Copyright © 2007 Dorling Kindersley
Chemical reactions take place when existing bonds between atoms are broken and new bonds form. When the gas ethene is heated at high pressure, its molecules link together in long chains to make the plastic polythene. Polythene is used to make plastic dishpans, squeeze bottles, and plastic bags.
Any iron object left in a damp location becomes covered with an orange-brown substance called rust. Rusting is a chemical reaction between iron, oxygen, and water.
A good glue is a substance that starts as a liquid, but transforms into a solid when exposed to air. As a liquid, the glue can flow into every nook and cranny of the surfaces where it is spread. The glue molecules form bonds with the molecules in the surfaces. As the glue sets, the surfaces are held firmly together.
Heat makes many solids, especially metals, softer and easier to shape. As the temperature rises, most solids eventually melt to the liquid state. But some materials react differently to heat. Heat can trigger chemical reactions between mixtures. In an oven, heat changes a cake mix from a sticky liquid into a fluffy solid.
Melting and boiling are reversible changes produced by heat. Steam from a boiling kettle condenses back into drops of water when it comes into contact with a cold surface, such as a window.
Candle wax melts at 140°F (60°C), lead melts at 621.5°F (327.5°C), and iron melts at 2,804°F (1,540°C). Even stone can melt. The material with the highest known melting temperature is the metal tungsten, which melts at 6,129°F (3,387°C). Tungsten wire is used to make the filaments of electric light bulbs and television tubes.
Burning, rusting, and cooking are permanent changes. They cannot be undone by reversing the conditions that brought them about.
Concrete is a mixture of sand, gravel, cement, and water. Cement powder contains calcium oxide (lime) and silica or similar chemical compounds (substances that are two or more elements). When cement is mixed with water, the compounds react and set into a solid. The setting cement glues the sand and gravel particles together to make a permanent solid structure.