CBSE Class 10 Science Carbon and its Compounds Quick Revision for Board Exams

As the CBSE Class 10 Science Board Exam draws near, students should prioritize quick revision of concept-heavy chapters like Carbon and Its Compounds. This chapter is highly important as it introduces the basics of organic chemistry and often includes theory-based, reasoning, and equation-based questions in the board exam.

This quick revision guide helps students recap essential concepts, important formulas, and frequently asked questions efficiently, boosting confidence for the final exam.

Carbon and Its Compounds 

Carbon is a foundational element, critical to life and organic chemistry. Its ability to form stable bonds with itself and other elements leads to millions of compounds, making it central to various industries and biological processes. 

Facts About Carbon

The study of carbon and its compounds is known as Organic Chemistry. 

Confirmatory Test for Carbon

1. Procedure: Burn a substance suspected of containing carbon in oxygen.

2. Observation: Carbon will produce carbon dioxide (CO₂), heat, and light.

3. Confirmation: Pass the gas through limewater (calcium hydroxide solution, Ca(OH)₂). It will turn milky or turbid due to insoluble calcium carbonate (CaCO₃) formation.

• • Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Introduction to Covalent Bonds

A covalent bond is a chemical bond formed by the sharing of electrons between non-metal atoms. Compounds with covalent bonds are called covalent compounds or molecular compounds.

Formation of Covalent Bonds: Examples

• Hydrogen (H₂): Each H atom shares 1 electron to form a single covalent bond (H-H), achieving a stable duplet.

• Oxygen (O₂): Each O atom shares 2 valence electrons to form a double covalent bond (O=O), achieving a stable octet.

Properties of Covalent Compounds

Covalent compounds are generally poor conductors of electricity because they lack free-moving ions or electrons. An exception is polar covalent compounds like HCl in water, which dissociate into ions.

Versatile Nature of Carbon

Carbon's ability to form a vast array of compounds comes from three unique properties:

1. Tetravalency: Carbon has a valency of four, forming four covalent bonds.

2. Multiple Bond Formation: Carbon can form single (C-C), double (C=C), and triple (C≡C) bonds with itself and other elements (e.g., C=O, C≡N).

3. Catenation: The unique property of an element's atoms bonding with other atoms of the same element to form long chains. Carbon exhibits extensive catenation, forming straight chains, branched chains, and closed rings.

Allotropes of Carbon

Allotropes are different physical forms of the same element in the same physical state. Carbon's main solid allotropes are Diamond, Graphite, and Fullerene.

Classification: Alkanes, Alkenes, and Alkynes

Structure of Benzene

Benzene is a cyclic, unsaturated hydrocarbon with the molecular formula C₆H₆.

• Structure Inspiration: Proposed by August Kekulé, who was inspired by a dream of a (snake seizing its own tail), suggesting a closed-loop structure.

• Structural Features: A closed ring of six carbon atoms, each bonded to one hydrogen atom. Carbon atoms are connected by alternating single and double bonds to satisfy tetravalency.

• Kekulé Structures: Benzene is represented by two resonance structures showing the alternating double bonds.

Classification of Functional Groups

• Halogens (Haloalkanes): F, Cl, Br, I.

• Oxygen-Containing Groups: Different arrangements of oxygen lead to classes like Alcohol (-OH), Aldehyde (-CHO), Ketone (>C=O), Carboxylic Acid (-COOH).

IUPAC Nomenclature: An Introduction

The IUPAC system provides a standardized way to name organic compounds.

Chemical Properties of Carbon Compounds

The chemical properties of carbon compounds:

1. Oxidation Reaction

Addition of oxygen to a substance. For organic compounds, it can convert an alcohol to a carboxylic acid.

• Example: Ethanol to Ethanoic Acid.

• Reagents: Strong oxidizing agents like Alkaline Potassium Permanganate (KMnO₄) or Acidified Potassium Dichromate (K₂Cr₂O₇), which provide nascent oxygen.

2. Addition Reaction

Characteristic of unsaturated hydrocarbons (double/triple bonds). A molecule adds across the multiple bond, making it saturated.

• Process: Catalytic Hydrogenation (reaction with H₂ in presence of catalysts like Nickel (Ni), Palladium (Pd), or Platinum (Pt)).

• Example: Vegetable oil (unsaturated) to vegetable ghee (saturated).

3. Combustion Reaction

Rapid reaction with an oxidant (usually oxygen), producing heat and light.

• General Equation: Hydrocarbon + O₂ → CO₂ + H₂O + Heat + Light

• Why Yellow Flame? Incomplete combustion produces glowing particles of unburnt carbon.

4. Substitution Reaction

An atom or group of atoms is replaced by another. Characteristic of saturated hydrocarbons.

• Example: Methane (CH₄) reacts with Chlorine (Cl₂) in sunlight. One H atom is substituted by Cl.

• CH₄ + Cl₂ → CH₃Cl (Chloromethane) + HCl

Important Carbon Compounds: Ethanol and Ethanoic Acid

Comparison of Physical Properties: Ethanol vs. Ethanoic Acid:

Chemical Properties of Ethanol

1. Reaction with Sodium

Ethanol reacts with sodium to produce sodium ethoxide and hydrogen gas, demonstrating the acidic nature of the -OH hydrogen.

• 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂↑

2. Dehydration Reaction

Ethanol undergoes dehydration in the presence of hot, concentrated Sulfuric Acid (H₂SO₄) at 443 K (170 °C). H₂SO₄ acts as a dehydrating agent, removing water to form ethene.

• CH₃CH₂OH ---(Hot conc. H₂SO₄, 443K)---> CH₂=CH₂ + H₂O

Chemical Properties of Ethanoic Acid

1. Esterification Reaction

Ethanoic acid reacts with an alcohol (e.g., ethanol) in the presence of concentrated H₂SO₄ (catalyst and dehydrating agent) to form an ester (e.g., Ethyl Ethanoate) and water. Esters have pleasant, fruity smells.

• CH₃COOH + CH₃CH₂OH ---(conc. H₂SO₄, Heat)---> CH₃COOCH₂CH₃ + H₂O

• (Memory Tip: Esters are named Alkyl Alkanoate. Alkyl part comes from the alcohol, Alkanoate from the acid.)

2. Saponification Reaction

This is the alkaline hydrolysis of an ester (e.g., Ethyl Ethanoate) with a strong base (e.g., NaOH), producing the parent alcohol (Ethanol) and the sodium salt of the carboxylic acid (Sodium Ethanoate, a soap).

• CH₃COOCH₂CH₃ (Ester) + NaOH → CH₃CH₂OH (Alcohol) + CH₃COONa (Sodium salt of acid)

3. Reaction with Metal Carbonates and Hydrogen Carbonates

Ethanoic acid, being an acid, reacts with metal carbonates or hydrogen carbonates to produce a salt, water, and carbon dioxide gas.

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

• CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Soaps and Detergents

Both are cleansing agents.

The Cleansing Action of Soap (Micelle Formation)

1. Orientation: In water, hydrophilic heads face water, hydrophobic tails point away.

2. Interaction with Dirt: Hydrophobic tails penetrate and dissolve in non-polar oil/grease droplets.

3. Micelle Formation: With agitation, soap molecules surround the oil, forming a micelle. The oil is trapped inside, while the water-soluble hydrophilic heads form the outer surface.

4. Washing Away: The micelle, with its water-soluble exterior, is easily lifted from the fabric and washed away with water, carrying the dirt.

Soaps vs. Detergents: Performance and Environmental Impact

1. Action in Hard Water

• Hard Water: Contains dissolved Ca²⁺ and Mg²⁺ salts.

• Soaps: Do not work effectively. They react with Ca²⁺/Mg²⁺ to form insoluble scum ((R-COO)₂Mg), wasting soap and preventing lather.

• Detergents: Work effectively as their calcium and magnesium salts are soluble and do not form scum.

2. Biodegradability

• Soaps: 100% biodegradable (straight-chain hydrocarbons easily broken down by bacteria).

Detergents: Many are not fully biodegradable (often branched-chain hydrocarbons), contributing to water pollution.