Carbon And Its Compounds

A pencil mark, a diamond and a drop of ethanol all contain carbon. Yet one leaves a soft trace, one is extremely hard and one flows as a liquid. Knowing the element is only the beginning: which atoms are connected, how they are bonded and how the resulting particles interact explain the differences.

These Class 10 Science notes follow NCERT's Carbon and Its Compounds, Chapter 4, in the 2026–27 reprint. The current CBSE syllabus includes covalent bonding, carbon's versatility, hydrocarbons, homologous series, naming, four reaction types, ethanol and ethanoic acid, and soaps and detergents. Detailed reactions of the two named compounds are included here for textbook understanding; CBSE describes their scope as properties and uses. The molecular models and questions are original practice, not past-paper questions.

• Count shared electron pairs, not just lines or neighbouring atoms.
• Distinguish a molecule's internal bonds from attractions between molecules.
• A molecular formula counts atoms; a structural formula also tells you how they connect.
• Identify the changing group and the reaction conditions before naming a reaction.
• Soap works because its hydrocarbon tail and ionic head interact differently with oil and water.

Bonding in carbon: the covalent bond

A neutral carbon atom has six electrons, arranged 2,4 in the shell model. Four are valence electrons. Forming an isolated C⁴⁺ ion would require removing all four; forming C⁴⁻ would require accommodating four more electrons around the small nucleus. Ordinary carbon compounds instead commonly use covalent bonding: atoms share electron pairs.

In methane, CH₄, carbon shares one pair with each of four hydrogen atoms. Each C–H bond contains two electrons, one contributed by each atom in the dot-and-cross bookkeeping model. Count each shared pair around both atoms: carbon has eight electrons in its outer-shell count and each hydrogen has two. There are eight valence electrons in the whole molecule, not sixteen. Shared electrons are counted twice only when checking the shells of the two bonded atoms.

The octet and hydrogen's duet are useful school models for these compounds, not a claim that every atom or molecule follows an exceptionless rule. Electrons do not have intentions; a stable bond corresponds to an energetically favourable arrangement of nuclei and electrons.

Read dots, crosses and bond lines

• Single bond: one shared pair, as in H–H and Cl–Cl. Each chlorine in Cl₂ also has three lone pairs.
• Double bond: two shared pairs, as in O=O. Each oxygen has two lone pairs.
• Triple bond: three shared pairs, as in N≡N. Each nitrogen has one lone pair.
• Water and ammonia: oxygen forms two O–H bonds and has two lone pairs in H₂O; nitrogen forms three N–H bonds and has one lone pair in NH₃.

A lone pair belongs to the atom's valence-shell count but is not shared in a bond. Dots and crosses distinguish where electrons were counted initially; they are not different species of electron. A flat Lewis drawing describes bonding, not the actual size, trajectory or three-dimensional placement of every particle.

Carbon is bonded to only two oxygen atoms in CO₂. Has tetravalency failed?

No. Its structure is O=C=O. Each double bond counts as two shared pairs, so the total bond order around carbon is four. Tetravalency does not always mean four neighbouring atoms. Each oxygen also has two lone pairs.

Molecules and networks behave differently

In many small molecular substances, strong covalent bonds hold each molecule together while weaker attractions act between molecules. Boiling ethanol mainly separates molecules from their neighbours; it does not normally break every molecule into carbon, hydrogen and oxygen atoms. This distinction explains why many molecular carbon compounds have relatively low melting and boiling points compared with extended solids.

It does not follow that “all covalent substances melt easily”. Diamond is an extended network of covalent bonds. Graphite is another network solid, with strongly bonded layers and weaker interactions between layers. Network structures need their own explanation.

Many pure molecular substances also lack mobile charged particles and conduct electricity poorly. But a covalent compound can form ions when it reacts with water: aqueous ethanoic acid contains some ions and conducts. Graphite conducts through mobile, delocalised electrons. The useful question is what can carry charge through this particular material?

The versatile nature of carbon

Catenation is the ability of an element to bond to itself. Carbon forms stable C–C bonds, allowing long chains, branches and rings. Tetravalency allows four units of bonding around an ordinary neutral carbon atom in the structures studied here. Together, these features permit many connected arrangements and many combinations with hydrogen, oxygen, nitrogen, sulphur and halogens.

A hydrocarbon contains only carbon and hydrogen. An organic compound may also contain other elements, so ethanol is organic but is not a hydrocarbon. Some familiar carbon-containing substances, including carbon dioxide and carbonates, are conventionally studied as inorganic compounds. “Contains carbon” is therefore not a complete definition of organic chemistry.

Allotropes: one element, different structures

• Diamond: each carbon is bonded to four others in a three-dimensional network. The rigid arrangement explains its great hardness. Pure diamond has no readily mobile electrons for ordinary electrical conduction.
• Graphite: each carbon is bonded to three neighbours within a layer. Delocalised electrons help carry current along the layers; weaker interactions between layers allow them to slide. Pencil cores contain graphite mixed with other materials, not metallic lead.
• Fullerenes: carbon atoms can form cage-like molecules. Buckminsterfullerene, C₆₀, has a cage containing pentagonal and hexagonal faces. This is a molecular form, unlike the extended diamond network.

A useful surprise: a laboratory-grown diamond is diamond, not automatically a glass imitation. GIA explains that it has essentially the same material properties as natural diamond, while growth features can reveal its origin. Carbon-rich gas can supply carbon during chemical vapour deposition. The lesson is that structure and origin are different questions. Examine GIA's comparison of natural and laboratory-grown diamonds.

Chains, multiple bonds and structural isomers

Start with two carbons. A single C–C bond uses one bonding unit at each carbon, leaving three for hydrogen: CH₃–CH₃, ethane, C₂H₆. A double C=C bond leaves two for hydrogen at each carbon: CH₂=CH₂, ethene, C₂H₄. A triple bond leaves one: HC≡CH, ethyne, C₂H₂. The carbon atoms have not changed their ordinary valency; the sharing pattern has changed.

Saturated hydrocarbons have only single carbon–carbon bonds. Unsaturated hydrocarbons contain carbon–carbon double or triple bonds. The C=O group in an aldehyde does not make it an unsaturated hydrocarbon: it contains oxygen, so it is not a hydrocarbon at all.

For open-chain hydrocarbons with no rings, alkanes follow CₙH₂ₙ₊₂. Those with one double bond and otherwise single bonds follow CₙH₂ₙ; those with one triple bond and otherwise single bonds follow CₙH₂ₙ₋₂. An alkene or alkyne needs at least two carbon atoms. These conditions matter: cyclohexane, C₆H₁₂, has only single bonds but closes into a ring. Its formula alone must not be used as proof of a double bond.

Structural isomers have the same molecular formula but different atom connectivity. Butane, CH₃–CH₂–CH₂–CH₃, and 2-methylpropane, CH₃–CH(CH₃)–CH₃, both contain four carbons and ten hydrogens. In the first, no carbon has more than two carbon neighbours; in the second, one has three. Bending or rotating a drawing while keeping all connections unchanged does not create another structural isomer.

Can you invent a third chain isomer of C₄H₁₀ by drawing the chain as a staircase?

A staircase with four carbons linked consecutively is still butane. Try counting carbon neighbours rather than looking at the outline. For these connected, saturated, open-chain four-carbon molecules, the only carbon skeletons are the unbranched chain and the three-branch centre. A ring would remove two hydrogens and give C₄H₈.

NCERT also introduces benzene, C₆H₆, through a ring drawing with alternating single and double bonds. That is a representation: benzene's electrons are delocalised around the ring, and its chemistry should not simply be treated as that of three isolated alkene double bonds. Detailed aromatic chemistry is beyond this chapter.

Homologous series

A homologous series is a family with the same characteristic functional group and general formula, whose successive members differ by a CH₂ unit. The first six open-chain alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂ and hexane C₆H₁₄. The corresponding name roots are meth-, eth-, prop-, but-, pent- and hex-.

The alcohol series begins CH₃OH, C₂H₅OH and C₃H₇OH. Using the school atomic masses C = 12 u, H = 1 u and O = 16 u, their molecular masses are 32 u, 46 u and 60 u. Each difference is 14 u, the mass of CH₂. If expressed as molar masses, the difference is 14 g/mol. Do not write “14” without saying what it measures.

The shared functional group gives related chemical behaviour, not identical properties in every setting. Physical properties change as chain size and shape change. Boiling points generally rise along a straight-chain series as attractions between molecules strengthen. Solubility can change substantially, and melting points need not increase smoothly at every step because packing also matters.

Are ethanol and ethanoic acid consecutive members of one homologous series?

No. Ethanol contains an alcohol group, whereas ethanoic acid contains a carboxyl group. They have the same number of carbon atoms but different functional groups. Compare ethanol with methanol or propanol when tracing the alcohol series.

Nomenclature of carbon compounds

A name is a compact description of structure. For the simple examples here, identify the carbon chain, its bond type and any functional group. A heteroatom is an atom other than carbon or hydrogen in an organic structure. A functional group is a characteristic atom or group associated with particular reactions; merely spotting an oxygen atom is not enough.

• Halogen: replacing H with Cl or Br gives a chloro- or bromo- prefix. CH₃–CH₂–Br is bromoethane.
• Alcohol, –OH: CH₃–CH₂–OH is ethanol, ending in -ol. The final e of ethane is removed.
• Aldehyde, –CHO: the carbonyl carbon is at an end and bonded to H. CH₃–CHO is ethanal, ending in -al.
• Ketone, –C(=O)–: the carbonyl carbon is bonded to carbon groups on both sides. CH₃–C(=O)–CH₃ is propanone, ending in -one.
• Carboxylic acid, –COOH: CH₃–COOH is ethanoic acid. The carbon in the functional group is included in the chain count.
• Multiple carbon bonds: CH₂=CH₂ is ethene and HC≡CH is ethyne; -ene and -yne replace the alkane ending.

Positions are needed when more than one arrangement is possible. CH₃–CH₂–CH₂–OH is propan-1-ol; CH₃–CH(OH)–CH₃ is propan-2-ol. Count from the appropriate end to give the group its lower position in these simple examples. This is an introduction, not the full set of IUPAC rules for molecules with several competing groups.

Why can propanone exist but an ordinary one-carbon ketone cannot?

A ketone's carbonyl carbon has carbon groups on both sides, so at least three carbons are needed. Methanal, H–C(=O)–H, is an aldehyde. The distinction comes from connectivity, not just the presence of C=O.

Chemical properties of carbon compounds

Read each reaction as a change in atoms and bonds. Conditions above an arrow are part of the information: they are not extra atoms that can be silently added to balance an equation. The following are explanations and paper models, not instructions to burn fuels or handle chlorine, sodium, oxidisers or concentrated acids.

Combustion: complete and incomplete

Complete combustion of a hydrocarbon in sufficient oxygen gives carbon dioxide and water, releasing energy. Complete combustion of carbon itself gives carbon dioxide:

• C + O₂ → CO₂
• CH₄ + 2O₂ → CO₂ + 2H₂O
• C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

The equations conserve atoms; released energy is not a new material substance. Limited oxygen can lead to carbon monoxide and soot as well as other products. Flame colour also depends on air supply and burning conditions. A yellow, sooty flame by itself does not prove that the original fuel was an unsaturated hydrocarbon. Carbon monoxide is hazardous; never investigate this by restricting a real burner or testing a flame indoors.

Oxidation: change one part of a molecule

Ethanol can be oxidised to ethanoic acid with suitable oxidising agents, such as acidified potassium dichromate or alkaline potassium permanganate in the school treatment. The condensed accounting equation is CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. Here [O] represents an oxygen equivalent supplied by the oxidiser, not a bottle of free oxygen atoms. The full reagent equation depends on conditions; alkaline solution initially favours the carboxylate salt, with the acid obtained on acidification.

Addition: use a multiple bond

In hydrogenation, hydrogen adds across a carbon–carbon multiple bond with a suitable catalyst. For ethene: CH₂=CH₂ + H₂ → CH₃–CH₃, using a catalyst such as nickel under suitable conditions. The C=C becomes C–C while new C–H bonds form. A catalyst provides another reaction pathway and is regenerated overall; it is not simply “an ingredient that never participates”. Hydrogenation is used to modify unsaturated oils. Fats and oils are mixtures, so “all plant oils are unsaturated and all animal fats saturated” is too absolute.

In a teacher-provided comparison of a simple alkene and an alkane under suitable conditions, loss of bromine colour can support an addition reaction at the alkene double bond. It is not a universal identification test for every unknown substance: other reactive groups can also consume bromine. Use the stated structures and controlled conditions, and analyse supplied observations instead of handling bromine.

Substitution: exchange an atom

Under ultraviolet light or suitable sunlight, chlorine can replace hydrogen in methane: CH₄ + Cl₂ → CH₃Cl + HCl. This is substitution: one H has been replaced by Cl. Further substitutions can occur, so this equation represents one stage, not a guarantee of a single pure product. The reaction is unsuitable for a home activity.

A fuel gives a sooty flame when air is restricted. Which conclusion is supported?

Its combustion under those conditions is incomplete. Its molecular structure has not been established. Change the explanation to match the observation: flame appearance depends on both the substance and its environment. Do not reproduce the restriction; reason from the stated observation.

Two important carbon compounds

Ethanol: CH₃CH₂OH

Ethanol is a colourless, flammable liquid at ordinary room temperature, mixes with water in all proportions and is a useful solvent and fuel. Its –OH group enables strong interactions with water; being a carbon compound does not automatically mean being water-insoluble. Ethanol is also present in alcoholic drinks, but laboratory or industrial alcohol must never be tasted.

Methanol is a different substance, CH₃OH, not a weaker name for ethanol. Methanol is highly toxic; CDC/NIOSH explains that its odour is not a reliable exposure warning. Denatured alcohol has additives that make it unsuitable for drinking; the formulation is not identical everywhere. No appearance, smell or classroom formula is a safe test of whether a liquid is drinkable.

In the NCERT reaction with sodium, hydrogen is displaced from ethanol's –OH group: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. The salt is sodium ethoxide. Ethanol's reaction with sodium does not make it chemically identical to ethanoic acid.

Heating ethanol with excess concentrated sulphuric acid at 443 K, approximately 170°C, can remove water: C₂H₅OH → C₂H₄ + H₂O. This is dehydration, yielding ethene. It differs from oxidation to ethanoic acid. These are teacher-controlled laboratory reactions to understand, not practical tasks to attempt independently.

Ethanoic acid: CH₃COOH

Ethanoic acid, also called acetic acid, contains the carboxyl group. Vinegar is a dilute aqueous solution containing acetic acid, whereas concentrated or glacial acetic acid is not a food substitute. Pure ethanoic acid freezes near 290 K, explaining the word glacial.

In the supplied classroom observations, aqueous ethanoic acid turns blue litmus red and mixes with water to form a uniform solution. Vinegar is used as a preservative in foods such as pickles; this use refers to food-grade vinegar, not concentrated laboratory acid. These are observations and uses to interpret, not instructions to smell, taste or mix chemicals.

Ethanoic acid is a weak acid: only some molecules ionise in water. “Weak” describes the extent of ionisation, not whether a sample is dilute, harmless or unable to react. Compare acids at specified concentrations; litmus alone does not measure acid strength.

Ethanoic acid reactions: follow the products

Neutralisation: with sodium hydroxide, ethanoic acid forms sodium ethanoate and water: CH₃COOH + NaOH → CH₃COONa + H₂O. Sodium ethanoate is also called sodium acetate.

Carbonate reactions: carbon dioxide is released along with salt and water. The coefficients depend on which carbonate is used:

• 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
• CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

The second reaction provides a classroom distinction from ethanol under comparable conditions: ethanoic acid gives effervescence with sodium hydrogencarbonate; ethanol does not give that same acid reaction. Identifying the evolved carbon dioxide with limewater is stronger evidence than calling every bubble “hydrogen”.

Esterification: ethanoic acid and ethanol can form ethyl ethanoate and water with an acid catalyst and warming: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The reversible arrow matters. Many esters contribute characteristic aromas and are used in fragrances and flavourings, but smell is not a safe chemical identification procedure.

Alkaline hydrolysis: the ester reacts with sodium hydroxide to give an alcohol and a carboxylate salt: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. This type of reaction is called saponification. To make ordinary soap, the starting fats or oils provide long-chain carboxylate groups. Sodium ethanoate from this short-chain example is not ordinary cleansing soap.

Why is “ester + alkali gives alcohol + acid” an incomplete answer?

Under alkaline conditions the carboxylic product is its salt. Ethyl ethanoate and sodium hydroxide give ethanol and sodium ethanoate. Obtaining the free acid would require a separate acidification step.

Soaps and detergents

Oil does not mix readily with water. A soap ion offers two different chemical regions: a long hydrocarbon tail that associates with oily material and a charged carboxylate head that interacts with water. Ordinary soaps are sodium or potassium salts of long-chain carboxylic acids. “Water-loving” and “water-avoiding” are memorable shorthand for these interactions, not conscious preferences.

Micelles and washing

In water, soap ions can assemble into micelles when conditions, including concentration, permit. Hydrocarbon tails gather inward and ionic heads face the water. Soap also helps disperse oily dirt: tails associate with the oil, while outward-facing heads allow small oil-rich droplets or aggregates to remain dispersed in water. Agitation helps loosen material from a surface, and rinsing carries dispersed dirt away.

The retained diagram compares an aggregate without a central oil droplet with one containing oily dirt. That distinction is useful: a micelle does not require a lump of dirt to exist. Drawings exaggerate particle sizes and simplify a dynamic process. Neither cloudiness nor the amount of foam alone measures cleaning performance.

Why hard water changes the result

Hard water contains appreciable dissolved calcium or magnesium ions. These can form poorly soluble salts with ordinary soap ions, producing scum and using up some soap before it can help clean. Many synthetic detergents have different head groups whose calcium and magnesium salts remain more soluble, making them useful in hard water. Product formulations still differ; “detergent” is not a promise of unlimited performance in every water sample.

A detergent produces foam in two water samples. Does that prove both samples are soft?

No. A detergent may work in hard water too. A fair comparison with ordinary soap would keep water volume, soap amount, temperature and shaking similar, then consider scum as well as lather. The observation must be connected to the chemistry of the cleaning agent.

Why it still matters

Coal, petroleum and natural gas store carbon from ancient organic matter transformed through geological processes over millions of years. Their formation is far slower than our present use, so they are called fossil fuels. This origin does not change the need to conserve atoms when explaining their combustion.

The same habit—follow the atoms—helps evaluate an environmental claim. Burning ethanol completely still produces CO₂. Calling it plant-derived does not remove carbon from the combustion equation. An overall environmental comparison must also consider how the feedstock grows, processing and transport energy, and what happens to land; a balanced equation alone cannot calculate the full result.

Try a small real-data reading task. NOAA's global monthly mean page reports 428.73 parts per million for May 2026, in its update dated 5 August 2026. Convert that number to percent: 428.73 ÷ 10,000 = 0.042873%. It is a global monthly mean, not a daily reading at one station, and the dated value is not a permanent atmospheric constant. Inspect NOAA's measurement page and its date.

Carbon connectivity, conservation of atoms, evidence and material properties are ideas used across science curricula worldwide. This page's chapter mapping remains specifically NCERT/CBSE. For the next step, explore Life Processes, where organisms transform and transport carbon-containing molecules, or revisit Chemical Reactions and Equations to practise atom accounting.

Sources

1. NCERT Science, Chapter 4: Carbon and Its Compounds, 2026–27 reprint — chapter concepts, models and textbook reaction scope.
2. CBSE Science curriculum, 2026–27 — current Class 10 scope and practical list.
3. OpenStax Chemistry 2e: Hydrocarbons — comparing molecular representations, chain isomers and the limits of general formulae.
4. Royal Society of Chemistry: Carbon — element and allotrope context.
5. GIA: Natural and laboratory-grown diamonds — sourced enrichment on structure and origin.
6. NOAA Global Monitoring Laboratory: Global CO₂ trends — the dated observation used in the conversion task.