Chemistry 1H

Chemistry 1H

1. Paints contain a pigment, a binding agent and a solvent

2. Paint is a colloid

3. Thermochromic paints change colour when heated

4. Phosphorescent pigments can glow in the dark

5. Key Processes

   • 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
   • 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
   • Oil paints have a hydrocarbon oil as the binding medium. This is dissolved in a solvent. The pigment is dispersed within this solution. When the paint dries the solvent evaporates away, leaving behind the pigment and oil binding medium. The oil hardens to form a continuous layer of dry paint. 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
   • Emulsion paints are water-based - their solvent is water. The solvent evaporates leaving the pigment and binding agent to dry
   • 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. Phosphorescent pigments are useful for watch faces that glow in the dark. They may also be used in fire safety signs placed near fire extinguishers. In the event of a fire, the position of the fire extinguishers can be seen, even if the lights fail