Hydrocarbons have different boiling
points. They can be solid, liquid or
gas at room temperature
Small hydrocarbons have low boiling
points and are gases.
Hydrocarbons with between 5 and
12 carbon atoms are liquids.
Large hydrocarbons have high boiling
points and are solids.
The fractionating column
A tall column is fitted above the mixture, with several condensers coming off at different heights.
The column is hot at the bottom and cool at the top.
Substances with high boiling points condense at the bottom and substances with lower boiling
points condense on the way to the top.
The crude oil is evaporated and its vapours condense at different temperatures in the fractionating
Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient.
They do not flow easily and are difficult to ignite. Crude oil often
contains too many large hydrocarbon molecules, and not enough
small hydrocarbon molecules, to meet demand. This is where cracking
Fractions containing large hydrocarbon molecules are vaporised
and passed over a hot catalyst. This breaks chemical bonds in the
molecules and forms smaller hydrocarbon molecules.
Cracking allows large hydrocarbon
molecules to be broken down into
smaller alkane and alkene
smaller hydrocarbons are more useful as
fuels, such as petrols
alkenes are useful, because they
are used to make polymers.
Cracking is an example of a thermal decomposition reaction.
Pigments are coloured substances used in paint.
They contain several things, including the pigment,
a solvent and a binding medium.
Emulsion paints are water-based. Their solvent is water and they dry when the water evaporates.
The pigments in oil paints are dispersed in oil, which may
itself be dissolved in a solvent.
Paints are a type of mixture called a colloid. In a colloid, particles of one
substance are mixed and dispersed with particles of another substance - but they
are not dissolved in it. The components do not separate out because their
particles are small enough not to settle at the bottom.
Esters are made by reacting an alcohol with an organic acid.
They are used in perfumes, and as solvents.
Nail varnish dissolves in nail varnish remover, but not in water.
Perfumes have a pleasant smell and they stimulate receptors in the nose. Some perfumes are
obtained from natural sources, such as lavender oil or sandalwood oil. Others are made
Commercial perfumes need particular properties to be successful. The table summarises these
properties and why they are needed.
Esters occur naturally, but can be made in the laboratory by reacting an alcohol with an organic acid