Carbon & Its Compounds
This page covers carbon's covalent bonding, catenation, allotropes, and the homologous series of hydrocarbons (alkanes-alkenes-alkynes), from the REET exam's point of view. REET repeatedly asks questions that require correctly identifying a compound's series from its general formula.
- 2 questions in REET 2024
- Class 6-8 level
- 10 min read
How this topic is asked in REET
REET 2024 asked two questions on this topic, and both were based on the general formula of the alkane-alkene-alkyne homologous series. The first question asked which of four given formulas is NOT a member of the alkene series (answer: C₂H₂, which is actually an alkyne). The second asked directly for an example of an alkene (answer: C₂H₄). This makes it clear that REET tests the ability to instantly apply the general formulas CₙH₂ₙ₊₂ / CₙH₂ₙ / CₙH₂ₙ₋₂ to any given formula and identify it correctly. Questions linked to carbon's allotropes, functional groups and practical topics like soap and detergents have also come up in previous years, so it pays to prepare the whole chapter solidly.
Covalent Bonding, Catenation and the Tetravalency of Carbon
A carbon atom has 4 electrons in its outermost shell. It can neither lose these 4 electrons to form a C⁴⁺ cation (this needs too much energy) nor gain 4 electrons to form a C⁴⁻ anion (pulling in 10 electrons is difficult for a nucleus with only 6 protons). So instead of forming an ionic bond, carbon shares its 4 valence electrons with other atoms to form a covalent bond — this is called tetravalency.
Carbon has a special property — catenation, that is, the ability of carbon atoms to form stable, long chains with each other (straight, branched or in a ring), a property not found to nearly the same degree in any other element. Together, tetravalency and catenation let carbon form an enormous number of compounds — the second most of any element after hydrogen — even though carbon makes up only a very small proportion (about 0.02%) of the Earth's crust.
Allotropes of Carbon
| Allotrope | Structure and Properties |
|---|---|
| Diamond | Each carbon atom is joined to 4 other carbon atoms in a rigid three-dimensional structure; the hardest substance in nature; a poor conductor of electricity |
| Graphite | Carbon atoms are joined in flat, hexagonal layers; the layers can slide over each other (soft, useful as a lubricant); a good conductor of electricity because of a free electron |
| Fullerene (C₆₀) | 60 carbon atoms joined in a football-like spherical structure — buckminsterfullerene is its famous example |
Homologous Series: Alkanes, Alkenes, Alkynes
| Series | General Formula | Bond | Example |
|---|---|---|---|
| Alkane | CₙH₂ₙ₊₂ | Saturated, only single bonds | Methane CH₄, ethane C₂H₆ |
| Alkene | CₙH₂ₙ | Unsaturated, one double bond (C=C) | Ethene C₂H₄, propene C₃H₆ |
| Alkyne | CₙH₂ₙ₋₂ | Unsaturated, one triple bond (C≡C) | Ethyne C₂H₂, propyne C₃H₄ |
Functional Groups
- -OH (alcohol)
- Example: ethanol C₂H₅OH (present in alcoholic drinks)
- -CHO (aldehyde)
- A functional group found at the end of a carbon chain
- C=O (ketone)
- A functional group found in the middle of a carbon chain
- -COOH (carboxylic acid)
- Example: ethanoic acid/acetic acid CH₃COOH (the main component of vinegar)
Everyday Uses of Carbon Compounds
- As fuel
- LPG (cooking gas) is mainly a mixture of butane and propane; CNG, used in vehicles, is mainly methane — both are saturated hydrocarbons (alkanes)
- In fruit ripening
- Ethene (C₂H₄) gas is used to artificially ripen raw fruit (such as mangoes and bananas)
- Uses of ethanol
- Ethanol is used as a solvent in tincture iodine, cough syrups and many cosmetic products
- Plastics and synthetic materials
- Most plastics, synthetic fibres and rubber are also long-chain carbon compounds (polymers), made possible by carbon's property of catenation
How Soap and Detergent Work
Soaps are the sodium or potassium salts of long-chain fatty acids. One end of a soap molecule is hydrophobic (water-repelling, a long carbon tail) that attaches to oil/grease, and the other end is hydrophilic (water-loving, an ionic head) that attaches to water. Because of this, soap forms round structures called micelles in water that trap oil-dirt particles inside them, so they dissolve in water and wash away, leaving cloth or skin clean. In hard water (which has dissolved calcium and magnesium salts), soap reacts with these salts to form an insoluble precipitate (scum), so it doesn't lather — while synthetic detergents lather normally even in hard water.
How to Teach This in Class
Activity
Have students build molecular models of methane, ethane and ethene using matchsticks and peas (or clay balls) to show the difference between single and double bonds.
Teaching aid
Mix a soap solution in hard water and in soft (clean) water and compare the lather — this simple, safe experiment demonstrates the difference between soap and detergent in a practical way.
Common misconception
Many students think diamond and graphite are different elements, or that all carbon compounds are dangerous/toxic — clarify that both are allotropes of pure carbon itself (only the arrangement of atoms differs, which changes their properties), and that sugar, starch and fat, all carbon compounds, are a major part of our food.
Assessment
Give students a few molecular formulas (such as C₃H₈, C₄H₈, C₅H₈) and ask them to classify each as an alkane/alkene/alkyne, then check practical understanding by asking the names of the hydrocarbons used in cooking gas and vehicle fuel, along with a quick oral question on why diamond and graphite differ in properties.
Memory trick / exam tip
A trick for the homologous series — the number of hydrogen atoms keeps decreasing across the three series: 2n+2 in alkanes, 2n in alkenes, 2n−2 in alkynes. To identify quickly, compare the number of H atoms in the formula with 2n — if it's 2 more than 2n, it's an alkane; if equal, an alkene; if 2 less, an alkyne. Remember that the n in the general formula is the number of carbon atoms, so find n first, then work out the expected number of H atoms and match it against the given formula.
Questions Asked in REET 2024 (With Solutions)
Question 129
Which of the following compound is not a member of alkene homologous series ?
- AC₅H₁₀
- BC₄H₈
- CC₃H₆
- DC₂H₂
Explanation
ऐल्कीन (alkene) का सामान्य सूत्र CₙH₂ₙ है: C₅H₁₀, C₄H₈ और C₃H₆ इस सूत्र में आते हैं। C₂H₂ (एथाइन/ऐसीटिलीन) का सूत्र CₙH₂ₙ₋₂ में आता है, अतः यह ऐल्काइन (alkyne) है, ऐल्कीन नहीं। (एथीन का सूत्र C₂H₄ होता है।)
Question 123
Example of alkene is
- AC₂H₆
- BC₂H₄
- CC₃H₈
- DC₂H₂
Explanation
ऐल्कीन वे हाइड्रोकार्बन हैं जिनमें कार्बन–कार्बन द्विबंध होता है और सामान्य सूत्र CₙH₂ₙ होता है। C₂H₄ (एथीन) इस सूत्र में आता है। C₂H₆ और C₃H₈ ऐल्केन (CₙH₂ₙ₊₂) हैं तथा C₂H₂ ऐल्काइन (CₙH₂ₙ₋₂) है।
Practice Questions
Question 1
The general formula CnH2n+2 belongs to which homologous series?
- Aऐल्केन
- Bऐल्कीन
- Cऐल्काइन
- Dऐल्कोहॉल
Explanation
CₙH₂ₙ₊₂ is the general formula of the alkane series — saturated hydrocarbons with only single bonds, such as methane (CH₄) and ethane (C₂H₆).
Question 2
C4H8 is a member of which homologous series?
- Aऐल्केन
- Bऐल्कीन
- Cऐल्काइन
- Dकार्बोक्सिलिक अम्ल
Explanation
Here n = 4, so by CₙH₂ₙ the number of H atoms should be 2×4 = 8, which matches C₄H₈ — so it belongs to the alkene series (CₙH₂ₙ).
Question 3
In diamond, each carbon atom forms covalent bonds with how many other carbon atoms?
- A2
- B3
- C4
- D6
Explanation
Because of its tetravalency, every carbon atom in diamond forms a strong covalent bond with 4 other carbon atoms, building a hard three-dimensional lattice structure.
Question 4
Why is graphite a conductor of electricity while diamond is not?
- Aग्रेफाइट में आयनिक बंध होता है
- Bग्रेफाइट धातु है
- Cहीरे में कार्बन परमाणु नहीं होते
- Dग्रेफाइट में परतों के बीच गति करने योग्य मुक्त इलेक्ट्रॉन होते हैं
Explanation
In graphite, each carbon is bonded to only 3 other carbon atoms, leaving one electron free to move between the layers, which allows it to conduct electricity — in diamond, all 4 electrons are tightly bonded.
Question 5
The formula of ethanoic acid (acetic acid), mainly found in vinegar, is:
- ACH₃OH
- BCH₃COOH
- CC₂H₅OH
- DHCOOH
Explanation
The main component of vinegar is ethanoic acid (acetic acid), with the formula CH₃COOH.
Question 6
Which functional group identifies an alcohol?
- A-COOH
- B-CHO
- C-OH
- DC=O
Explanation
The -OH (hydroxyl) functional group identifies an alcohol, such as in ethanol C₂H₅OH.
Question 7
Which part of a soap molecule is hydrophilic?
- Aलंबी कार्बन शृंखला (पूँछ)
- Bइसमें कोई भाग जल-स्नेही नहीं होता
- Cपूरा अणु समान रूप से
- Dआयनिक सिरा (सिर)
Explanation
The ionic head of a soap molecule (such as -COO⁻Na⁺) is hydrophilic and attaches to water, while the long carbon tail is hydrophobic and attaches to oil/grease.
Question 8
A better option than soap for washing clothes in hard water is a detergent, because:
- Aअपमार्जक (detergent) कठोर जल में भी आसानी से झाग बनाता है
- Bअपमार्जक साबुन से सस्ता होता है
- Cअपमार्जक में कार्बन नहीं होता
- Dसाबुन जल में घुलता ही नहीं
Explanation
A detergent does not form a precipitate with the calcium-magnesium salts in hard water, so it lathers normally even in hard water and cleans effectively, while soap stops lathering because it forms a precipitate.
Question 9
The most effective teaching aid to explain the structure of carbon compounds (difference between single and double bonds) to class 8 students is:
- Aकेवल सूत्र श्यामपट्ट पर लिखना
- Bबॉल-एंड-स्टिक/क्ले मॉडल से आण्विक संरचना बनवाना
- Cकेवल परिभाषा रटवाना
- Dकेवल नाम याद कराना
Explanation
At the class 6-8 level, having students build molecular structures with ball-and-stick/clay models lets them see and understand the real difference between single and double bonds for themselves.
Question 10
What is the total number of carbon atoms in a fullerene (C60) molecule?
- A12
- B32
- C60
- D70
Explanation
In the famous form of fullerene, buckminsterfullerene, 60 carbon atoms are arranged in a football-like spherical structure, which is why it is called C₆₀.
Frequently Asked Questions
Why does carbon only form covalent bonds, and what is catenation?+
Carbon has 4 electrons in its outermost shell, which are energetically difficult to lose or gain, so it shares electrons to form covalent bonds instead. Catenation is carbon's ability to join with itself into long, stable chains (straight, branched or ring-shaped), a property found far more strongly in carbon than in other elements, giving rise to an enormous number of organic compounds.
What is the difference between diamond and graphite?+
In diamond, every carbon atom is joined to 4 other carbon atoms in a three-dimensional structure, making it extremely hard and a poor conductor of electricity. In graphite, carbon is joined in layered hexagonal structures, making it soft and a good conductor of electricity.
What is the difference between alkanes, alkenes and alkynes?+
Alkanes (CₙH₂ₙ₊₂) have only single bonds, alkenes (CₙH₂ₙ) have one double bond, and alkynes (CₙH₂ₙ₋₂) have one triple bond — alkenes and alkynes are called unsaturated hydrocarbons.
Why doesn't soap lather in hard water?+
Soap reacts with the dissolved calcium and magnesium salts in hard water to form an insoluble precipitate (scum), which uses up soap's useful part and stops it from lathering.
What is fullerene?+
Fullerene is an allotrope of carbon in which 60 carbon atoms are joined in a football-like spherical structure — it is also called buckminsterfullerene (C₆₀).
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Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked