1. A compound is called a hydrocarbon only when its composition is limited to which elements?
ⓐ. Carbon and hydrogen only
ⓑ. Carbon and oxygen only
ⓒ. Carbon, hydrogen, and oxygen only
ⓓ. Carbon, hydrogen, and nitrogen only
Correct Answer: Carbon and hydrogen only
Explanation: A hydrocarbon is an organic compound made only of carbon and hydrogen atoms. The word itself points to this composition: “hydro” for hydrogen and “carbon” for carbon. If oxygen, nitrogen, chlorine, or any other element is also present, the compound is no longer classified as a hydrocarbon. For example, \( \mathrm{CH_4} \) is a hydrocarbon, but \( \mathrm{CH_3OH} \) is not because it contains oxygen. This composition rule is the first boundary for the whole hydrocarbon family.
2. Among the following substances, the hydrocarbon is the one represented by
ⓐ. \( \mathrm{CH_3OH} \)
ⓑ. \( \mathrm{C_2H_6} \)
ⓒ. \( \mathrm{CCl_4} \)
ⓓ. \( \mathrm{CO_2} \)
Correct Answer: \( \mathrm{C_2H_6} \)
Explanation: \( \mathrm{C_2H_6} \) contains only carbon and hydrogen, so it satisfies the definition of a hydrocarbon. \( \mathrm{CH_3OH} \) has oxygen, \( \mathrm{CCl_4} \) has chlorine, and \( \mathrm{CO_2} \) has oxygen, so they do not belong to hydrocarbons. A compound may be carbon-containing without being a hydrocarbon. The decisive check is not whether carbon is present, but whether only carbon and hydrogen are present. This avoids confusing hydrocarbons with the wider group of organic or carbon compounds.
3. Methane, \( \mathrm{CH_4} \), is a hydrocarbon because
ⓐ. it contains one carbon atom
ⓑ. it contains four hydrogen atoms
ⓒ. it is a gas at ordinary conditions
ⓓ. it contains only carbon and hydrogen atoms
Correct Answer: it contains only carbon and hydrogen atoms
Explanation: Methane has the formula \( \mathrm{CH_4} \), so its atoms are only carbon and hydrogen. Having one carbon atom alone does not define a hydrocarbon, because many one-carbon compounds contain oxygen, chlorine, or other elements. Having hydrogen alone is also not enough, since many non-hydrocarbons contain hydrogen. Physical state is not part of the definition; some hydrocarbons are gases, some are liquids, and some are solids. The composition of the molecule, not its state, decides this classification.
4. The role of carbon tetravalency in hydrocarbons is best shown by carbon forming
ⓐ. four covalent bonds in stable neutral molecules
ⓑ. only one bond with hydrogen
ⓒ. two ionic bonds in every hydrocarbon
ⓓ. only metallic bonds with other carbon atoms
Correct Answer: four covalent bonds in stable neutral molecules
Explanation: Carbon is tetravalent, so it commonly forms four covalent bonds in stable organic molecules. In \( \mathrm{CH_4} \), carbon forms four \( \mathrm{C-H} \) covalent bonds. In larger hydrocarbons, carbon can bond to hydrogen and also to other carbon atoms while still completing four bonds. Hydrocarbon structures are built mainly through covalent bonding, not ionic or metallic bonding. This tetravalency is why carbon can make chains, branches, and rings.
5. The expression “carbon skeleton” in a hydrocarbon mainly refers to
ⓐ. the hydrogen-count pattern of the molecule
ⓑ. the carbon-chain or ring framework
ⓒ. the oxygen-bearing part of the compound
ⓓ. the colour change seen during combustion
Correct Answer: the carbon-chain or ring framework
Explanation: The carbon skeleton is the framework made by carbon atoms bonded to one another. It may be a straight chain, branched chain, ring, or part of a more complex structure. Hydrogen atoms are attached to this framework to satisfy the valency of carbon. The carbon skeleton helps distinguish compounds that may have similar compositions but different structures. This idea becomes important when comparing straight-chain, branched, cyclic, and aromatic hydrocarbons.
6. A saturated hydrocarbon is identified by the presence of
ⓐ. at least one carbon-carbon double bond
ⓑ. at least one carbon-carbon triple bond
ⓒ. only carbon-carbon single bonds
ⓓ. a benzene-like aromatic ring
Correct Answer: only carbon-carbon single bonds
Explanation: A saturated hydrocarbon contains only single bonds between carbon atoms. Such a molecule has no \( \mathrm{C=C} \) or \( \mathrm{C\equiv C} \) multiple bond in its carbon skeleton. Alkanes are the common open-chain saturated hydrocarbons. The word saturated means the carbon framework has the maximum number of hydrogen atoms possible for that skeleton. A multiple bond reduces the number of hydrogens and makes the hydrocarbon unsaturated.
7. A hydrocarbon molecule containing a \( \mathrm{C=C} \) or \( \mathrm{C\equiv C} \) bond is described as
ⓐ. unsaturated
ⓑ. saturated
ⓒ. inorganic
ⓓ. ionic
Correct Answer: unsaturated
Explanation: Unsaturated hydrocarbons contain at least one carbon-carbon multiple bond such as \( \mathrm{C=C} \) or \( \mathrm{C\equiv C} \). Alkenes contain \( \mathrm{C=C} \), while alkynes contain \( \mathrm{C\equiv C} \). These multiple bonds mean the molecule has fewer hydrogen atoms than the corresponding fully single-bonded chain. The term does not mean that the compound has a non-carbon element; it still contains only carbon and hydrogen. The bonding pattern inside the carbon skeleton decides saturation or unsaturation.
8. Match each formula with the most suitable first-level description.
| Formula | Description |
|---|
| P. \( \mathrm{CH_4} \) | 1. Hydrocarbon with a triple bond |
| Q. \( \mathrm{C_2H_4} \) | 2. Simplest hydrocarbon with only single \( \mathrm{C-H} \) bonds |
| R. \( \mathrm{C_2H_2} \) | 3. Hydrocarbon with a double bond |
ⓐ. P-1, Q-2, R-3
ⓑ. P-2, Q-3, R-1
ⓒ. P-3, Q-1, R-2
ⓓ. P-2, Q-1, R-3
Correct Answer: P-2, Q-3, R-1
Explanation: \( \mathrm{CH_4} \) is methane, the simplest hydrocarbon, and its carbon is bonded only to hydrogen atoms. \( \mathrm{C_2H_4} \) is ethene, which contains a \( \mathrm{C=C} \) double bond. \( \mathrm{C_2H_2} \) is ethyne, which contains a \( \mathrm{C\equiv C} \) triple bond. The comparison is based on bonding pattern, not only on the number of carbon atoms. The formulas give the entry-level examples for alkane, alkene, and alkyne families.
9. A molecule written as \( \mathrm{C_6H_6} \) contains only carbon and hydrogen. At this stage, the safest classification from composition alone is that it is
ⓐ. definitely an alkane
ⓑ. definitely an alkyne
ⓒ. a hydrocarbon
ⓓ. an alcohol
Correct Answer: a hydrocarbon
Explanation: The formula \( \mathrm{C_6H_6} \) contains only carbon and hydrogen, so it is certainly a hydrocarbon. Composition alone does not prove that it is an alkane or alkyne, because those names depend on bonding and structure. Benzene has the formula \( \mathrm{C_6H_6} \), but it behaves as an aromatic hydrocarbon rather than an ordinary alkene or alkyne. It cannot be an alcohol because alcohols contain oxygen in the \( \mathrm{-OH} \) group. Formula reading must be combined with structural information for deeper classification.
10. Consider the following statements about hydrocarbon bonding.
I. Hydrocarbons contain covalent bonds.
II. Carbon atoms in hydrocarbons may join to other carbon atoms.
III. A hydrocarbon must contain oxygen if it is organic.
ⓐ. I only
ⓑ. II and III only
ⓒ. I, II, and III
ⓓ. I and II only
Correct Answer: I and II only
Explanation: Hydrocarbons are covalent compounds made from carbon and hydrogen atoms. Carbon can bond to other carbon atoms, allowing chains, branches, rings, and aromatic frameworks to form. Statement III is false because a hydrocarbon, by definition, does not contain oxygen. Organic chemistry includes many oxygen-containing compounds, but hydrocarbons form the simpler carbon-hydrogen-only class. The main idea is that all hydrocarbons are organic compounds, but not all organic compounds are hydrocarbons.
11. Open-chain hydrocarbons such as straight-chain alkanes, alkenes, and alkynes are usually grouped under
ⓐ. aromatic hydrocarbons
ⓑ. inorganic hydrides
ⓒ. metal carbides
ⓓ. aliphatic hydrocarbons
Correct Answer: aliphatic hydrocarbons
Explanation: Aliphatic hydrocarbons include open-chain hydrocarbons, whether their chains are straight or branched. This broad group includes alkanes, alkenes, and alkynes when they are not aromatic. Aromatic hydrocarbons contain benzene-like ring systems with special stability and reaction behaviour. Metal carbides and inorganic hydrides are not ordinary hydrocarbon classes. The first classification separates chain-type hydrocarbons from cyclic and aromatic systems.
12. A hydrocarbon has carbon atoms joined in a ring, but the ring is not benzene-like. The most suitable broad description is
ⓐ. alicyclic hydrocarbon
ⓑ. aromatic hydrocarbon
ⓒ. alcohol
ⓓ. alkyl halide
Correct Answer: alicyclic hydrocarbon
Explanation: An alicyclic hydrocarbon has carbon atoms arranged in a ring but does not have the special aromatic benzene-like system. The word combines “ali” from aliphatic behaviour and “cyclic” for ring structure. Such compounds are still hydrocarbons if they contain only carbon and hydrogen. Aromatic hydrocarbons are a separate category because their rings have special delocalised bonding and unusual stability. A ring alone does not automatically make a compound aromatic.
13. The table gives four descriptions. Select the row that gives the best broad classification.
| Row | Description | Broad class |
|---|
| P | Open-chain hydrocarbon | Aliphatic |
| Q | Non-aromatic ring hydrocarbon | Alicyclic |
| R | Benzene-ring hydrocarbon | Aromatic |
| S | Hydrocarbon containing oxygen | Saturated |
ⓐ. P and S only
ⓑ. Q and S only
ⓒ. R and S only
ⓓ. P, Q, and R only
Correct Answer: P, Q, and R only
Explanation: Open-chain hydrocarbons are classified as aliphatic, so row P is suitable. Non-aromatic ring hydrocarbons are alicyclic, so row Q is also suitable. Benzene-ring hydrocarbons belong to aromatic hydrocarbons, making row R suitable. Row S is not acceptable because a compound containing oxygen is not a hydrocarbon at all. Saturated describes the absence of carbon-carbon multiple bonds, not the presence of oxygen.
14. A hydrocarbon is described as open-chain and contains only \( \mathrm{C-C} \) single bonds. Its most likely broad reaction family at the introductory level is
ⓐ. alkane-type saturated hydrocarbon
ⓑ. alkene-type unsaturated hydrocarbon
ⓒ. alkyne-type unsaturated hydrocarbon
ⓓ. aromatic hydrocarbon
Correct Answer: alkane-type saturated hydrocarbon
Explanation: An open-chain hydrocarbon with only \( \mathrm{C-C} \) single bonds belongs to the alkane-type saturated family. Alkenes require a \( \mathrm{C=C} \) bond, while alkynes require a \( \mathrm{C\equiv C} \) bond. Aromatic hydrocarbons have benzene-like ring systems and special stability. The words “open-chain” and “only single bonds” together point toward the alkane family. This early classification helps connect structure with later reaction behaviour.
15. A learner claims, “Every hydrocarbon with fewer hydrogen atoms than the corresponding alkane must behave like a simple alkene.” The best response is that
ⓐ. the claim is always true because fewer hydrogen atoms always mean \( \mathrm{C=C} \)
ⓑ. the claim is false because unsaturation or ring structure can arise in different ways
ⓒ. the claim is true only for compounds containing oxygen
ⓓ. the claim is false because hydrocarbons cannot have rings
Correct Answer: the claim is false because unsaturation or ring structure can arise in different ways
Explanation: A lower hydrogen count may come from a double bond, a triple bond, a ring, or an aromatic system. An alkene has a \( \mathrm{C=C} \) bond, but that is not the only reason a hydrocarbon may contain fewer hydrogen atoms. Benzene, \( \mathrm{C_6H_6} \), is highly unsaturated by formula but behaves as an aromatic hydrocarbon, not as a simple alkene. Cyclic hydrocarbons also show formula patterns that can resemble open-chain unsaturated compounds. Formula clues are useful, but structure is needed before assigning a precise family.
16. A simple orientation of hydrocarbon reactions pairs the family and common reaction tendency as
ⓐ. alkanes: addition; alkenes: substitution; benzene: ordinary addition
ⓑ. alkanes: polymerisation only; alkynes: combustion only; benzene: no reaction
ⓒ. alkanes: substitution; alkenes: addition; benzene: mainly substitution
ⓓ. alkenes: no reaction; alkynes: no reaction; alkanes: aromatic substitution
Correct Answer: alkanes: substitution; alkenes: addition; benzene: mainly substitution
Explanation: Alkanes are saturated, so they generally undergo substitution reactions rather than addition under ordinary hydrocarbon chemistry conditions. Alkenes contain a \( \mathrm{C=C} \) bond, and the \( \pi \)-bond makes addition across the double bond common. Benzene contains a special aromatic ring, so it usually undergoes substitution in a way that preserves aromatic stability. This is only an introductory orientation, because exact products depend on reagents and conditions. The reaction type follows from the bonding pattern and stability of the hydrocarbon family.
17. A hydrocarbon sample is described as a chain compound with one carbon-carbon triple bond. Its first broad placement should be
ⓐ. saturated aliphatic hydrocarbon
ⓑ. unsaturated aliphatic hydrocarbon
ⓒ. aromatic hydrocarbon
ⓓ. alicyclic saturated hydrocarbon
Correct Answer: unsaturated aliphatic hydrocarbon
Explanation: A chain hydrocarbon belongs to the aliphatic group when it is not a benzene-like aromatic compound. The presence of a \( \mathrm{C\equiv C} \) bond makes it unsaturated, because the carbon skeleton does not contain the maximum possible number of hydrogen atoms. Alkynes are the common hydrocarbon family with a carbon-carbon triple bond. It is not alicyclic because no ring is described. It is not aromatic because a triple bond in a chain does not create the special benzene-like aromatic system.
18. In the classification map below, the missing entry at \(X\) is most suitable for which hydrocarbon type?
| Hydrocarbon feature | Broad class |
|---|
| Open-chain, only \( \mathrm{C-C} \) single bonds | Alkane |
| Open-chain, at least one \( \mathrm{C=C} \) | \(X\) |
| Open-chain, at least one \( \mathrm{C\equiv C} \) | Alkyne |
ⓐ. Cycloalkane
ⓑ. Arene
ⓒ. Alkyl halide
ⓓ. Alkene
Correct Answer: Alkene
Explanation: An open-chain hydrocarbon with at least one \( \mathrm{C=C} \) bond is classified as an alkene. The double bond makes it unsaturated and gives it addition-type reaction behaviour in many cases. A cycloalkane is a saturated ring hydrocarbon and does not require a double bond. An arene contains an aromatic ring, while an alkyl halide is not a hydrocarbon because it contains a halogen. The missing term is decided by the carbon-carbon bonding feature, not merely by the molecular formula.
19. Assertion: A hydrocarbon ring is not automatically aromatic.
Reason: Aromatic hydrocarbons require a special benzene-like electronic arrangement, not just cyclic carbon atoms.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Both Assertion and Reason are true, and Reason explains Assertion
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: A hydrocarbon can be cyclic without being aromatic. Alicyclic hydrocarbons contain carbon rings but do not have the special aromatic stabilisation associated with benzene-like systems. Aromatic hydrocarbons are treated separately because their bonding and reaction behaviour are unusual compared with ordinary alkenes or cycloalkanes. The Reason explains why the presence of a ring alone is insufficient for aromatic classification. The distinction prevents the mistake of calling every carbon ring an aromatic compound.
20. A compound has the formula \( \mathrm{C_3H_6} \), and its structure is not given. The most careful conclusion at this point is that it
ⓐ. open-chain alkene only
ⓑ. cyclic alkane only
ⓒ. either alkene or cycloalkane
ⓓ. oxygen-containing compound
Correct Answer: either alkene or cycloalkane
Explanation: \( \mathrm{C_3H_6} \) contains only carbon and hydrogen, so it is certainly a hydrocarbon. The same molecular formula can represent propene, an open-chain alkene, or cyclopropane, a cyclic saturated hydrocarbon. Formula alone does not always reveal whether the lower hydrogen count comes from a double bond or a ring. This is why structural information is needed before assigning a precise class. The safest conclusion keeps both possibilities open.
