Objects and materials
Everything around us is an object, and every object is made of one or more materials. A tumbler can be made of glass, steel or plastic. A chair can be made of wood, plastic or metal. One material can also make many different objects: wood is used for tables, doors, pencils and boats.
We choose a material for a job because of its properties. A frying pan is made of metal because metal can take heat; its handle is often plastic or wood because those don't get hot quickly.
Why group materials?
Grouping materials with similar properties makes them easier to study and to choose. We group them in several ways.
Appearance: lustre
Freshly cut or polished metals such as iron, copper, aluminium, gold and silver have a shine. This property is called lustre, and such materials are lustrous. Wood, cloth, paper and plastic have no shine and are non-lustrous.
Metals lose their shine over time because air and moisture react with their surface. Iron rusts, and silver turns black. Rubbing the surface with sandpaper brings the shine back.
Hardness
A material that is difficult to compress or scratch is hard; one that can be easily pressed or scratched is soft. Iron and stone are hard. Cotton, a sponge and rubber are soft. Hardness is a comparison: wood is harder than chalk but softer than iron.
Soluble or insoluble in water
Stir different substances into water and watch:
Soluble (disappear in water) | Insoluble (do not mix) |
|---|---|
sugar | sand |
salt | sawdust |
lemon juice | chalk powder |
vinegar | kerosene, mustard oil |
Some gases dissolve in water too. Oxygen dissolves in water, and fish and other water animals breathe this dissolved oxygen.
Liquids behave in the same way. Vinegar mixes completely with water, but oil floats as a separate layer.
Floating and sinking
Objects that are lighter than the same volume of water float; heavier ones sink. Dry leaves, a piece of wood, a plastic ball and oil float. A stone, an iron nail and a coin sink.
Transparency
How much light passes through a material decides whether we can see through it.
Type | What happens to light | Examples |
|---|---|---|
Transparent | passes through almost completely; we can see clearly through it | clear glass, water, air, cellophane |
Translucent | passes through partly; we see things only dimly | frosted glass, butter paper, oiled paper, thin tissue |
Opaque | does not pass through; we cannot see through it | wood, cardboard, metal, a brick wall |
Shop owners keep sweets in transparent jars so customers can see them. Bathroom windows use translucent glass to let in light while keeping privacy.
Summary
Objects are made of materials, chosen for their properties.
Materials can be grouped as lustrous or non-lustrous, hard or soft, soluble or insoluble, floating or sinking, and transparent, translucent or opaque.
Grouping helps us study materials and choose the right one for a job.