Carbon and its Compounds

A blue flame indicates that the fuel is receiving sufficient oxygen and is undergoing complete combustion. Complete combustion of cooking gas generally produces carbon dioxide and water with little or no soot. In contrast, insufficient oxygen causes incomplete combustion, which may produce a yellow flame and soot.

A yellow, smoky flame indicates that the fuel is not receiving enough oxygen, so it undergoes incomplete combustion. Fine carbon particles, called soot, are produced and make the flame yellow and smoky. In contrast, complete combustion with sufficient oxygen generally produces a cleaner blue flame.

Saturated hydrocarbons, such as alkanes, contain only single covalent bonds. They therefore generally undergo substitution reactions, in which a hydrogen atom is replaced by another atom or group. For example, methane reacts with chlorine in sunlight to form chloromethane. In contrast, addition reactions are mainly shown by unsaturated hydrocarbons because they contain double or triple bonds.

Unsaturated hydrocarbons contain carbon-carbon double or triple bonds. These multiple bonds can open to add atoms or groups such as hydrogen or halogens; therefore, addition is their characteristic reaction. In contrast, substitution reactions are more common in saturated hydrocarbons such as alkanes.

Vegetable oils commonly contain carbon-carbon double bonds and are therefore unsaturated. In the presence of a nickel catalyst, hydrogen adds across these double bonds, making the oil more saturated and potentially converting it into a solid fat. This process is called hydrogenation. Neutralisation is an acid-base reaction, so it does not apply here.

Alkaline potassium permanganate is a strong oxidising agent. It oxidises the primary alcohol ethanol to ethanoic acid: \(\mathrm{CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O}\). Ethanal can be an intermediate product, but further oxidation gives ethanoic acid.

Ethanol is an alcohol and ethanoic acid is a carboxylic acid. They react, usually in the presence of concentrated sulphuric acid, to form an ester and water. The ester formed is ethyl ethanoate, which has a pleasant smell. This reaction is called esterification. In neutralisation, an acid and a base form a salt and water, not an ester.

When an ester is hydrolysed with an alkali such as sodium hydroxide or potassium hydroxide, it forms the sodium or potassium salt of a carboxylic acid. Such salts formed from fats or oils are soaps. This process is called saponification. In contrast, neutralisation is the direct reaction of an acid with a base to form salt and water; it does not involve hydrolysis of an ester.

The carboxylate ionic head, \\(\mathrm{-COO^-}\), of a soap molecule is hydrophilic, so it attracts water molecules. In contrast, the long hydrocarbon tail is non-polar and hydrophobic; it attaches to oil or grease. Therefore, the carboxylate ionic head is the correct answer.

The hydrophobic tail of a soap molecule is a long hydrocarbon chain that is attracted to non-polar substances such as oil and grease. Its hydrophilic head remains in water. Thus, the tails surround the oily dirt to form micelles, which are washed away with water. Exam tip: the part that attaches to oil is the hydrophobic tail.

The hydrophobic end of a soap molecule faces the oily dirt, while the hydrophilic end faces the water. Many soap molecules surround the dirt and form a micelle. This disperses the dirt in water so that it can be washed away. An emulsion is a mixture of two immiscible liquids, whereas the specific structure formed around dirt by soap is a micelle.

Hard water contains calcium and magnesium ions. These ions react with soap to form insoluble calcium or magnesium soaps, called scum. Soap is used up in forming this scum, so less soap remains to produce lather. In contrast, dissolved oxygen in water is not responsible for poor lather formation.

Hard water contains calcium and magnesium ions. Soap reacts with these ions to form insoluble scum, reducing lather and cleaning action. Detergents generally do not form such insoluble scum with calcium and magnesium ions, so they remain effective in hard water.

Soaps are sodium or potassium salts of long-chain fatty acids, such as stearic acid. Their long hydrocarbon chain associates with oil or grease, while the ionic end interacts with water, enabling cleansing action. In contrast, salts of short-chain carboxylic acids lack the long hydrophobic chain characteristic of soaps.

Isomerism is the phenomenon in which compounds have the same molecular formula but differ in the arrangement of their atoms or in their structure. For example, \(C_4H_{10}\) has two structural isomers. Catenation is the ability of carbon atoms to link to one another to form chains; it does not mean having different structures with the same molecular formula.

Carbon is tetravalent, so it can form four covalent bonds. It also exhibits catenation: carbon atoms join to one another to form long chains, branched chains, and rings. These two properties lead to a very large variety of carbon compounds. In contrast, carbon mainly forms covalent bonds, not only ionic bonds.

Carbon has four valence electrons in its outermost shell. To complete its octet by ionic bonding, it would have to lose four electrons or gain four electrons. Losing four electrons requires very high energy, while gaining four electrons would create a large negative charge on a small carbon atom. Electron transfer is therefore unfavourable, so carbon usually shares electrons to form covalent bonds. Option B is incorrect because carbon has four valence electrons.

Most carbon compounds are covalent and exist as separate molecules. The forces of attraction between these molecules are relatively weak, so less heat is needed to separate them; hence, their melting and boiling points are generally low. Substances with a strong ionic lattice usually have high melting points. Exam tip: The forces between covalent molecules are called intermolecular forces.

Most carbon compounds are covalent. Their electrons are shared in bonds between atoms, so free ions or free electrons are not available to conduct electric current. Therefore, they are generally poor conductors of electricity. Option B describes metals, which conduct because they have free electrons.

Propane has the molecular formula \(C_3H_8\) and contains only single covalent bonds between its carbon atoms. Saturated hydrocarbons with only single bonds are called alkanes; therefore, propane is an alkane. An alkene has at least one carbon-carbon double bond, which propane does not have. An ester is not a hydrocarbon because it also contains oxygen.