A homologous series is a family of compounds whose molecules have the same functional group. The functional group is the atom or group of atoms that decides how the molecules react. In alcohols it is the hydroxyl group, -OH. In carboxylic acids it is the carboxyl group, -COOH. In alkenes it is the C=C double bond.
Because members of a series have the same functional group, they have similar chemical reactions. Every alcohol burns to make carbon dioxide and water, and every alcohol can be oxidised to a carboxylic acid. Every carboxylic acid reacts with a carbonate to make a salt, water and carbon dioxide. Every alkene decolourises bromine water, turning it from orange to colourless. Physical properties such as boiling point change gradually as the chains get longer, with the boiling point increasing.
This lets you predict. Ethanol is oxidised to ethanoic acid, so butanol is oxidised to butanoic acid. Ethanoic acid reacts with sodium carbonate to make sodium ethanoate, so propanoic acid reacts with sodium carbonate to make sodium propanoate. Hexene has a C=C bond like ethene, so it will also turn bromine water colourless.