Welcome dear students! Today we are going to learn about Carbon and its Compounds from Class 10 Science. In the last chapter, we studied many important compounds. Now, we will explore more interesting compounds and their properties, focusing on carbon, an element of immense significance in both its elemental and combined forms. Let us begin with Activity 4.1. Make a list of ten things you have used or consumed since morning. Compile this with your classmates and sort the items into three categories: things made of metal, things made of glass or clay, and others. If an item uses multiple materials, place it in all relevant columns. Look at the last column. Your teacher will explain that most items are made of carbon compounds. Can you think of a test to confirm this? What product forms when a carbon compound burns? Do you know a test to identify it? Food, clothes, medicines, and books all rely on this versatile element. All living structures are carbon-based. Surprisingly, carbon in the earth’s crust is only 0.02 percent, found in minerals like carbonates, hydrogen-carbonates, coal, and petroleum. The atmosphere holds just 0.03 percent carbon dioxide. Despite this small amount, carbon’s importance is immense. [CHECKPOINT]
Now, let us examine section 4.1, Bonding in Carbon – The Covalent Bond. Previously, we learned that ionic compounds have high melting and boiling points and conduct electricity in solution or molten state. Most carbon compounds are poor conductors. Table 4.1 shows carbon compounds have low melting and boiling points compared to ionic compounds. For instance, acetic acid melts at 290 K and boils at 391 K. Chloroform melts at 209 K and boils at 334 K. Ethanol melts at 156 K and boils at 351 K. Methane melts at 90 K and boils at 111 K. This indicates weak inter-molecular forces. Since they are non-conductors, their bonding does not produce ions. Carbon has an atomic number of 6, with an electron distribution of two in the K shell and four in the L shell, giving it four valence electrons. Elements react to attain a noble gas configuration. If carbon gained four electrons to form a C⁴⁻ anion, its six-proton nucleus would struggle to hold ten electrons. If it lost four electrons to form a C⁴⁺ cation, removing them would require excessive energy. Carbon solves this by sharing valence electrons with other atoms. These shared electrons belong to both outermost shells, allowing both atoms to attain noble gas configuration. [CHECKPOINT]
Consider simple molecules formed by electron sharing. Hydrogen, atomic number 1, has one K-shell electron. Two hydrogen atoms share electrons to form H₂, attaining helium’s configuration. We depict this with dots or crosses for valence electrons. The shared pair forms a single covalent bond, shown as a line. Chlorine, atomic number 17, forms Cl₂ by sharing one electron each. Oxygen, atomic number 8, has six L-shell electrons and needs two more. Each oxygen shares two electrons with another, forming a double bond of two shared pairs. Nitrogen, atomic number 7, shares three electrons each to form a triple bond. Ammonia, NH₃, has single bonds between nitrogen and three hydrogens. Now, examine methane, CH₄. Carbon is tetravalent. It shares four electrons with four hydrogen atoms. Bonds formed by sharing an electron pair are covalent bonds. Covalent molecules have strong intra-molecular bonds but weak inter-molecular forces, causing low melting and boiling points. Since no charged particles form, they are poor electricity conductors. [CHECKPOINT]
Carbon occurs in nature in different forms with varying physical properties. Both diamond and graphite consist of carbon atoms. In diamond, each carbon bonds to four others, forming a rigid three-dimensional structure. In graphite, each carbon bonds to three others in the same plane, creating a hexagonal array. One of these bonds is a double-bond, satisfying carbon’s valency. Graphite consists of these hexagonal arrays stacked in layers. These structural differences cause vastly different physical properties. Diamond is the hardest known substance. Graphite is smooth, slippery, and a good electricity conductor. Synthetic diamonds are made by subjecting pure carbon to high pressure and temperature. Fullerenes are another allotrope. The first identified was C-60, with carbon atoms arranged like a football, named after architect Buckminster Fuller. Let us answer the questions after section 4.1. First, the electron dot structure of carbon dioxide, CO₂. Carbon shares two electrons with each oxygen, forming two double bonds. Second, for a sulphur molecule of eight atoms, the textbook hint states the eight atoms are joined together in the form of a ring, with each atom sharing electrons to satisfy valency. [CHECKPOINT]
Moving to section 4.2, Versatile Nature of Carbon. The covalent bond allows carbon to form millions of compounds due to two factors. First, carbon exhibits catenation, the unique ability to bond with other carbon atoms, forming large molecules. These can be long chains, branched chains, or rings, linked by single, double, or triple bonds. Compounds with only single carbon-carbon bonds are saturated. Those with double or triple bonds are unsaturated. Silicon forms chains of up to eight atoms, but they are highly reactive. Carbon-carbon bonds are strong and stable. Second, carbon’s tetravalency allows it to bond with four other atoms, including oxygen, hydrogen, nitrogen, sulphur, and chlorine. These bonds are exceptionally strong due to carbon’s small size, which allows the nucleus to hold shared electron pairs tightly. Historically, organic compounds were thought to require a vital force. Friedrich Wöhler disproved this in 1828 by synthesizing urea from ammonium cyanate. Today, carbon compounds, except carbides, oxides, carbonates, and hydrogencarbonates, are studied as organic chemistry. [CHECKPOINT]
Section 4.2.1 discusses Saturated and Unsaturated Carbon Compounds. Ethane, C₂H₆, is formed by linking two carbons with a single bond, then satisfying remaining valencies with hydrogen. Propane is C₃H₈. Ethene, C₂H₄, requires a double bond to satisfy carbon valencies. Ethyne, C₂H₂, requires a triple bond. Unsaturated compounds are more reactive than saturated ones. Section 4.2.2 covers Chains, Branches, and Rings. Table 4.2 lists saturated hydrocarbons: Methane CH₄, Ethane C₂H₆, Propane C₃H₈, Butane C₄H₁₀, Pentane C₅H₁₂, and Hexane C₆H₁₄. Butane has two possible carbon skeletons: a straight chain and a branched chain. Both share the formula C₄H₁₀. Compounds with identical molecular formulas but different structures are structural isomers. Cyclohexane, C₆H₁₂, has a ring structure. Benzene, C₆H₆, has a ring with alternating double bonds. Compounds containing only carbon and hydrogen are hydrocarbons. Saturated hydrocarbons are alkanes. Unsaturated hydrocarbons with double bonds are alkenes, and those with triple bonds are alkynes. [CHECKPOINT]
Section 4.2.3, Will you be my Friend? Carbon bonds with halogens, oxygen, nitrogen, and sulphur. Replacing hydrogen in a hydrocarbon chain with these elements maintains carbon’s valency. The replacing element is a heteroatom. Heteroatoms often appear in functional groups, which confer specific properties regardless of chain length. Table 4.3 lists key functional groups: Haloalkane with Cl or Br, Alcohol with –OH, Aldehyde with –CHO, Ketone with >C=O, and Carboxylic acid with –COOH. Section 4.2.4 explains Homologous Series. A series where the same functional group replaces hydrogen in a carbon chain is a homologous series. CH₄ and C₂H₆ differ by a –CH₂– unit. C₂H₆ and C₃H₈ also differ by –CH₂–. Propane and butane differ similarly. The molecular mass difference is 14 u. For alkenes, ethene is C₂H₄, followed by C₃H₆, C₄H₈, and C₅H₁₀, all differing by –CH₂–. The general formula for alkenes is CnH2n. For alkanes, it is CnH2n+2. For alkynes, it is CnH2n-2. As molecular mass increases, melting and boiling points rise, but chemical properties remain similar due to the functional group. Activity 4.2 asks you to calculate differences for CH₃OH, C₂H₅OH, C₃H₇OH, and C₄H₉OH. Each differs by –CH₂– and 14 u, forming a homologous series. [CHECKPOINT]
Section 4.2.5 covers Nomenclature. Names derive from the carbon chain, modified by prefixes or suffixes. Methanol, ethanol, propanol, and butanol are examples. Naming rules: First, count carbon atoms. Three carbons is propane. Second, indicate the functional group. Third, if the suffix starts with a vowel, drop the final e from the chain name. A three-carbon ketone is propanone. Fourth, for unsaturated chains, replace ane with ene or yne. A three-carbon chain with a double bond is propene; with a triple bond, it is propyne. Table 4.4 provides examples: Haloalkanes use chloro or bromo prefixes. Alcohols use the ol suffix. Aldehydes use al. Ketones use one. Carboxylic acids use oic acid. Alkenes use ene. Alkynes use yne. Questions after this section: Pentane has three structural isomers. Carbon’s vast compound count stems from catenation and tetravalency. Cyclopentane has the formula C₅H₁₀, with five carbons in a ring, each bonded to two hydrogens. [CHECKPOINT]
Section 4.3 covers Chemical Properties. Section 4.3.1 is Combustion. Carbon burns in oxygen to yield carbon dioxide, heat, and light. Equations: C + O₂ → CO₂ + heat and light. CH₄ + 2O₂ → CO₂ + 2H₂O + heat and light. CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + heat and light. Activity 4.3 requires teacher assistance. Burn naphthalene, camphor, and alcohol on a spatula. Observe flames and smoke. Place a metal plate above. Saturated hydrocarbons yield clean flames. Unsaturated compounds yield yellow, sooty flames with black smoke. Activity 4.4: Adjust a bunsen burner’s air hole. Limited air causes a yellow, sooty flame. Sufficient air yields a blue flame. Incomplete combustion soots vessels. Blocked stove air holes waste fuel. Coal and petroleum combustion releases sulphur and nitrogen oxides, major pollutants. The Do You Know box explains flames occur only when gaseous substances burn. Wood volatiles vaporize and burn with a flame. Candle flames glow yellow due to unburnt carbon particles. [CHECKPOINT]
Section 4.3.2 covers Oxidation. Activity 4.5: Warm 3 mL ethanol in a water bath. Add 5 percent alkaline potassium permanganate dropwise. The purple colour disappears initially as ethanol oxidizes. Excess permanganate retains its colour as oxidation completes. Alcohols convert to carboxylic acids. Substances adding oxygen are oxidising agents, like alkaline potassium permanganate or acidified potassium dichromate. Section 4.3.3 is Addition Reaction. Unsaturated hydrocarbons add hydrogen using palladium or nickel catalysts to form saturated hydrocarbons. Catalysts alter reaction rates without being consumed. This hydrogenates vegetable oils, converting unsaturated chains to saturated fats. Unsaturated fatty acids are healthier. Section 4.3.4 is Substitution Reaction. Saturated hydrocarbons are inert but react rapidly with chlorine in sunlight. Chlorine replaces hydrogen atoms sequentially. Equation: CH₄ + Cl₂ → CH₃Cl + HCl in sunlight. This is a substitution reaction. Questions after 4.3: Ethanol to ethanoic acid is oxidation because oxygen is added. Oxygen and ethyne burn for welding due to extremely high temperatures, unlike ethyne and air which cause incomplete combustion. [CHECKPOINT]
Section 4.4 covers Ethanol and Ethanoic Acid. Section 4.4.1 details Ethanol. It is a liquid at room temperature, commonly called alcohol, and is the active ingredient in alcoholic drinks. It is a good solvent for tincture iodine, cough syrups, and tonics. It mixes with water in all proportions. Dilute consumption causes drunkenness. Pure ethanol is lethal. Long-term use causes health issues. Reaction with sodium: Activity 4.6 is a teacher demo. Drop sodium into ethanol. Hydrogen gas evolves. Equation: 2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂, forming sodium ethoxide. Dehydration: Heating ethanol at 443 K with concentrated sulphuric acid removes water to form ethene. Equation: CH₃CH₂OH → CH₂=CH₂ + H₂O. Sulphuric acid acts as a dehydrating agent. The Do You Know box notes ethanol depresses the central nervous system. Methanol oxidizes to methanal in the liver, coagulating protoplasm and causing blindness. Industrial ethanol is denatured with methanol and blue dye to prevent drinking. [CHECKPOINT]
Section 4.4.2 covers Ethanoic Acid. Commonly called acetic acid, a 5 to 8 percent solution is vinegar. Pure ethanoic acid melts at 290 K, freezing in winter as glacial acetic acid. Activity 4.7: Ethanoic acid smells like vinegar and turns blue litmus red. Activity 4.8: Mix 1 mL ethanol, 1 mL glacial acetic acid, and concentrated sulphuric acid. Warm for 5 minutes and pour into water. It smells sweet, indicating esterification. Equation: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. The product is ethyl ethanoate. Reaction with base: NaOH + CH₃COOH → CH₃COONa + H₂O. Activity 4.9: Add dilute ethanoic acid to sodium carbonate. Effervescence occurs. Passing the gas through lime-water turns it milky, confirming carbon dioxide. Same occurs with sodium hydrogencarbonate. Equations: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. Questions after 4.4: Distinguish alcohol and carboxylic acid using sodium carbonate. Acid produces CO₂ bubbles; alcohol does not. Oxidising agents add oxygen to other substances. [CHECKPOINT]
Section 4.5 covers Soaps and Detergents. Activity 4.10: Add oil to water in two tubes. Add soap to tube B. Shake. Tube A separates quickly. Tube B forms an emulsion. Soap molecules have a hydrophilic ionic end and a hydrophobic carbon chain. They form micelles, with tails trapping oil inward and heads facing water. This emulsifies dirt for removal. The More to Know box explains micelles align at water surfaces and form clusters inside water, stabilized by ion-ion repulsion. They scatter light, making solutions cloudy. Activity 4.11: Soap in distilled water lathers well. In hard water, it forms a white curdy precipitate. Activity 4.12: Detergent in hard water lathers well. Soap forms curdy solid. Detergents are sodium salts of sulphonic acids. Their charged ends do not precipitate with calcium or magnesium ions, making them effective in hard water. [CHECKPOINT]
Questions after 4.5: Detergents cannot test water hardness as they lather in both. Agitation is necessary to break dirt into smaller pieces and allow micelles to surround and remove it effectively. Review summary: Carbon’s versatility stems from tetravalency and catenation. Covalent bonds share electrons. Carbon forms chains, branches, rings, and multiple bonds. Homologous series share functional groups and chemical properties. Carbon compounds fuel our world. Ethanol and ethanoic acid are vital. Soaps and detergents clean via hydrophobic and hydrophilic interactions. Now, the Exercises. Question 1: Ethane has 7 covalent bonds. Answer b. Question 2: Butanone contains a ketone group. Answer c. Question 3: Blackened vessels indicate incomplete fuel combustion. Answer b. Question 4: In CH₃Cl, carbon shares one electron with chlorine and three with hydrogens, achieving noble gas configurations. Question 5: Electron dot structures: Ethanoic acid has a methyl group bonded to a carboxyl group. H₂S has sulphur single-bonded to two hydrogens with two lone pairs. Propanone has a central carbon double-bonded to oxygen. F₂ has a single bond with three lone pairs per fluorine. Question 6: A homologous series shares a functional group across varying chain lengths, like methanol, ethanol, and propanol. [CHECKPOINT]
Question 7: Ethanoic acid smells of vinegar, freezes at 290 K, turns litmus red, and releases CO₂ with carbonates. Ethanol smells pleasant, freezes at 156 K, does not affect litmus, and does not release CO₂. Question 8: Micelles form in water to shield hydrophobic tails. They do not form well in ethanol, which dissolves both ends. Question 9: Carbon compounds release substantial heat and light upon combustion, making them excellent fuels. Question 10: Scum forms when soap reacts with calcium and magnesium salts in hard water, creating insoluble precipitates. Question 11: Soap is basic and turns red litmus blue. Question 12: Hydrogenation adds hydrogen to unsaturated hydrocarbons using a catalyst. Industrially, it converts liquid oils to solid fats. Question 13: C₃H₆ and C₂H₂ undergo addition reactions. Question 14: Add bromine water. Unsaturated hydrocarbons decolourise it; saturated ones do not. Question 15: Soap micelles trap oily dirt with hydrophobic tails. Agitation disperses dirt. Hydrophilic heads keep micelles suspended in water for easy rinsing. [CHECKPOINT]
Group Activity I: Use molecular model kits to construct three-dimensional models of methane, ethane, ethene, ethyne, ethanol, and ethanoic acid. This visualizes spatial arrangements and bond angles. Group Activity II: Take 20 mL of castor, cotton seed, linseed, or soyabean oil in a beaker. Add 30 mL of 20 percent sodium hydroxide solution. Heat with continuous stirring until thickened. Add 5 to 10 g of common salt. Stir well and cool. Cut into shapes and add perfume before setting. This demonstrates saponification, the chemical process of soap preparation. Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]