a) Methane is a hydrocarbon which has a single bond. Its formula is CH4.
The characteristic chemical property of methane is the substitution reaction. It is a reaction in which one or more hydrogen atoms are replaced by some other atoms.
Ethene is a hydrocarbon containing a double bond. Its formula is C2H4.
The characteristic chemical property of ethene is the addition reaction. It is a reaction in which unsaturated compound (ethene) is combined with another substance to form a single product (saturated compound).
(b) Detergent, a sodium salt of a long chain benzene sulphonic acid, is a cleansing agent. It has cleansing properties in water. A detergent has two parts, a large non-ionic hydrocarbon group and an ionic group such as sulphonate group SO3-Na+. Name of a detergent is sodium n-dodecyl benzene sulphonate.
(c) Detergents have replaced soap as a washing agent because they do not form insoluble calcium and magnesium salts (scum) in hard water. Solubility of detergents, even in hard water, is better than a soap. Detergents also froth well.