CHANGING MATERIALS

We can use chemical reactions and heat to change materials and their properties to meet our needs. Some changes are PERMANENT, others are REVERSIBLE.

HOW DO CHEMICAL REACTIONS CHANGE MATERIALS?

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.

RUSTING IRON

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.

WHAT MAKES GLUE STICK?

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.

HOW DOES HEAT CHANGE MATERIALS?

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.

REVERSIBLE CHANGES

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.

CAN STONE MELT?

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.

PERMANENT CHANGES

Burning, rusting, and cooking are permanent changes. They cannot be undone by reversing the conditions that brought them about.

HOW DOES CONCRETE CHANGE FROM A LIQUID TO A SOLID?

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.

Copyright © 2007 Dorling Kindersley