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Q1. Replacement of hydrogen does not change:
In organic compounds, when a hydrogen atom is replaced by another atom or group (like Cl, OH, NH₂), the carbon atom still forms four bonds. The valency of carbon remains 4 always. However, atomic mass changes (if a heavier atom replaces H), reactivity changes (different functional groups behave differently), and structure changes (new compound is formed). So only valency of carbon remains unchanged.
Q2. Carbon compounds arranged in straight chains, branched chains or rings can be:
Carbon chains (straight, branched) and rings can be either saturated (all single bonds, e.g., cyclohexane C₆H₁₂) or unsaturated (containing double or triple bonds, e.g., cyclohexene C₆H₁₀). So both types exist. They are not ionic because carbon compounds are covalent. The arrangement (chain or ring) does not force saturation or unsaturation.
Q3. Hydrocarbons may exist in:
Hydrocarbons (compounds of only C and H) show great structural diversity. They can exist as straight chains (e.g., n-butane), branched chains (e.g., isobutane), and rings (e.g., cyclopentane, cyclohexane, benzene). So all three arrangements are possible. This is due to carbon’s catenation property.
Q4. Number of hydrogen atoms bonded to each carbon atom in ethane is:
Ethane has the molecular formula C₂H₆ and structure H₃C–CH₃. Each carbon atom is bonded to three hydrogen atoms and one other carbon atom. So each carbon has three hydrogen atoms attached. In methane (CH₄), each carbon has four H. In ethene (C₂H₄), each carbon has two H. In ethyne (C₂H₂), each carbon has one H.
Q5. Carbon compounds containing oxygen, nitrogen or halogens show:
When carbon compounds contain elements like oxygen (alcohols, acids), nitrogen (amines), or halogens (chloroform), they acquire specific properties due to the functional group present. For example, -OH gives alcohol properties, -COOH gives acidic properties, -NH₂ gives basic properties. They do not become metallic, ionic, or all have the same properties. Each functional group gives unique characteristics.
Q6. Which two characteristic features of carbon give rise to a large number of compounds?
The two most important properties of carbon responsible for millions of compounds are: (1) Tetravalency — carbon can form four covalent bonds; (2) Catenation — carbon atoms can bond with other carbon atoms to form long chains, branches, and rings. Atomic size and mass are not unique (silicon is similar but shows limited catenation). Conductivity, reactivity, and acidity are not the primary reasons.
Q7. Carbon compounds outnumber all other element compounds because:
The vast number of carbon compounds (over 3 million) is because carbon can form long, stable chains and rings through catenation. Carbon is not metallic, not very abundant in the crust (only 0.02%), and not radioactive (except trace C-14). The stability of carbon-carbon bonds allows countless combinations.
Q8. Ethyne has the molecular formula:
Ethyne (also called acetylene) is an alkyne with a triple bond between two carbon atoms. Its molecular formula is C₂H₂. Each carbon is bonded to one hydrogen. C₂H₆ is ethane (alkane), CH₄ is methane, C₂H₄ is ethene (alkene). Ethyne is used in welding torches because it burns very hot.
Q9. Straight chain and branched chain compounds are examples of:
Compounds with the same molecular formula but different arrangement of atoms are called structural isomers. For example, butane (C₄H₁₀) has two isomers: n-butane (straight chain) and isobutane (branched chain). Allotropes are different forms of the same element (diamond and graphite). Isotopes are atoms of same element with different neutrons. Salts are ionic compounds.
Q10. Carbon–carbon bond is:
The bond between two carbon atoms is a strong covalent bond. Because carbon atoms are very small, the shared electrons are held tightly between the two nuclei. The bond energy of a C–C single bond is about 348 kJ/mol. This strength allows carbon to form stable long chains. The bond is not weak, temporary, or ionic.
Q11. Carbon compounds studied under organic chemistry exclude:
Organic chemistry traditionally studies carbon compounds except for certain simple ones. Carbonates (like CaCO₃), hydrogencarbonates (like NaHCO₃), and oxides of carbon (CO, CO₂) are considered inorganic compounds. So all three are excluded from organic chemistry. The answer “All of these” means all three options (B, C, D) are excluded.
Q12. Number of possible structures for butane is:
Butane (C₄H₁₀) has two structural isomers: (1) n-butane — straight chain with four carbons in a row; (2) isobutane (2-methylpropane) — branched chain with three carbons in a row and one carbon attached to the middle carbon. No other structures are possible for C₄H₁₀. So the number is two.
Q13. Saturated hydrocarbons are known as:
Saturated hydrocarbons are those in which all carbon-carbon bonds are single bonds. They are called alkanes (e.g., methane CH₄, ethane C₂H₆, propane C₃H₈). Alkenes have double bonds (unsaturated), alkynes have triple bonds (unsaturated), and arenes are aromatic hydrocarbons (like benzene, which is unsaturated).
Q14. The versatility of carbon is due to:
Carbon’s versatility (ability to form millions of compounds) comes from two properties: catenation (self-linking to form chains and rings) and tetravalency (ability to form four strong covalent bonds with many elements). Carbon is not metallic, does not have a particularly high melting point (it sublimes), and does not form ionic bonds typically.
Q15. Compounds containing double or triple bonds are called:
Unsaturated compounds are those that contain at least one double bond (C=C) or triple bond (C≡C) between carbon atoms. They are called unsaturated because they can add more atoms (like hydrogen) without removing anything. Alkanes are saturated (only single bonds). Organic salts and saturated compounds are different categories.
Q16. Organic compounds were initially believed to be formed only:
Before 1828, scientists believed in the “vital force theory” — that organic compounds could only be produced by living organisms (plants and animals) using a special “vital force.” They thought these compounds could not be made in laboratories. This belief was disproved by Friedrich Wöhler. Industries, atmosphere, and laboratories were not believed to produce organic compounds naturally.
Q17. Friedrich Wohler prepared urea from:
In 1828, Friedrich Wöhler accidentally synthesized urea (an organic compound found in urine) by heating ammonium cyanate (NH₄CNO), which was considered inorganic. This experiment disproved the vital force theory. He did not use sodium cyanate, ammonia alone, or ammonium chloride directly. This was a landmark discovery in organic chemistry.
Q18. Compounds in which all valencies are satisfied by single bonds are called:
Saturated compounds are those in which all carbon atoms use only single covalent bonds to satisfy their valency. No double or triple bonds are present. For example, alkanes like methane and ethane are saturated. Aromatic compounds (like benzene) have double bonds (unsaturated). Unsaturated compounds have double/triple bonds. Ionic compounds are completely different.
Q19. Unsaturated hydrocarbons with double bonds are called:
Unsaturated hydrocarbons are classified based on the type of multiple bond: Alkenes contain at least one carbon-carbon double bond (C=C), e.g., ethene C₂H₄. Alkynes contain triple bonds (C≡C), e.g., ethyne C₂H₂. Alkanes are saturated (only single bonds). Alcohols contain -OH group and are not just hydrocarbons.
Q20. Strong attraction of shared electrons is due to:
Carbon atoms are very small. When two carbon atoms share electrons, the shared pair is close to both nuclei. The positive charge of the nuclei (6 protons each) strongly attracts the negatively charged electrons. This strong attraction makes the covalent bond strong. A large nucleus would have more protons but also more shells, making the bond weaker. Weak nucleus and presence of ions are incorrect.
Q21. Cyclohexane has the molecular formula:
Cyclohexane is a cyclic (ring) saturated hydrocarbon. It has 6 carbon atoms arranged in a ring. Since it is saturated (all single bonds) and cyclic, the general formula is CₙH₂ₙ. For n=6, C₆H₁₂. C₆H₁₄ is hexane (straight chain alkane). C₆H₆ is benzene (aromatic). C₅H₁₂ is pentane.
Q22. Number of bonds between two carbon atoms in ethyne is:
Ethyne (C₂H₂, also called acetylene) has a triple bond between the two carbon atoms. A triple bond consists of three covalent bonds: one sigma bond and two pi bonds. So the number of bonds between the two carbon atoms is three. Ethene has a double bond (two bonds), ethane has a single bond (one bond). Four bonds would be a quadruple bond (extremely rare).
Q23. Alkanes differ from alkenes because alkanes:
Alkanes are saturated hydrocarbons — they contain only single covalent bonds (C–C and C–H). Alkenes are unsaturated hydrocarbons — they contain at least one double bond (C=C). Alkanes are not aromatic (benzene is aromatic). Alkanes do not contain triple bonds (alkynes do). Alkanes are saturated, not unsaturated. So the key difference is single bonds vs double bonds.
Q24. Carbon is called a friendly element because it forms bonds with:
Carbon is called “friendly” or “versatile” because it can form stable covalent bonds with many different elements — hydrogen, oxygen, nitrogen, halogens (chlorine, fluorine), sulphur, phosphorus, and many metals in organometallic compounds. It does not bond only with carbon, only hydrogen, or only oxygen. This wide bonding ability creates millions of compounds.
Q25. Molecular formula of butane is:
Butane is an alkane with 4 carbon atoms. The general formula for alkanes is CₙH₂ₙ₊₂. For n=4, C₄H₁₀. Butane has two isomers: n-butane and isobutane. C₂H₆ is ethane, C₃H₈ is propane, C₅H₁₂ is pentane. So C₄H₁₀ is correct for butane.
Q26. Ethane is a hydrocarbon with the formula:
Ethane is the second member of the alkane family (saturated hydrocarbons). Its molecular formula is C₂H₆, with structure H₃C–CH₃. CH₄ is methane, C₂H₄ is ethene (alkene with double bond), C₂H₂ is ethyne (alkyne with triple bond). Ethane is a colourless gas and a component of natural gas.
Q27. Carbon skeleton refers to:
The term “carbon skeleton” (or carbon backbone) refers to the arrangement of carbon atoms in an organic molecule — whether they are in straight chains, branched chains, or rings. Hydrogen atoms are attached to this skeleton but are not part of the skeleton itself. Molecular mass and ionic structure are not related to the term “carbon skeleton.”
Q28. Who disproved the vital force theory?
In 1828, Friedrich Wöhler (a German chemist) accidentally synthesized urea (an organic compound) from ammonium cyanate (an inorganic compound). This proved that organic compounds can be made in a laboratory without any “vital force” from living organisms. Lavoisier is father of modern chemistry, Dalton proposed atomic theory, Rutherford discovered nucleus.
Q29. Compounds with same molecular formula but different structures are called:
Structural isomers (or constitutional isomers) are compounds that have the same molecular formula but different arrangements of atoms (different structures). For example, butane (C₄H₁₀) has two structural isomers. Allotropes are different forms of the same element. Homologues are members of a homologous series. Isotopes are atoms with same protons but different neutrons.
Q30. First step in drawing structure of simple carbon compounds is:
When drawing the structure of a carbon compound, the first step is to identify and link the carbon atoms together (the carbon skeleton). After the carbon skeleton is ready, then hydrogen atoms and other atoms (oxygen, nitrogen, halogens) are attached to satisfy the valency of carbon (4 bonds per carbon). Drawing electron dots comes later if needed.
Q31. Molecular formula of ethene is:
Ethene (also called ethylene) is an alkene with a double bond between two carbon atoms. Its molecular formula is C₂H₄, with structure H₂C=CH₂. C₂H₂ is ethyne (alkyne), C₂H₆ is ethane (alkane), CH₄ is methane. Ethene is used to ripen fruits and make plastics like polyethylene.
Q32. Chains of carbon atoms may contain:
Carbon chains can be very long — from 1 carbon atom to hundreds or even thousands of carbon atoms (as in polymers like polyethylene, which has tens of thousands of carbons). Chains are not restricted to only even numbers, only three atoms, or only five atoms. They can contain any number, including tens, hundreds, or more.
Q33. The belief that a special force was required for synthesis of organic compounds was called:
The “vital force theory” was the belief that organic compounds could only be produced by living organisms because a special “vital force” was needed. Scientists thought it was impossible to synthesize organic compounds in laboratories. This theory was disproved by Friedrich Wöhler in 1828 when he made urea from ammonium cyanate.
Q34. Butane shows isomerism because:
Isomerism occurs when compounds have the same molecular formula but different structural arrangements. Butane (C₄H₁₀) has two isomers: n-butane (straight chain) and isobutane (branched chain). Butane is saturated (no double bonds), contains only C and H (no oxygen), and does not have double bonds. The reason for isomerism is same formula but different structures.
Q35. Bonds formed by larger atoms are generally:
Larger atoms have more electron shells, so the outer electrons are farther from the nucleus. When two large atoms form a covalent bond, the shared electrons are not held as tightly. This makes the bond weaker. For example, the I–I bond (iodine) is weaker than the Cl–Cl bond (chlorine) because iodine atoms are larger. Stronger bonds are formed by smaller atoms.
Q36. Unsaturated carbon compounds are:
Unsaturated compounds (those with double or triple bonds) are generally more reactive than saturated compounds. The double and triple bonds have pi electrons that are more exposed and can easily participate in addition reactions. For example, ethene (C₂H₄) readily adds bromine, while ethane (C₂H₆) does not. Unsaturated compounds are not inert, less reactive, or simply neutral — they actively participate in reactions.
Q37. Main reason for the vast number of carbon compounds is:
Carbon forms very strong covalent bonds with other carbon atoms (catenation) and with other elements. Because these bonds are strong, long chains and rings can be formed without breaking easily. This allows millions of stable carbon compounds. Carbon atoms are small, not large. Carbon has low density (graphite ~2.2 g/cm³). Weak bonding would not produce stable compounds.
Q38. Methane, ethane and propane contain respectively:
Methane (CH₄) has 1 carbon atom. Ethane (C₂H₆) has 2 carbon atoms. Propane (C₃H₈) has 3 carbon atoms. So the number of carbon atoms increases by one each time: 1, 2, 3. They do not have 1,2,3 hydrogen atoms (methane has 4 H). They do not have 3,4,5 or 2,3,4 carbon atoms — that would be incorrect ordering.
Q39. Carbon forms stable compounds mainly due to:
Carbon has a very small atomic size (atomic radius about 77 pm). Because of its small size, the nucleus holds the shared pair of electrons very tightly. This results in strong, stable covalent bonds. Carbon is non-metallic, not metallic. Its electronegativity (2.55) is moderate, not very high. Its atomic radius is small, not large.
Q40. Ethene contains which type of bond between carbon atoms?
Ethene (C₂H₄) has a double bond between the two carbon atoms (C=C). It is an alkene. Ethane has a single bond, ethyne has a triple bond. There is definitely a bond between carbon atoms in ethene, not “no bond”. So double bond is correct.
Q41. Unsaturated hydrocarbons with triple bonds are called:
Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond (C≡C). The simplest alkyne is ethyne (C₂H₂, also called acetylene). Alkenes have double bonds, alkanes have single bonds (saturated), and ethers contain an oxygen atom between two carbon atoms (R–O–R). So alkynes is the correct term.
Q42. Saturated carbon compounds are generally:
Saturated compounds (alkanes) are generally less reactive because they have only strong single bonds (C–C and C–H) and no exposed pi electrons. They do not easily undergo addition reactions. They are not ionic, not typically explosive (except under extreme conditions), and not highly reactive. For example, methane is quite stable and burns only when ignited.
Q43. Arrangement of carbon atoms in cyclohexane is:
The prefix “cyclo-” in cyclohexane indicates a ring structure. Cyclohexane has 6 carbon atoms arranged in a closed ring (cyclic structure). It is not random, not a straight chain (that would be hexane), and not a branched chain (that would be an isomer like methylcyclopentane or dimethylbutane). So ring is correct.
Q44. In ethane, carbon atoms are joined by:
Ethane (C₂H₆) is an alkane. The two carbon atoms are joined by a single covalent bond (C–C). Each carbon also forms three single bonds with three hydrogen atoms. Ethene has a double bond, ethyne has a triple bond. Ethane does not have ionic bonds (it is covalent). So single bond is correct.
Q45. Compounds containing only carbon and hydrogen are called:
Hydrocarbons are organic compounds that contain only carbon and hydrogen atoms. Examples include methane (CH₄), ethane (C₂H₆), ethene (C₂H₄), ethyne (C₂H₂), and benzene (C₆H₆). Alcohols contain oxygen (-OH), esters contain oxygen (COO), and salts are ionic compounds. So hydrocarbons is the correct term.
Q46. Benzene has the molecular formula:
Benzene is an aromatic hydrocarbon with 6 carbon atoms and 6 hydrogen atoms. Its molecular formula is C₆H₆. It has a ring structure with alternating double bonds (resonance). C₆H₁₂ is cyclohexane (saturated ring), C₆H₁₄ is hexane (straight chain alkane), C₅H₆ is not a common stable hydrocarbon. So C₆H₆ is correct.
Q47. Carbon forms compounds with all except:
Neon is a noble gas with a completely filled outer shell (2,8). It is chemically inert and does not form any compounds with carbon or any other element. Carbon readily forms compounds with oxygen (CO, CO₂), sulphur (CS₂, carbonyl sulphide), and nitrogen (cyanides, proteins, amines). So carbon cannot form compounds with neon.
Q48. In a hydrocarbon chain, hydrogen atoms can be replaced by:
In organic chemistry, hydrogen atoms in a hydrocarbon can be replaced by other atoms or groups (called functional groups) such as chlorine (-Cl), hydroxyl (-OH), amino (-NH₂), etc. These replacements maintain the valency of carbon (4). Noble gases do not react, metals only is incorrect (non-metals also replace H), neutrons are subatomic particles and cannot replace H atoms.
Q49. Alkenes and alkynes are classified as:
Alkenes (with double bonds) and alkynes (with triple bonds) are both unsaturated hydrocarbons. They are called unsaturated because they can add more atoms (like hydrogen) without removing anything. Saturated hydrocarbons are alkanes (only single bonds). Alkenes and alkynes are not metallic or ionic — they are covalent organic compounds.
Q50. Year in which urea was synthesised artificially is:
In 1828, Friedrich Wöhler synthesized urea (an organic compound) from ammonium cyanate (an inorganic compound). This was the first artificial synthesis of an organic compound from inorganic starting materials. It disproved the vital force theory. The year is 1828, not 1840, 1802, or 1815. This is a landmark date in the history of organic chemistry.