Properties of materials

What is a property?

Freshly cut wood looks quite different from iron, and iron looks different from copper or aluminium. Each material has its own properties: how it looks, how it feels and how it behaves. Grouping materials by their properties, or classification, helps us study them and spot patterns, much as a grocer keeps spices in one corner and pulses in another.

Appearance: lustrous or non-lustrous

Some materials shine when light falls on them. They are lustrous, and they are usually metals: iron, copper, zinc, aluminium and gold. Air and moisture can dull some metals, so the shine is clearest on a freshly cut surface. Materials without a shiny surface, such as paper, wood, rubber and jute, are non-lustrous. As the old saying goes, all that glitters is not gold: polishing, or a thin coat of plastic or wax, can make other materials shine too.

Feel: hard or soft

Press a stone and it does not change; press an eraser and it gives. Scratch wood, aluminium, stone, iron, a candle and chalk with a metal key: some scratch easily. Materials that are easy to compress or scratch are soft, and those that are difficult are hard. These properties are relative: rubber is harder than sponge but softer than iron. A brick (baked clay), a table and a tumbler feel hard; a pillow and a sweater feel soft.

Seeing through

Materials can be transparent (glass, water, air), translucent (frosted glass, butter paper) or opaque (wood, cardboard, metal), depending on how clearly we can see through them. Read more in transparent, translucent and opaque.

In water

Some materials, like sugar and salt, dissolve in water and are soluble; others, like sand, chalk powder and sawdust, are insoluble. Read more in soluble and insoluble substances.

Mass and volume: properties of all materials

Not every material has every property, but two are shared by all. Mass tells how much matter something has; the heavier object has more mass, measured in grams and kilograms. Volume is the space it occupies, measured in millilitres and litres. Anything with mass and volume is matter.

Grouping by properties

Classification is everywhere. A kitchen keeps similar utensils together, a grocer keeps spices in one corner and pulses and grains in another, and a chemist arranges medicines in an order that makes them easy to find. Scientists group rocks, plants and animals in the same way, by their properties. Ayurveda, one of India’s systems of medicine, describes all matter by 20 properties in ten pairs of opposites, such as heavy and light, cold and hot, soft and hard, and smooth and rough.

Properties decide uses

We pick materials by their properties and the job the object must do:

• Tumbler: must hold water, so glass, steel, copper, brass, terracotta or plastic, not cloth or paper.
• Cooking pots: must stand heat, so metal or clay; paper-like materials would burn or go soggy.
• Sports balls: a tennis ball must bounce high, while a cricket ball must be hard and heavy, so each uses different materials.
• Chairs: wood, iron, plastic, bamboo or stone, chosen for strength, light weight, warmth or easy cleaning.

For the textbook’s questions, see the Chapter 6 NCERT solutions; for the whole chapter, see the Chapter 6 notes.