Key Points

Carbon has four valence electrons and achieves a stable electron configuration by sharing these electrons with other atoms, forming strong covalent bonds. It does not typically form ions like $C^{4+}$ or $C^{4-}$ due to high energy requirements.

Carbon's ability to form a vast number of compounds is due to two unique properties: catenation (self-linking to form long chains, branches, and rings) and tetravalency (forming four covalent bonds).

Carbon exists in different physical forms called allotropes. Key examples include diamond (a rigid three-dimensional structure), graphite (hexagonal layers, a good conductor of electricity), and Buckminsterfullerene ($C_{60}$).

Hydrocarbons containing only single covalent bonds between carbon atoms are called alkanes. They are relatively unreactive and have the general formula $C_nH_{2n+2}$. Example: Ethane ($C_2H_6$).

Hydrocarbons with one or more double bonds are alkenes (general formula $C_nH_{2n}$), and those with one or more triple bonds are alkynes (general formula $C_nH_{2n-2}$). They are more reactive than alkanes. Examples: Ethene ($C_2H_4$) and Ethyne ($C_2H_2$).

Compounds that have the identical molecular formula but different structural arrangements of atoms are called structural isomers. For example, butane ($C_4H_{10}$) exists as a straight chain (n-butane) and a branched chain (isobutane).

A functional group is an atom or a group of atoms, like alcohol ($-\text{OH}$) or carboxylic acid ($-\text{COOH}$), that replaces a hydrogen atom in a hydrocarbon and determines the compound's chemical properties.

A homologous series is a sequence of compounds with the same functional group and similar chemical properties, in which successive members differ by a $-\text{CH}_2-$ group. Physical properties like boiling point show a gradual change within the series.

Carbon compounds burn in oxygen to produce carbon dioxide, water, heat, and light. Complete combustion of saturated hydrocarbons produces a clean blue flame, while incomplete combustion of unsaturated hydrocarbons often produces a yellow, sooty flame.

Unsaturated hydrocarbons react with hydrogen in the presence of a catalyst like nickel or palladium to form saturated hydrocarbons. This reaction is called hydrogenation and is used to convert vegetable oils into solid fats.

Saturated hydrocarbons undergo substitution reactions, where one or more hydrogen atoms are replaced by another atom, typically a halogen. For example, methane reacts with chlorine in sunlight: $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$.

Ethanol is a liquid solvent used in alcoholic beverages and medicines. It reacts with sodium to produce hydrogen gas and undergoes dehydration with concentrated sulfuric acid at 443 K to form ethene ($CH_2=CH_2$).

Ethanoic acid ($CH_3COOH$) reacts with an alcohol like ethanol in the presence of an acid catalyst to form a sweet-smelling compound called an ester. This reaction is known as esterification: $CH_3COOH + C_2H_5OH \rightarrow CH_3COOC_2H_5 + H_2O$.

Saponification is the process of making soap by heating fats or oils (which are esters) with an alkali like sodium hydroxide ($NaOH$). The reaction produces soap (sodium salt of a carboxylic acid) and glycerol (an alcohol).

Soap molecules have a hydrophilic (water-attracting) head and a hydrophobic (oil-attracting) tail. They form structures called micelles that trap oily dirt at the center, allowing it to be washed away with water.

Soaps are less effective in hard water because they react with calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions to form an insoluble precipitate called scum. Detergents do not form scum and remain effective in hard water.