Dyes, Paints, Flavours and Perfumes

Fibre dyes and food colourings

A dye is a coloured organic compound that can fix itself on a fibre (cotton, wool, silk, nylon) or dissolve in a liquid. A dye must be fast: its colour should not wash out or fade quickly in sunlight. Dyes work because their molecules have parts that absorb some colours of light and reflect others, and parts that grip the fibre.

Natural dyes come from plants and insects, such as indigo, turmeric and madder. Synthetic dyes are made in factories and give a wider range of bright and long-lasting colours.

Food colourings are dyes that are safe to eat in small amounts. They are checked by food laws, and only approved ones can be used. Natural examples: saffron, beetroot, turmeric, caramel. Some artificial colours are allowed in limited amounts; non-food dyes must never go into food.

Composition of paints

Pigments are fine coloured powders that do not dissolve. In paint they are spread through a liquid. A paint has three main parts:

• Pigment: gives colour and covering power (for example titanium dioxide for white).
• Binder: the sticky part (oil or resin) that holds the pigment to the wall and forms a smooth film as it dries.
• Solvent (thinner): water or an organic liquid that makes paint easy to spread. It evaporates as the paint dries.

Small amounts of additives are added: driers, anti-fungus agents and others. Water-based paints are better for indoors because they smell less; oil-based paints are tougher outdoors.

Aromas, essences and perfumes

Smells come from volatile compounds: small molecules that evaporate easily and travel through air to the nose. Many are esters, which smell fruity (for example, ethyl butanoate smells like pineapple). Others are alcohols, aldehydes or ketones.

An essence (or essential oil) is the concentrated fragrant liquid taken from flowers, spices or fruit, for example by steam distillation. A flavour gives taste with smell, as in vanilla. A perfume is a blend of fragrance oils in alcohol. Light molecules reach you first (top notes), then medium ones (middle notes), and heavy ones last longest (base notes).

Try it

1. In the last 3D step, drag the amount slider from low to high with each colour. Which colours look darker when more dye is used?

2. At home: put a drop of food colour in two glasses of water, one hot and one cold. In which does the colour spread faster? (The hot one, because particles move faster.)

Key formulas and definitions

• Paint = pigment + binder + solvent (+ additives)
• Dye: dissolves or fixes on fibre; pigment: insoluble coloured powder
• Perfume smell order: top notes → middle notes → base notes

Worked examples

Common mistakes

• Mixing up dyes and pigments. Dyes dissolve; pigments do not.
• Thinking solvent stays in dry paint. It evaporates.
• Thinking any dye can go in food. Only approved food colours are safe.
• Believing perfume is only alcohol. It is a blend of fragrance compounds in alcohol.