –OH |
| Carboxylic acids | HCOOH (methanoic), CH₃COOH (ethanoic) | –COOH |
| Halogenated | CH₃Cl (chloromethane), C₂H₅Cl (chloroethane) | –Cl |
6. Nomenclature of Carbon Compounds
A name tells you exactly what the molecule is made of. For Class 10 you learn the essentials: a prefix for the number of carbons and a suffix for the functional group. This is a simplified IUPAC naming at school level.
Step 1 – prefix: count the longest carbon chain
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Prefix | Meth– | Eth– | Prop– | But– | Pent– | Hex– |
Step 2 – suffix: the functional group or bond type
| Functional group / bond | Suffix | Example |
|---|---|---|
| Single bonds only (alkane) | –ane | Ethane C₂H₆ |
| Double bond (alkene) | –ene | Ethene C₂H₄ |
| Triple bond (alkyne) | –yne | Ethyne C₂H₂ |
| Alcohol –OH | –ol | Ethanol C₂H₅OH |
| Aldehyde –CHO | –al | Ethanal CH₃CHO |
| Ketone >C=O | –one | Propanone CH₃COCH₃ |
| Carboxylic acid –COOH | –oic acid | Ethanoic acid CH₃COOH |
Put the two together
- A 2-carbon single-bond chain → ethane (C₂H₆).
- A 2-carbon chain with –OH → ethanol (C₂H₅OH).
- A 2-carbon chain with –COOH → ethanoic acid (CH₃COOH).
- A 3-carbon chain with a ketone group → propanone (CH₃COCH₃).
- A 1-carbon chain with –COOH → methanoic acid (HCOOH).
"Acetic acid" and "ethanoic acid" are the same compound: CH₃COOH. Vinegar is only a dilute solution of it, not the pure substance. And remember: an alkane ending in <ane> with three carbons is propane, not propene — one letter at the end changes the whole molecule.
7. Functional Groups – The Reactive Handles of Carbon Compounds
Hydrogen in a hydrocarbon chain is boring — C–H bonds hardly react. But replace one hydrogen by another atom or group and the molecule suddenly has a personality. That personality is called its functional group.
A functional group is an atom or a group of atoms, other than hydrogen, attached to the carbon skeleton of a molecule. It decides the chemical behaviour of the whole compound. The rest of the chain only influences physical properties.
The five functional groups you must know for Class 10
| Functional group | Symbol (in formula) | Family name | Example |
|---|---|---|---|
| Alcohol | –OH (hydroxyl) | Alcohol | Ethanol CH₃CH₂OH |
| Aldehyde | –CHO (formyl) | Aldehyde | Ethanal CH₃CHO |
| Ketone | >C=O (carbonyl, ketone) | Ketone | Propanone CH₃COCH₃ |
| Carboxylic acid | –COOH (carboxyl) | Carboxylic acid | Ethanoic acid CH₃COOH |
| Halogen | –Cl, –Br (halo) | Haloalkane | Chloroethane C₂H₅Cl |
- Ethanol = ethyl group + –OH. Fix the –OH and the name ends in <ol>.
- Ethanoic acid = CH₃COOH. Fix the –COOH and the name ends in <oic acid>.
- Propanone = CH₃COCH₃ — a carbonyl group C=O sitting between two CH₃ groups.
- Chloromethane = CH₃Cl — one hydrogen of methane replaced by a chlorine atom.
Board questions love the phrase "name the functional group present in ..." The answer must be the family, not an element: –OH is an alcohol group (not "OH"), –COOH is a carboxylic acid group. One word can cost a mark.
8. Physical Properties of Carbon Compounds
Because carbon compounds are covalent, their molecules are held together only by weak forces. That shows up in a handful of predictable physical properties.
| Property | Typical behaviour of carbon compounds | Reason |
|---|---|---|
| State at room temperature | Gases and liquids are common; solids have low melting point | Weak intermolecular forces |
| Melting / boiling points | Low compared with ionic compounds | Little energy needed to separate molecules |
| Electrical conductivity | Insulators in pure liquid or molten state | No free electrons or ions |
| Solubility | Soluble in organic solvents, sparingly in water | Non-polar character; does not mix with polar water |
| Dissociation | Do not dissociate into ions | Electrons are shared, not transferred |
Common salt (NaCl) is ionic: high melting point (801°C), conducts electricity when molten or dissolved. Paraffin wax or sugar is covalent: melts around 50–186°C and never conducts. That contrast is the easiest proof of the theory in exams.
Ethanol and ethanoic acid – the two star liquids
Two everyday carbon compounds appear again and again in boards: ethanol (C₂H₅OH) and ethanoic acid (CH₃COOH). Both are directly linked to food, drink, industry and soap. Their properties and reactions make up their own big sections later in these notes.
Ethanol is the alcohol in drinks; the petroleum alcohol methanol (CH₃OH) is poisonous. And anywhere you read "alcohol burns with a blue flame", that is ethanol — not methanol, and not ethanoic acid.
9. Chemical Properties of Carbon Compounds
Carbon compounds are valued because they can burn, oxidise, add atoms and substitute atoms. Master these four classes of reactions and you have covered the chemistry of this chapter.
1. Combustion – burning in air
Combustion is the reaction of a substance with oxygen, releasing heat and light. Most carbon compounds burn in oxygen to give carbon dioxide, water and heat.
\[ CH_4(g) + 2O_2(g) \xrightarrow{\text{ignition}} CO_2(g) + 2H_2O(l) + heat + light \]
\[ C_2H_5OH(l) + 3O_2(g) \xrightarrow{\text{ignition}} 2CO_2(g) + 3H_2O(l) + heat + light \]
If the oxygen supply is small, burning is incomplete and gives carbon monoxide (CO), a poisonous gas, along with soot (unburnt carbon). A yellow, sooty flame means incomplete burning; a clean blue flame means the hydrocarbon is saturated and burning completely.
"Explain why candle flames are yellow but methane burns with a blue flame." Methane (saturated, CNG) burns completely with a blue flame; a candle has a mixture of hydrocarbons with more carbon, so parts burn incompletely and give a yellow sooty flame. Unsaturated compounds like ethene and ethyne give a smoky yellow flame when burnt in air.
2. Oxidation – controlled reactions with oxygen
Combustion is a violent, uncontrolled oxidation. But with a careful oxidising agent we can oxidise a compound gently and get a useful product. Alcohols oxidise to carboxylic acids.
\[ CH_3CH_2OH \xrightarrow[\text{oxidation}]{\text{alkaline KMnO}_4 / \text{acidified K}_2\text{Cr}_2\text{O}_7} CH_3COOH \]
An oxidising agent like alkaline potassium permanganate(KMnO₄, purple) or acidified potassium dichromate (K₂Cr₂O₇, orange) is added to hot ethanol. Ethanol is oxidised to ethanoic acid. The KMnO₄ loses its purple colour — that colour change proves the oxidation.
3. Addition reaction – only for unsaturated compounds
In an addition reaction, atoms or groups are added across a carbon–carbon double or triple bond, converting an unsaturated compound into a saturated one. Nothing is removed.
\[ CH_2=CH_2 + H_2 \xrightarrow{\text{Ni}} CH_3-CH_3 \]
\[ CH \equiv CH + 2H_2 \xrightarrow{\text{Ni}} CH_3-CH_3 \]
Vegetable oils are unsaturated fats (they have C=C bonds). Passing hydrogen over them with a nickel catalyst converts them to saturated fats — this is how vanaspati ghee is made and how oil is hardened. This is the industrial shining example of an addition reaction.
4. Substitution reaction – for saturated compounds
In a substitution reaction, one atom or group in the molecule is replaced by another atom or group. It happens readily in saturated compounds such as alkanes.
\[ CH_4 + Cl_2 \xrightarrow{\text{sunlight}} CH_3Cl + HCl \]
\[ CH_3Cl + Cl_2 \xrightarrow{\text{sunlight}} CH_2Cl_2 + HCl \]
Unsaturated compounds (alkenes, alkynes) do addition reactions across their double/triple bonds. Saturated compounds (alkanes) do substitution reactions, one hydrogen at a time. Do not write "ethene + Cl₂ → chlorination products" — ethene adds Cl₂ to give dichloroethane. Methane substitutes to give chloromethane.
Bromine water test: shake a compound with bromine water (orange). Unsaturated compounds decolourise it (addition of Br₂); saturated compounds do not. A very common one-marker in recent papers.
10. Ethanol – C₂H₅OH, the Alcohol
Ethanol is the second member of the alcohol homologous series. It is the alcohol found in sanitisers and drinks, and one of the most useful liquids in a chemistry lab.
Structure and physical properties
- Formula: C₂H₅OH or CH₃–CH₂–OH (also written C₂H₆O).
- State: colourless liquid with a characteristic smell.
- Solubility: completely miscible with water.
- Boiling point: 351 K (78°C) — low because it is a covalent compound. The –OH group allows hydrogen bonding, which is why it is higher than comparable alkanes.
Chemical reactions of ethanol
(a) Combustion
\[ C_2H_5OH + 3O_2 \to 2CO_2 + 3H_2O + heat + light \]
(b) Reaction with sodium – displacement of hydrogen
\[ 2Na + 2C_2H_5OH \to 2C_2H_5ONa + H_2 \]
Sodium ethoxide forms and hydrogen gas bubbles off. The –OH hydrogen is the one replaced; ethanol behaves as a weak acid towards sodium without being acidic enough to affect litmus.
(c) Oxidation to ethanoic acid
\[ C_2H_5OH + 2[O] \xrightarrow[\text{oxidising agent}]{\Delta} CH_3COOH + H_2O \]
Alkaline KMnO₄ or acidified K₂Cr₂O₇ supplies the oxygen. The purple KMnO₄ loses colour as ethanol becomes ethanoic acid.
(d) Dehydration to ethene
\[ CH_3CH_2OH \xrightarrow[H_2O]{\text{conc. } H_2SO_4, 443K} CH_2=CH_2 \]
Concentrated sulphuric acid removes a molecule of water, and ethanol dehydrates to ethene. This is an elimination reaction — the opposite of an addition reaction.
- As a solvent (e.g. in paints, tincture iodine, perfumes).
- As an antiseptic; 70% ethanol solution kills germs more reliably than pure ethanol.
- In alcoholic beverages — but harmful when abused; it depresses the nervous system.
- As a fuel or fuel additive (bio-ethanol blended with petrol).
Consumption of ethanol in excess damages the liver, brain and heart, and causes addiction, accidents and family harm. Diluted ethanol is used in drinks, but it is a depressant — never a safe "energy" drink.
11. Ethanoic Acid – CH₃COOH, the Vinegar Acid
Ethanoic acid (also called acetic acid) is the second member of the carboxylic acid series. Dilute, it is the acid in vinegar; pure, it freezes to an ice-like solid called glacial acetic acid.
Structure and physical properties
- Formula: CH₃COOH, with the –COOH carboxylic acid group.
- Common name: acetic acid; its 5–8% solution in water is vinegar.
- State: colourless liquid with the sharp, pungent smell of vinegar.
- Boiling point: 391 K (118°C).
- Melting point: 290 K (17°C); below this it solidifies to glacial acetic acid, which looks like ice.
- Solubility: fully miscible with water.
Is ethanoic acid a strong or weak acid?
Ethanoic acid is a weak acid. In water it ionises only partially into H+ and acetate ions, CH₃COO−. It turns blue litmus red and reacts with bases and metals, but its H+ release is much smaller than that of strong mineral acids like HCl or H₂SO₄.
\[ CH_3COOH + H_2O \rightleftharpoons CH_3COO^- + H_3O^+ \]
Reactions of ethanoic acid
(a) With sodium – a metal
\[ 2CH_3COOH + 2Na \to 2CH_3COONa + H_2 \]
Sodium ethanoate and hydrogen gas form. The hydrogen comes from the –COOH group.
(b) With sodium hydroxide – a base (neutralisation)
\[ CH_3COOH + NaOH \to CH_3COONa + H_2O \]
(c) With sodium bicarbonate – a carbonate/hydrogencarbonate
\[ CH_3COOH + NaHCO_3 \to CH_3COONa + H_2O + CO_2 \]
Add ethanoic acid to sodium bicarbonate solution. Brisk effervescence of carbon dioxide appears — pass the gas through lime water, which turns milky. This CO₂ test is the standard practical attached to this reaction.
(d) With alcohol – esterification
\[ CH_3COOH + C_2H_5OH \underset{\text{conc. } H_2SO_4}{\rightleftharpoons} CH_3COOC_2H_5 + H_2O \]
Ethyl acetate (an ester) with a fruity smell is produced; the reaction is reversible. Esterification gets its own spotlight in the next section.
- Acid + base → salt + water (neutralisation).
- Acid + carbonate/hydrogencarbonate → salt + water + CO₂ (fizz!).
- Acid + metal → salt + H₂.
- Acid + alcohol → ester + water (fruity smell).
12. Esterification and Saponification
These two reactions are the doorway from pure chemistry to the chemistry of smells and soaps. Both involve esters — the fragrant compounds made when an acid meets an alcohol.
An ester is a compound formed from a carboxylic acid and an alcohol, with the general structure R–COO–R’. Esters are responsible for the fruity smells of many flowers and fruits.
Esterification
Esterification is the reaction of a carboxylic acid with an alcohol in the presence of concentrated sulphuric acid to form an ester and water. Because it is reversible, it is written with a double arrow (⇌).
\[ CH_3COOH + C_2H_5OH \underset{\text{catalyst: conc. } H_2SO_4}{\rightleftharpoons} CH_3COOC_2H_5 + H_2O \]
Ethanoic acid has a sharp smell, ethanol a mild alcoholic one, but the ester ethyl ethanoate (ethyl acetate) smells sweet and fruity — like nail polish remover or ripened banana. Esters inside fruits and flowers produce their natural fragrance, which is why esters are used in perfumes and synthetic flavourings.
Saponification – making soap
Saponification is the reaction of an ester with sodium or potassium hydroxide (a base) to give an alcohol and the salt of a carboxylic acid. When the ester is a fat (an ester of glycerol with long-chain acids), this reaction produces soap (fatty acid salt) plus glycerol.
\[ CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \]
\[ \text{Fat/oil (ester of glycerol)} + NaOH \to \text{soap (fatty acid salt)} + \text{glycerol} \]
Esterification makes an ester (acid + alcohol, acid catalyst). Saponification breaks an ester back down into salt + alcohol using a base. Esterification needs concentrated H₂SO₄; saponification needs NaOH or KOH.
Questions pair these two: "Why is the reaction written with a reversible arrow?" (because water formed in esterification can hydrolyse the ester back to acid and alcohol) and "What happens when an ester reacts with NaOH?" (saponification → soap + glycerol).
13. Soaps and Detergents – How Do They Clean?
A soap is a sodium (or potassium) salt of a long-chain carboxylic acid. Sodium stearate, C₁₇H₃₅COONa, is a typical soap. The magic of cleaning lies in the soap molecule having two very different ends.
The two-faced soap molecule
A soap molecule has a long non-polar hydrocarbon tail (hydrophobic, water-hating, oil-loving) and a short polar ionic head –COO−Na+ (hydrophilic, water-loving). Water dissolves the head; oil and grease dissolve the tail.
Cleansing action of soap
The hard water problem – why soap fails
Hard water contains calcium (Ca2+) and magnesium (Mg2+) ions. Soap reacts with them to form an insoluble, sticky precipitate called scum (calcium/magnesium stearate). The soap is wasted, lather is poor, and clothes get a grey film.
\[ 2C_{17}H_{35}COONa + Ca^{2+} \to (C_{17}H_{35}COO)_2Ca + 2Na^+ \]
Scum is the white, sticky, insoluble precipitate of calcium or magnesium stearate formed when soap reacts with hard water. It sticks to clothes and makes cleaning difficult and wasteful.
Detergents – clean in hard water too
A detergent is a synthetic cleansing agent — the sodium salt of a long-chain alkyl sulphonate or alkyl hydrogen sulphate, e.g. sodium lauryl sulphate. Detergents work in both soft and hard water.
Why? The calcium and magnesium salts of detergent sulphonates are soluble in water, unlike the insoluble stearate salts of soap. So no scum, and effective cleaning even in hard water.
Soaps are natural and biodegradable, but fail in hard water. Detergents work everywhere, but many are non-biodegradable — they persist in rivers and lakes, foaming up and harming aquatic life. This is the "soap vs detergent" debate made simple for boards.
14. Comparison Tables – The Exam Shortcuts
Comparative questions are guaranteed in some form. Memorise these five tables and you can answer any of them in a single paragraph each.
1. Saturated vs. unsaturated compounds
| Point | Saturated | Unsaturated |
|---|---|---|
| Bond type | Only C–C single bonds | Contains C=C or C≡C |
| General formula | CnH2n+2 | CnH2n or CnH2n-2 |
| Hydrogen content | Maximum | Less |
| Addition reactions | No | Yes (double/triple bond opens) |
| Bromine water test | No decolourisation | Decolourises quickly |
| Example | Ethane C₂H₆ | Ethene C₂H₄, ethyne C₂H₂ |
2. Soap vs. detergent
| Point | Soap | Detergent |
|---|---|---|
| Nature | Na/K salt of long-chain carboxylic acid | Na salt of alkyl sulphonate / alkyl hydrogen sulphate |
| In hard water | Forms scum with Ca2+/Mg2+; poor lather | Works well; salts are soluble, no scum |
| Biodegradable | Yes (natural) | Often no (synthetic, persist) |
| Skin / environment | Mild | Can irritate skin; pollutes water |
3. Ethanol vs. ethanoic acid
| Point | Ethanol | Ethanoic acid |
|---|---|---|
| Formula | C₂H₅OH | CH₃COOH |
| Functional group | –OH (alcohol) | –COOH (carboxylic acid) |
| Smell | Alcoholic | Sharp, pungent (vinegar) |
| Litmus | Neutral (no change) | Turns blue litmus red (acid) |
| Reaction with NaHCO₃ | No reaction | Effervescence, CO₂ released |
| Common use | Sanitiser, solvent, fuel | Vinegar, food preservative |
4. Addition vs. substitution reaction
| Point | Addition | Substitution |
|---|---|---|
| Which compounds | Unsaturated (alkenes, alkynes) | Saturated (alkanes) |
| What happens | Atoms added across C=C / C≡C | One atom replaced by another |
| Example | C₂H₄ + H₂ → C₂H₆ | CH₄ + Cl₂ → CH₃Cl + HCl |
| Catalyst/condition | Oft. Ni / catalyst | Usually sunlight for halogens |
5. Covalent vs. ionic compounds
| Point | Covalent compounds | Ionic compounds |
|---|---|---|
| Formation | Sharing of electrons | Transfer of electrons |
| State of matter | Gases, liquids or low-melting solids | Usually crystalline solids |
| Melting/boiling point | Low | High |
| Electrical conductivity | Poor conductor | Conducts when molten/aqueous |
| Solubility | More in organic solvents | More in water |
15. Important Chemical Equations – Ready Reckoner
Recite these until you can write them blindfolded. Balance is the water; one missing coefficient is a lost mark.
| # | Reaction | Equation |
|---|---|---|
| 1 | Combustion of methane | \[ CH_4 + 2O_2 \to CO_2 + 2H_2O + heat + light \] |
| 2 | Combustion of ethanol | \[ C_2H_5OH + 3O_2 \to 2CO_2 + 3H_2O + heat + light \] |
| 3 | Ethanol with sodium | \[ 2Na + 2C_2H_5OH \to 2C_2H_5ONa + H_2\uparrow \] |
| 4 | Ethanol oxidised to ethanoic acid | \[ C_2H_5OH + 2[O] \xrightarrow[\text{alkaline KMnO}_4]{} CH_3COOH + H_2O \] |
| 5 | Ethanol dehydrates to ethene | \[ C_2H_5OH \xrightarrow[\text{conc. } H_2SO_4, 443K]{-H_2O} CH_2=CH_2 \] |
| 6 | Ethanoic acid with sodium | \[ 2CH_3COOH + 2Na \to 2CH_3COONa + H_2\uparrow \] |
| 7 | Neutralisation with NaOH | \[ CH_3COOH + NaOH \to CH_3COONa + H_2O \] |
| 8 | With sodium bicarbonate | \[ CH_3COOH + NaHCO_3 \to CH_3COONa + H_2O + CO_2\uparrow \] |
| 9 | Esterification | \[ CH_3COOH + C_2H_5OH \underset{\text{conc. } H_2SO_4}{\rightleftharpoons} CH_3COOC_2H_5 + H_2O \] |
| 10 | Saponification | \[ CH_3COOC_2H_5 + NaOH \to CH_3COONa + C_2H_5OH \] |
| 11 | Hydrogenation of ethene | \[ CH_2=CH_2 + H_2 \xrightarrow{\text{Ni}} CH_3-CH_3 \] |
| 12 | Hydrogenation of ethyne | \[ CH\equiv CH + 2H_2 \xrightarrow{\text{Ni}} CH_3-CH_3 \] |
| 13 | Substitution with chlorine | \[ CH_4 + Cl_2 \xrightarrow{\text{sunlight}} CH_3Cl + HCl \] |
In exams, always write physical states (s), (l), (g) and the catalyst/condition above or below the arrow when the question asks for it. Arrows for reversible reactions must be double-headed (⇌).
16. Key Points – Must Remember
- Carbon: atomic number 6, configuration 2, 4 — tetravalent.
- Carbon shares electrons (covalent bonding) because it can neither lose nor gain 4 electrons easily.
- Catenation (C–C bonds) + tetravalency = millions of carbon compounds.
- Saturated = only single C–C bonds (alkanes, CnH2n+2).
- Unsaturated = C=C (alkenes, CnH2n) or C≡C (alkynes, CnH2n−2).
- Homologous series: neighbouring members differ by –CH₂– (14 u); same chemistry.
- Functional groups: –OH (alcohol), –CHO (aldehyde), >C=O (ketone), –COOH (carboxylic acid), –Cl/–Br (haloalkane).
- Carbon compounds: low m.p./b.p., poor conductors, weak intermolecular forces.
- Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light.
- Addition reactions = unsaturated; substitution reactions = saturated.
- Ethanol (C₂H₅OH) → ethanoic acid (CH₃COOH) by oxidation with alkaline KMnO₄.
- Esterification: acid + alcohol ⇌ ester + water (conc. H₂SO₄); saponification: ester + NaOH → salt + alcohol (soap!).
- A soap = Na/K salt of a long-chain carboxylic acid; micelle lifts dirt; scum appears in hard water; detergents work even in hard water.
Covalent bond: shared electron pair. Catenation: bonding to like atoms. Homologous series: CH₂ family. Functional group: the reactive atom/group. Ester: fruity R–COO–R’. Scum: Ca/Mg soap precipitate. Micelle: soap cluster around dirt.
17. Common Mistakes to Avoid in Exams
- Writing propane as C₃H₆: propane is an alkane C₃H₈; C₃H₆ is propene. Keep the general formula in mind before writing any formula.
- Calling vinegar "ethanoic acid": vinegar is a dilute 5–8% aqueous solution of ethanoic acid, not the pure acid.
- Swapping ethanol and ethanoic acid formulas: ethanol C₂H₅OH; ethanoic acid CH₃COOH. Write the OH and COOH groups deliberately.
- Balancing by copying blindly: every equation must be balanced — check atoms on both sides, especially oxygen and hydrogen.
- Forgetting the condition: hydrogenation needs Ni catalyst; esterification needs conc. H₂SO₄; chlorination needs sunlight. A missing condition costs marks.
- Using a single arrow for reversible esterification: esterification is reversible — use the double arrow ⇌.
- Confusing addition and substitution: alkenes add, alkanes substitute. Ethene + Cl₂ gives a dichloroethane (addition), not chloroethane.
- Saying covalent compounds conduct electricity: they do not — no free ions/electrons. That is an ionic property.
- Thinking carbon forms four ions: carbon shares, it never forms C4+ or C4− easily.
- Writing CH₂ = CH₂ for ethane: ethane is CH₃–CH₃; ethene is CH₂=CH₂. Two entirely different molecules.
- Confusing methane and methanol: methane CH₄ is a gas (CNG); methanol CH₃OH is a poisonous liquid.
- Omitting [O] in oxidation of ethanol: the product is written with [O] from an oxidising agent; don’t balance oxygen purely with O₂.
- Calling soap a detergent and vice versa in hard water questions: soap → scum; detergent → works fine. Know why (soluble salts of detergent).
- Forgetting glycerol in saponification: soap + glycerol, not soap alone.
18. Board Exam Tips for This Chapter
- Draw electron-dot structures neatly: one dot per electron, shared pairs between atoms, labels for each element.
- Write names next to formulas in your answer (e.g. C₂H₆O — ethanol); examiners reward identification.
- Use symbols and abbreviations privately in rough work; final answer should be tidy, especially around arrows and coefficients.
- For "name the functional group" questions, write the name (alcohol, carboxylic acid), not just the symbols.
- For esterification, always mention the concentrated sulphuric acid catalyst and the reversible arrow.
- For soap vs detergent, connect every difference to Ca2+/Mg2+ ions in hard water and solubility of their salts.
- Learn two standard diagrams cold: the electron-dot structure of methane/ethane/ethene/ethyne, and the micelle formation diagram.
- Quote physical states in balanced equations where possible — it shows confidence and saves marks.
- Revise the "common mistakes" list above; almost every one has appeared as a board distracter.
Quick Self-test (answer in your head)
(a) What is the general formula of alkanes? (b) Which functional group is in ethanoic acid? (c) What is added across a double bond during hydrogenation? (d) Why does soap not lather in hard water? (e) Name the catalyst used in hydrogenation.
19. Chapter Summary
Carbon, atomic number 6, is tetravalent. Instead of losing or gaining four electrons, it shares them, forming covalent bonds. Because carbon links to itself (catenation) in straight chains, branched chains and rings, and connects with hydrogen, oxygen, nitrogen and halogens, it forms millions of compounds.
Hydrocarbons that contain only single C–C bonds are saturated (alkanes, CnH2n+2); those with C=C or C≡C bonds are unsaturated (alkenes, alkynes). Compounds differing by one –CH₂– unit and sharing a functional group form a homologous series with the same general formula and similar chemistry.
Carbon compounds burn (combustion), get oxidised (ethanol → ethanoic acid), add atoms if unsaturated (addition, e.g. hydrogenation of vegetable oils to vanaspati), and substitute atoms if saturated (substitution, e.g. chlorination of methane in sunlight).
Ethanol, C₂H₅OH, is the drink/sanitiser alcohol; ethanoic acid, CH₃COOH, is the vinegar acid. Their key reactions are with sodium, bases, carbonates, and each other (esterification). An ester reacting with NaOH (saponification) yields soap and glycerol.
Soap’s hydrophobic tail dissolves grease while its hydrophilic head stays in water; aggregation around oily dirt forms a micelle. In hard water, Ca2+/Mg2+ ions turn soap into insoluble scum; detergents avoid this because their calcium/magnesium salts are soluble.
20. Quick Revision Table
| Topic | One-line memory |
|---|---|
| Covalent bond | Sharing of electron pairs between atoms |
| Catenation | C–C chains — why carbon makes millions of compounds |
| Tetravalency | Carbon makes 4 bonds (configuration 2, 4) |
| Alkanes | Saturated, CnH2n+2, e.g. CH₄, C₂H₆ |
| Alkenes / alkynes | Unsaturated: CnH2n and CnH2n-2, C=C / C≡C |
| Homologous series | CH₂– steps, same functional group, similar chemistry |
| Functional groups | –OH alcohol, –COOH acid, >C=O ketone, –CHO aldehyde, –Cl halo |
| Ethanol | C₂H₅OH → burns, reacts with Na, oxidises to acid, dehydrates to ethene |
| Ethanoic acid | CH₃COOH; weak acid; vinegar; frothy with NaHCO₃ |
| Esterification / saponification | Acid+alcohol ⇌ ester+water; ester+NaOH → soap+glycerol |
| Soap / detergent | Soap fails in hard water (scum); detergent works (soluble salts) |
| Micelle | Tails in grease, heads in water — lifts dirt away |