Chapter 11 — Carbon compounds, fuels and polymers
Why carbon makes so many compounds
Carbon has four valence electrons in the basic school model and commonly forms four covalent bonds. Carbon atoms can bond to one another in chains and rings and can form single, double or triple bonds. These features help explain the large number of carbon compounds. A formula alone does not always tell us the arrangement of atoms. C₂H₆O can represent ethanol or dimethyl ether: the same elemental counts arranged differently. Such substances are structural isomers, and their properties can differ. This is a strong reason to read a structural formula rather than treating every molecular formula as a unique substance name.
The simplest saturated hydrocarbon family is the alkanes, in which carbon atoms are linked by single bonds in the elementary open-chain model. Methane is CH₄, ethane C₂H₆ and propane C₃H₈. In this family, the formula CₙH₂ₙ₊₂ applies to an acyclic saturated molecule. It is not a universal formula for all carbon compounds or even all hydrocarbons. An alkene contains a carbon–carbon double bond in its defining structure; ethene is C₂H₄. An alkyne has a carbon–carbon triple bond; ethyne is C₂H₂. The suffix and the bond type should be matched together in a question.
Saturated and unsaturated describe bonding, not whether a substance is “full” of oxygen or dissolved in water. Unsaturation in this simple comparison involves a multiple bond between carbon atoms. A compound can contain oxygen and still have a carbon–carbon double bond. The bromine-water test is often taught as evidence for a suitable unsaturated compound under stated conditions, but it is not a unique identification of a named molecule. Multiple substances can affect a coloured reagent. Choose the inference the observation supports, not a more specific identity the question has not established.
Functional groups organise reactions
A functional group is a characteristic group of atoms that influences a molecule's chemistry. In the elementary list, –OH attached in the alcohol context identifies an alcohol such as ethanol, CH₃CH₂OH. The –COOH group identifies a carboxylic acid such as ethanoic acid, CH₃COOH. A carbonyl group in an aldehyde is at the end of a suitable chain; in a ketone it is within the chain. A police-level question may ask only which formula bears the alcohol or acid group. Do not count an –OH written inside a carboxylic acid as proof that the whole substance belongs to the ordinary alcohol family; the group is part of –COOH and the classification differs.
Ethanol and ethanoic acid have different familiar roles and properties despite both having two carbon atoms. Ethanol is an alcohol; ethanoic acid is a carboxylic acid. Ethanoic acid can react with a suitable alcohol under appropriate conditions to form an ester and water, an example of esterification. Saponification is the alkaline hydrolysis of an ester in the common school explanation of soap-making from fats or oils. Naming the reaction is enough here; this book does not supply mixing or heating instructions. In an MCQ, do not call every substance with a pleasant smell an ester without further evidence.
Soap molecules have a water-attracting end and an oil-attracting hydrocarbon tail in the simplified model. In water they can help surround oily material, allowing it to disperse and be washed away. Hard water contains dissolved ions such as calcium and magnesium that can react with ordinary soap to form scum and reduce lather. Some synthetic detergents behave differently under such conditions. “More foam always means better cleaning” is not a dependable scientific conclusion; foam and removal of a specific soil are distinct observations.
Combustion and fuel comparisons
A hydrocarbon fuel can react with oxygen to release energy. For methane's complete combustion, CH₄ + 2O₂ → CO₂ + 2H₂O. Check C: 1=1; H: 4=4; O: 4=4. If oxygen is insufficient, products can include carbon monoxide or soot. Whether a fuel is suitable in practice depends on energy density, storage, emissions, cost, distribution and safety conditions. A single label such as “natural” does not establish that a fuel has zero environmental impact. In an exam item, read whether the question asks about complete combustion products, fuel storage or pollution; they require different facts.
Calorific value describes energy released per unit quantity of fuel under a stated convention, often per unit mass. It is not the same as a fuel's total energy in a particular cylinder or tank; the total depends on how much fuel is present. A synthetic exam example: if fuel A releases 40 MJ/kg, then 2 kg under the same idealised convention releases 80 MJ. If fuel B has a larger calorific value per kilogram, that does not automatically make it cheaper or easier to store. Compare only quantities with matching denominators.
Fossil fuels include coal, petroleum and natural gas formed through geological processes over long times. They are finite resources on human timescales. Biomass fuels come from recently living material, but calling a biomass source “renewable” requires attention to regrowth and management. Burning biomass still produces emissions. Solar and wind energy are different renewable inputs and are not carbon compounds used as fuels in the same way. A well-written MCQ specifies whether it asks for an energy source, fuel chemistry, emissions or renewability.
Polymers and repeating units
A polymer is a very large molecule made of repeating smaller units or related structural segments. The starting small molecules are monomers in many common polymerisation descriptions. Polyethylene is made from ethene-derived repeating units in the simple addition-polymer example. The double bond of ethene is used during polymer formation, so the polymer's repeating chain is not just a row of intact C₂H₄ molecules held side by side. A drawing of the repeating unit can show bracketed –CH₂–CH₂– with a subscript n, indicating many repeats. The actual chain length can vary.
Not every polymer is a plastic, and not every plastic has identical environmental behaviour. Cellulose and proteins are natural polymers; polyethylene is synthetic. Nylon is a family of synthetic polyamides. A question asking “natural or synthetic” should name the exact material. One should also distinguish polymer from composite: a composite combines materials to obtain useful properties, while the polymer name describes molecular structure. A fibreglass object may combine a polymer matrix and glass fibres; the entire object is not one pure polymer molecule.
Thermoplastics soften on heating and can be reshaped under suitable processing conditions; thermosetting plastics form structures that do not simply remelt and reshape after setting. This is a materials-class distinction, not a statement that a discarded thermoplastic can always be recycled economically or indefinitely. Recycling depends on collection, contamination, sorting and processing. The code number on a package, if present, identifies a resin category in a specified system; it does not guarantee local recycling access. The exam-safe concept is the difference in heat-response classes.
Some polymers biodegrade under particular conditions, while others persist. “Biodegradable” alone gives no time frame, location or assurance of harmlessness. A material could fragment into small pieces without being mineralised by biological processes. Likewise, microplastics are small plastic particles, but the term describes a size/material class rather than one chemical formula. Avoid inferring health effects from the mere existence of a fragment; a claim needs exposure and evidence. In a static-science MCQ, focus on polymer source, structure and disposal categories rather than unsourced current statistics.
Carbon's different forms
Diamond and graphite are allotropes of carbon: different structural forms of the same element. Diamond has a three-dimensional covalent network that makes it very hard. Graphite has layered structure and can conduct electricity because of its electronic arrangement. Their contrasting properties show that elemental identity alone does not determine a material's behaviour. Coal is not simply a pure allotrope of carbon; its composition and origin make it a more complex material. A question that groups “coal, diamond and graphite” as three identical pure forms of carbon is therefore suspect.
Carbon dioxide and carbon monoxide each contain carbon and oxygen but have different formulas and properties. Carbon dioxide is CO₂; carbon monoxide is CO. A change of one subscript changes the compound. In air-pollution questions, carbon monoxide from incomplete combustion should not be described as a harmless version of carbon dioxide. In climate questions, carbon dioxide's atmospheric role should not be used to imply it is always poisonous at ordinary outdoor concentrations. Keep chemistry, exposure and environmental context separate.
Use the family formula only after checking the structure
For an acyclic alkane, CₙH₂ₙ₊₂ gives the hydrogen count. With four carbon atoms, the formula is C₄H₁₀. A simple acyclic hydrocarbon with one carbon–carbon double bond has the alkene pattern CₙH₂ₙ, giving C₄H₈ for four carbons. However, C₄H₈ can also describe a saturated ring structure. The molecular formula by itself does not prove that a double bond is present. A question that asks for a specific family must provide the structural condition or a named compound. This is the same general lesson as isomerism: atom counts and connectivity carry different information.
A homologous series groups compounds with a common functional-group pattern and a systematic change in composition between successive members. Successive simple alkanes differ by CH₂: methane CH₄, ethane C₂H₆ and propane C₃H₈. With rounded atomic masses, that difference is 14 mass units. Their related chemistry does not make all physical properties identical; boiling points and other properties vary along the series. A question about the difference between neighbouring members can use the CH₂ rule, while a question about an individual compound still requires its own formula and structure.
Separate molecular and empirical formulas
A molecular formula reports actual atom counts in a molecule. An empirical formula gives the simplest whole-number ratio. For ethene, C₂H₄ reduces to CH₂ as its empirical formula. For ethane, C₂H₆ reduces to CH₃. Methane's CH₄ is already in the simplest ratio. An empirical formula does not tell molecular size: several different molecules can share the same simplest ratio. If an item supplies only CH₂, it cannot identify one particular hydrocarbon without additional information such as molar mass or structure.
Using rounded masses C=12 and H=1, ethene has molar mass 2×12+4×1=28 g/mol, while its empirical-formula mass is 12+2=14. The molecular mass is twice the empirical-formula mass, so the molecular formula contains two CH₂ units. The arithmetic ratio is useful only when the composition and molecular mass are correctly supplied. It does not imply that a stable isolated CH₂ molecule must be the starting substance or that every compound with that ratio is ethene.
Explain cleaning with two molecular regions
The hydrocarbon part of a soap ion associates with oily material, while its ionic end interacts favourably with water. Groups of these molecules can form structures that disperse oil in water, helping remove it from a surface. Cleaning is therefore linked to molecular structure and to the surrounding solution. The presence of calcium and magnesium ions in hard water can form poorly soluble salts with ordinary soap, producing scum. This consumes some soap that would otherwise participate in cleaning. Hardness is caused by particular dissolved ions; muddy appearance is a different property and cannot diagnose hardness by itself.
Temporary hardness and permanent hardness are traditional categories based on the dissolved salts and their response to treatment. The chemistry should be read from the named ions or process rather than inferred from the word “permanent” as meaning impossible to remove. Different detergents have different environmental properties as well as cleaning properties. A material's usefulness in hard water does not by itself establish biodegradability or environmental harmlessness. Questions about cleaning performance and disposal therefore require separate evidence.
Follow energy through a fuel comparison
Suppose fuel A has an assumed calorific value of 30 MJ/kg and an appliance converts 60% of that input to useful heating. One kilogram supplies 18 MJ of useful heat in that model. Fuel B at 25 MJ/kg used in an 80% efficient appliance supplies 20 MJ useful heat per kilogram. A higher calorific value alone therefore does not guarantee more useful heat in a particular device. Compute input energy first, then apply the stated efficiency. Comparing cost would additionally require prices; comparing storage practicality would require still more information.
A combustion equation also connects oxygen requirement with fuel amount. In CH₄+2O₂→CO₂+2H₂O, one mole of methane needs two moles of oxygen for the specified complete reaction. If only one mole of oxygen is available, the stated complete reaction cannot consume one mole of methane without another oxygen supply. The balanced equation does not automatically identify all products under oxygen shortage; it only states the ratio for the reaction written. In a multiple-choice item, distinguish checking a proposed equation from predicting a complex real flame.
Read a polymer drawing as a chain
In a polyethylene repeat unit, the bracketed –CH₂–CH₂– segment connects to further chain segments at both ends. The n outside the bracket means the segment repeats many times. It is not an element symbol, and it does not mean the polymer contains nitrogen. A chain of 100 such ideal repeat units contains 200 carbon atoms in the repeating backbone count, but the exact full molecular formula also depends on its ends. For exam questions on repeat-unit ratios, state whether end groups are being ignored rather than silently treating an approximation as an exact formula for every chain.
Addition polymerisation can join unsaturated monomers without eliminating a small molecule in the elementary example. Some condensation-polymer routes link multifunctional molecules while eliminating a small molecule such as water. Polymer classification is broader than these two school examples, so the monomers and reaction described should guide the answer. Nylon and polyester are different material families; a trade name or the fact that both can be fibres does not make their chemical linkages identical. Structure explains the family, while a manufactured use describes what the material has been made into.
Retrieval exercise with worked checks
What is the acyclic alkane formula for five carbons? C₅H₁₂. Does C₅H₁₀ alone prove the presence of a double bond? No, because ring structures can share that formula. What is the empirical formula of C₆H₁₂? CH₂. How much useful heat results from 2 kg of fuel at 20 MJ/kg and 75% efficiency? Input is 40 MJ and useful output 30 MJ. Finally, what does n mean after a bracketed polymer repeat unit? It represents the number of repeats in the chain description. Each check links a formula or number to the physical quantity it represents.
Worked structure and formula route
A question offers CH₄, C₂H₄, C₂H₆ and C₂H₂ and asks for the two-carbon saturated alkane. The two-carbon condition removes CH₄. Acyclic saturated alkane formula CₙH₂ₙ₊₂ gives H=6 when n=2, so C₂H₆ is ethane. C₂H₄ and C₂H₂ fit the familiar unsaturated examples. This numerical route works only because the item states the simple acyclic family. A cyclic saturated hydrocarbon can have a different formula pattern; do not silently broaden the rule.
Another item states a reaction uses 16 g of methane and 64 g of oxygen and produces 44 g carbon dioxide and 36 g water in an idealised closed system. The input total is 80 g; the output total is also 80 g. The balanced equation has a 1:2:1:2 molecule ratio, but the stated masses follow molar masses, not direct copying of those coefficients as grams. A solution that writes “1 g methane plus 2 g oxygen” from the coefficients alone confuses amount of substance with mass. The arithmetic is included to expose that conceptual trap.
Recall and review
State why carbon can form chains and compare single, double and triple carbon–carbon bonds. Classify an alkane, alkene and alkyne from formula and structure with the acyclic condition stated. Identify –OH and –COOH in their proper contexts. Balance complete methane combustion and distinguish it from incomplete combustion. Explain what a repeating unit means in polyethylene and why thermoplastic is a processing class, not a recycling guarantee. Finally, contrast diamond and graphite as structures of the same element. Attempt the chapter practice set, then use the explanations to revisit any uncertain concepts.
Figure 11. Original chapter diagram.
Practice 43 questions
Choose one answer for each item. The answer key and explanations follow this chapter set.
1. Which hydrocarbon is burnt with oxygen to produce the extremely hot flame used as the standard fuel in gas welding and cutting of metals?
A. Methane B. Ethane C. Ethene (ethylene) D. Ethyne (acetylene)
2. Which alcohol is commonly called 'wood alcohol' and is poisonous to drink?
A. Ethanol (C₂H₅OH) B. Propan-2-ol C. Methanol (CH₃OH) D. Glycerol
3. Which chemist reported the 1828 preparation of urea from ammonium cyanate, a milestone against the claim that organic molecules require living organisms?
A. Antoine Lavoisier B. Friedrich Wöhler C. Dmitri Mendeleev D. John Dalton
4. Which molecular formula does NOT fit CₙH₂ₙ₋₂ for an open-chain hydrocarbon with one carbon–carbon triple bond and no other multiple bonds?
A. C₂H₂ B. C₃H₄ C. C₄H₈ D. C₄H₆
5. Teflon, used as the non-stick coating on cookware, is chemically:
A. Polyvinyl chloride (PVC) B. Polystyrene (PS) C. Polytetrafluoroethene (PTFE) D. Polypropylene (PP)
6. Polypropylene (PP), used for car parts and containers, is made by the addition polymerisation of which monomer?
A. Ethene B. Vinyl chloride C. Styrene D. Propene
7. Charcoal and carbon black (used in inks and tyres) are examples of which form of carbon?
A. Amorphous carbon B. Diamond C. Fullerene D. Graphene
8. A simple acyclic hydrocarbon rapidly decolourises bromine water in the dark. In the standard school test, which feature does this support?
A. Only carbon–carbon single bonds B. A carbon–carbon double or triple bond C. A carboxylic acid group only D. An ether linkage
9. On dehydration under suitable conditions, ethanol (C₂H₅OH) can form which two-carbon alkene?
A. Ethane B. Ethene C. Ethyne D. Methane
10. In graphite, each carbon atom is covalently bonded to how many other carbon atoms?
A. 4 B. 2 C. 6 D. 3
11. Formalin, used to preserve biological specimens, is an aqueous solution of:
A. Acetaldehyde (ethanal) B. Acetone (propanone) C. Formaldehyde (methanal) D. Formic acid
12. The –CO–NH– linkage formed, with loss of a water molecule, when the carboxyl group of one amino acid reacts with the amino group of another is called a:
A. Peptide bond B. Glycosidic bond C. Phosphodiester bond D. Ester bond
13. Buckminsterfullerene, the football-shaped allotrope of carbon discovered in 1985, contains how many carbon atoms per molecule?
A. 50 B. 70 C. 60 D. 80
14. How many structural isomers are possible for butane (C₄H₁₀)?
A. 3 B. 2 C. 1 D. 4
15. Which systematic name describes glycerol, a molecule with three –OH groups on a three-carbon chain?
A. Propan-1-ol B. Ethane-1,2-diol C. Propane-1,2,3-triol D. Propan-2-ol
16. An open-chain hydrocarbon has the molecular formula C₅H₁₀. To which homologous series does it belong?
A. Alkenes B. Alkanes C. Alkynes D. Arenes
17. Aniline (C₆H₅NH₂), widely used in making dyes, contains which functional group?
A. Hydroxyl group (–OH) B. Carboxyl group (–COOH) C. Amino group (–NH₂) D. Nitro group (–NO₂)
18. What is the correct IUPAC name of the compound CH₃–CHBr–CH₂–CH(CH₃)–CH₃?
A. 4-Methyl-2-bromopentane B. 2-Bromo-4-methylpentane C. 4-Bromo-2-methylpentane D. 2-Bromo-4-methylhexane
19. Which gaseous hydrocarbon acts as a natural plant hormone that promotes the ripening of fruits?
A. Methane B. Ethane C. Butane D. Ethene
20. Which of the following compounds is a functional-group isomer of propanal (CH₃CH₂CHO)?
A. Propanone (CH₃COCH₃) B. Propan-1-ol (CH₃CH₂CH₂OH) C. Propanoic acid (CH₃CH₂COOH) D. Propane (CH₃CH₂CH₃)
21. Which of the following compounds shows cis–trans (geometrical) isomerism?
A. But-1-ene B. Propene C. Ethene D. But-2-ene
22. In a common esterification reaction, which pair of reactants forms an ester and water?
A. An aldehyde and a ketone B. An alcohol and an ether C. An amine and an alkane D. A carboxylic acid and an alcohol
23. The reaction CH₄ + Cl₂ → CH₃Cl + HCl, which takes place in the presence of sunlight, is an example of:
A. Addition reaction B. Substitution reaction C. Polymerisation D. Hydrogenation
24. What product forms when ethene reacts with hydrogen over a suitable metal catalyst in an addition reaction?
A. Ethene oxide B. Ethyne C. Ethane D. Ethanol
25. Propan-1-ol (CH₃CH₂CH₂OH) and propan-2-ol (CH₃CH(OH)CH₃) are examples of:
A. Position isomerism B. Chain isomerism C. Functional group isomerism D. Geometrical isomerism
26. The alkaline hydrolysis of fats and oils (esters) with sodium hydroxide to form soap is called:
A. Saponification B. Esterification C. Hydrogenation D. Fermentation
27. The unique ability of carbon atoms to link with one another to form long chains and rings is called:
A. Isomerism B. Catenation C. Allotropy D. Hybridisation
28. Consider the following statements:
Statement I: Ethyne (acetylene) is a polar molecule because of its carbon–carbon triple bond.
Statement II: One carbon–carbon triple bond of an alkyne can add two molecules of bromine.
Which of the above is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
29. Consider the following statements:
Statement I: Primary alcohols on oxidation give aldehydes.
Statement II: Ketones are easily oxidised to carboxylic acids by mild oxidising agents.
Which of the above is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
30. Consider the statements. I: An acyclic hydrocarbon with one C≡C bond and no other multiple bonds has formula CₙH₂ₙ₋₂. II: Polythene can form by addition polymerisation of ethene. Which statement(s) is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
31. Which of the following compounds does NOT contain a carbonyl (C=O) group?
A. Acetone B. Ethanol C. Formaldehyde D. Acetaldehyde
32. Which of the following hydrocarbons is NOT unsaturated?
A. Ethene B. Ethyne C. Propene D. Ethane
33. Which covalent linkage joins adjacent nucleotide units along the sugar–phosphate backbone of DNA?
A. Glycosidic bond B. Peptide bond C. Phosphodiester bond D. Ester bond between fatty acids and glycerol
34. On hydrolysis, sucrose (table sugar) gives which pair of monosaccharides?
A. Glucose and galactose B. Glucose and glucose C. Glucose and fructose D. Fructose and galactose
35. Lactose, the sugar present in milk, is a disaccharide made up of which two monosaccharide units?
A. Glucose and fructose B. Glucose and galactose C. Two glucose units D. Fructose and galactose
36. Nylon 6,6, made from a diamine and a dicarboxylic acid, belongs to which polymer class?
A. Polyester B. Polyolefin C. Polyamide D. Polycarbonate
37. Bakelite is a thermosetting resin historically formed by reacting phenol with which compound?
A. Formaldehyde B. Ethene C. Tetrafluoroethene D. Propene
38. Match each biological material with its constituent building blocks. List I: (a) protein; (b) nucleic acid; (c) starch; (d) fat (triglyceride). List II: (i) glucose; (ii) nucleotide; (iii) glycerol and fatty acids; (iv) amino acid.
A. a-ii, b-iv, c-i, d-iii B. a-iv, b-ii, c-iii, d-i C. a-iv, b-i, c-ii, d-iii D. a-iv, b-ii, c-i, d-iii
39. Match List-I (polymer) with List-II (monomer).
List-I: (a) Polythene (b) PVC (c) Teflon (d) Polypropylene
List-II: (i) Tetrafluoroethene (ii) Propene (iii) Ethene (iv) Vinyl chloride
A. a-iv, b-iii, c-i, d-ii B. a-iii, b-iv, c-ii, d-i C. a-ii, b-iv, c-i, d-iii D. a-iii, b-iv, c-i, d-ii
40. Match the plastic recycling (resin identification) code in List-I with the polymer in List-II.
List-I: (a) 1 (b) 2 (c) 3 (d) 6
List-II: (i) High-density polyethylene (HDPE) (ii) Polyethylene terephthalate (PET) (iii) Polystyrene (PS) (iv) Polyvinyl chloride (PVC)
A. a-ii, b-i, c-iv, d-iii B. a-i, b-ii, c-iv, d-iii C. a-ii, b-i, c-iii, d-iv D. a-ii, b-iv, c-i, d-iii
41. Humans cannot digest cellulose, although it is made of glucose, because:
A. Humans lack the enzyme cellulase to break its β-glycosidic bonds B. Cellulose contains nitrogen C. Cellulose is a synthetic polymer D. Cellulose dissolves in stomach acid
42. Chitin, which forms the exoskeleton of insects and crustaceans, is a polymer of:
A. Glucose, as in cellulose B. N-acetylglucosamine C. Amino acids D. Nucleotides
43. Which polysaccharide is the main short-term carbohydrate storage material in many animal tissues?
A. Cellulose B. Glycogen C. Chitin D. Pectin
Answers and explanations
1. D. An oxy-acetylene torch burns ethyne (acetylene, HC≡CH) with oxygen to produce a very hot flame used in welding and cutting. The exact flame temperature depends on mixture and operating conditions; identifying the C₂H₂ fuel is the exam fact. Ethene is an alkene, whereas ethyne is the two-carbon alkyne.
2. C. Methanol was originally obtained by destructive distillation of wood, hence 'wood alcohol'. It is highly toxic — it can cause blindness and death, as in spurious-liquor tragedies. Ethanol is grain alcohol, the alcohol in beverages.
3. B. Friedrich Wöhler reported the conversion of ammonium cyanate to urea in 1828. The result challenged the strict idea that a living organism was required to make an organic molecule. The history of vitalism is more nuanced than a single experiment instantly ending every such belief.
4. C. For an acyclic hydrocarbon containing one C≡C bond, H = 2n − 2. Thus C₂H₂, C₃H₄ and C₄H₆ fit, but C₄H₈ does not. The CₙH₂ₙ₋₂ relation can describe other structures as well, so the specified open-chain one-triple-bond context matters.
5. C. Teflon is polytetrafluoroethene, the addition polymer of tetrafluoroethene (CF₂=CF₂). Its strong C–F bonds make it slippery, chemically inert and heat-resistant, ideal for non-stick pans. PVC is used for pipes and polystyrene for foam cups.
6. D. Polypropylene is formed when many propene (CH₃–CH=CH₂) molecules add together through their double bonds. Ethene gives polyethylene, vinyl chloride gives PVC and styrene gives polystyrene.
7. A. Charcoal, coke, lamp black and carbon black are amorphous forms of carbon without a regular crystal lattice. Diamond and graphite are crystalline, and fullerenes like C₆₀ are molecular cage-shaped forms.
8. B. The usual inference for a simple acyclic hydrocarbon is a C=C or C≡C bond, across which bromine can add. An alkane ordinarily does not decolourise bromine water rapidly in the dark. The test is not universally specific across all organic compounds because some other functional groups can also react with bromine.
9. B. Removing H and OH as water from neighbouring atoms of ethanol forms CH₂=CH₂, ethene. Its molecular formula is C₂H₄. Ethane is saturated C₂H₆, ethyne is C₂H₂, and methane has only one carbon.
10. D. In graphite each carbon is bonded to three others in flat hexagonal layers; the fourth electron is delocalised, making graphite a conductor. In diamond each carbon is bonded to four others in a rigid 3-D tetrahedral network.
11. C. Formalin is an aqueous formaldehyde (methanal) solution, commonly around 37% formaldehyde by mass; formulations can differ and may include stabiliser. It is used for specimen preservation. Acetone is a ketone, ethanal another aldehyde, and formic acid a carboxylic acid.
12. A. Amino acids join by condensation: –COOH of one and –NH₂ of the next lose H₂O to form a –CO–NH– peptide bond, building polypeptides and proteins. Glycosidic bonds link sugars, phosphodiester bonds link nucleotides and ester bonds join acids to alcohols (as in fats).
13. C. Buckminsterfullerene is C₆₀: 60 carbon atoms arranged in 12 pentagons and 20 hexagons like a football ('buckyball'). Its discoverers Kroto, Curl and Smalley received the 1996 Nobel Prize in Chemistry.
14. B. C₄H₁₀ has two structural (chain) isomers: n-butane CH₃CH₂CH₂CH₃ and isobutane (2-methylpropane) CH₃CH(CH₃)CH₃. Pentane (C₅H₁₂) is the first alkane with three isomers.
15. C. Glycerol is propane-1,2,3-triol, HO–CH₂–CH(OH)–CH₂–OH. “Triol” marks three alcohol groups. Propan-1-ol and propan-2-ol have only one –OH, while ethane-1,2-diol has two carbons and two –OH groups.
16. A. Alkenes have the general formula CₙH₂ₙ; for n = 5 this gives C₅H₁₀ (pentene). An alkane with 5 carbons would be C₅H₁₂ (CₙH₂ₙ₊₂) and an alkyne C₅H₈ (CₙH₂ₙ₋₂).
17. C. Aniline is phenylamine, an aromatic amine with an –NH₂ group on a benzene ring; it is weakly basic. It is prepared by reducing nitrobenzene, which carries the –NO₂ group, a common confusion.
18. B. The parent chain has 5 carbons (pentane). Numbering from either end gives substituent locants {2, 4}; in such a tie the substituent cited first alphabetically (bromo) gets the lower number. Prefixes are also written alphabetically, so the name is 2-bromo-4-methylpentane.
19. D. Ethene (ethylene, C₂H₄) is a gaseous plant hormone that promotes ripening of many fruits. Methane, ethane and butane do not have the same established plant-hormone role. Avoid the absolute claim that no other gaseous signalling compound exists in plant biology.
20. A. Propanal and propanone (acetone) share the formula C₃H₆O but one is an aldehyde and the other a ketone, so they are functional isomers. Propan-1-ol is C₃H₈O, propanoic acid C₃H₆O₂ and propane C₃H₈ — different molecular formulae, hence not isomers.
21. D. Geometrical isomerism needs restricted rotation about C=C and two different groups on each doubly bonded carbon. In but-2-ene (CH₃CH=CHCH₃) each carbon carries H and CH₃, giving cis and trans forms; but-1-ene, propene and ethene have a CH₂= end.
22. D. A carboxylic acid and an alcohol can react to form an ester and water, often with acid catalysis. For example, ethanoic acid and ethanol can produce ethyl ethanoate and water. Many small esters have fruity odours, but an odour cannot define the entire class.
23. B. A hydrogen atom of methane is replaced by a chlorine atom, so this is a substitution reaction, typical of saturated alkanes. Addition reactions occur across C=C or C≡C bonds, which methane does not have.
24. C. Hydrogen adds across the C=C bond of ethene: CH₂=CH₂ + H₂ → CH₃–CH₃, ethane. A suitable catalyst such as nickel may be used. This is hydrogenation, not oxidation or hydration.
25. A. Both have the same formula C₃H₈O, the same carbon chain and the same –OH group; only the position of –OH differs (carbon 1 vs carbon 2), which is position isomerism. Chain isomers differ in skeleton; functional isomers differ in group.
26. A. Saponification: fat + NaOH → sodium salt of fatty acid (soap) + glycerol. Esterification is the reverse type of reaction forming esters, hydrogenation converts oils to vanaspati ghee, and fermentation is microbial conversion of sugars.
27. B. Catenation is self-linking of atoms of the same element; carbon shows it to the greatest extent because C–C bonds are strong. Isomerism is same formula–different structure, and allotropy is different physical forms of one element.
28. B. Statement I is wrong: ethyne is linear and symmetric (H–C≡C–H), so its dipole moment is zero; alkynes, like other hydrocarbons, are essentially non-polar. Statement II is correct: HC≡CH + Br₂ → CHBr=CHBr, then + Br₂ → CHBr₂–CHBr₂.
29. A. Only I is correct. Primary alcohols oxidise to aldehydes (e.g. ethanol → ethanal) and further to acids. Ketones resist oxidation because the C=O carbon carries no hydrogen; this is why mild oxidants (Tollens', Fehling's) distinguish aldehydes from ketones.
30. C. Both are correct with Statement I’s specified structural scope. Ethyne and propyne fit CₙH₂ₙ₋₂. Ethene molecules add across their double bonds to form repeating –CH₂–CH₂– units without elimination of a small molecule, producing polyethene (polythene).
31. B. Ethanol (CH₃CH₂OH) is an alcohol with a hydroxyl group and no C=O. Acetone (CH₃COCH₃) is a ketone, while formaldehyde (HCHO) and acetaldehyde (CH₃CHO) are aldehydes — all carbonyl compounds.
32. D. Ethane (C₂H₆) has only C–C single bonds, so it is saturated (an alkane). Ethene and propene have a C=C double bond and ethyne has a C≡C triple bond, making all three unsaturated.
33. C. A phosphodiester linkage connects the sugar of one nucleotide to the phosphate associated with the next, forming the DNA backbone. Peptide bonds join amino acids in proteins, while glycosidic bonds link sugar units in carbohydrates.
34. C. Sucrose (C₁₂H₂₂O₁₁) = glucose + fructose. Lactose (milk sugar) gives glucose + galactose, and maltose (malt sugar) gives two glucose units — these are the usual distractors.
35. B. Lactose (milk sugar) = glucose + galactose. Sucrose (table sugar) = glucose + fructose, and maltose (malt sugar) = glucose + glucose. People lacking the enzyme lactase cannot digest lactose properly.
36. C. Nylon 6,6 is a polyamide. Its repeating units contain amide (–CO–NH–) links formed by condensation of a diamine and a dicarboxylic acid. The name is specific because other nylons can be prepared from different starting materials. PET is a polyester.
37. A. Bakelite is a phenol–formaldehyde resin. Condensation and cross-linking make a hard thermoset that cannot simply be remelted like a thermoplastic. Ethene forms polyethylene, tetrafluoroethene forms PTFE, and propene forms polypropylene.
38. D. Proteins contain amino-acid residues; nucleic acids contain nucleotide residues; starch contains glucose units; and a triglyceride contains glycerol esterified with three fatty acids. Thus a-iv, b-ii, c-i, d-iii. A triglyceride is not a repeating-chain polymer in the same sense as the first three, so “building blocks” is the appropriate umbrella term.
39. D. Polythene from ethene, PVC from vinyl chloride (CH₂=CHCl), Teflon (PTFE) from tetrafluoroethene (CF₂=CF₂) and polypropylene from propene. All four are addition polymers.
40. A. The resin identification codes are 1 PET, 2 HDPE, 3 PVC and 6 PS; therefore a-ii, b-i, c-iv, d-iii. Codes identify the resin type and do not by themselves guarantee local recyclability, which depends on collection and processing facilities.
41. A. Cellulose glucose units are joined by β-1,4-glycosidic links, which need cellulase; humans lack it, so cellulose passes as dietary fibre. Ruminants digest it with the help of gut microbes that produce cellulase.
42. B. Chitin resembles cellulose, but in each unit the –OH at carbon-2 of glucose is replaced by an acetamido group (–NHCOCH₃), so its monomer is N-acetylglucosamine. It is a polysaccharide, not a protein (amino acids) or nucleic acid (nucleotides).
43. B. Glycogen is the branched storage polysaccharide found especially in liver and muscle. Cellulose provides plant-cell-wall structure, chitin helps build arthropod exoskeletons and fungal walls, and pectin is a plant-cell-wall polysaccharide.