OCR Gateway Science - Rocks and Metals C2

Pigments are coloured substances used in paint. Paints are a type of mixture called a colloid. They contain several components, including the pigment, a solvent and a binding medium.

Thermochromic and phosphorescent pigments are very useful, and extend the normal properties of paints.

Dyes are substances that can be used to colour fabrics. In the past, people usually used natural dyes from animals and plants.

Paints contain these ingredients:

• a pigment - gives the paint its colour
• a binding medium – a liquid polymer that hardens to form a continuous layer when the paint dries
• a solvent - dissolves the binding medium and makes the paint more fluid

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.

In an paint the pigment is dispersed in the liquid formed from the binding medium and solvent solution.

Thermochromic pigments are sensitive to temperature. They change colour when they are heated up or cooled down. This property is useful for indicating if the drink in a cup is hot, or if the water in a kettle is hot.

Phosphorescent pigments glow in the dark. They are able to absorb light energy and store it. This stored energy is released as light energy over a period of time.

The components of a colloid are dispersed throughout the mixture. They do not separate out because their particles are small enough not to settle at the bottom.

When oil paints dry, the oil oxidises to form a hard film. This happens as the solvent evaporates and the oil reacts with oxygen in the air.

In the past, glow-in-the-dark watch faces used radioactive paints. Phosphorescent pigments are much safer, though.

Granite, limestone and marble are used as construction materials. Cement, which is made from limestone, is used to make concrete. But reinforced concrete has better properties for construction work than concrete alone.

Metal carbonates such as calcium carbonate break down when heated strongly.

Metals high up in the reactivity series - such as calcium - have carbonates that need a lot of energy to decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to see.

Concrete is made by mixing together:

• cement - made by heating powdered limestone and clay together
• gravel - crushed rocks
• sand
• water

Reinforced concrete is a composite material made from concrete and steel. It is a better construction material than concrete alone because:

• concrete is hard and strong when squashed, but weak when stretched
• steel is flexible and strong when stretched

The composite material combines the best properties of both materials, so that it is hard and strong when squashed or stretched. This makes…