Materials Around Us

Opening

A chair can be wood, iron, plastic, bamboo, or stone. The chair is the object. Wood or iron is the material. Once you hear that split, the room becomes a set of decisions. Somebody chose a material because of a property, even if they never said the word property aloud.

Sorting by what shows

Grouping materials is the same mental tool as grouping plants. You pick a feature, and the piles appear.

Lustre is a shine. Metals such as copper, iron, and aluminium are lustrous when the surface is fresh. Leave many of them in damp air and they dull. The lustre was a property of the material, and the dull skin is a story that happened later. An eraser, wood, and paper are non-lustrous. Gold stays famously willing to shine, which is one reason people trusted it.

Hard and soft are about whether a material gives way under a push you can reasonably make. Stone and iron are hard. A sponge, a cotton pillow, and fresh dough are soft. Hard is not the same as strong in every engineer's sense, but for this class it means the material resists being scratched or pressed out of shape.

Colour is a feature too, and a weak one for sorting science, because paint can lie. A red plastic cup and a red apple share a colour and almost nothing else.

Seeing through, and mixing with water

Transparent materials let you see clearly through them. Clear glass, clean water, and air are the teachers. A shopkeeper uses a transparent jar so the grain is visible without opening the lid.

Opaque materials block the view. Wood, a metal sheet, a textbook, a brick wall. You see the object, not what is behind it.

Translucent materials let light through in a confused way. You know something is behind them, and you cannot name its details. Butter paper and frosted glass are the examples to keep. Oiled paper sometimes behaves this way.

So the false sentence "wood is translucent and glass is opaque" is upside down. Ordinary wood is opaque. Clear glass is transparent. Frosted glass would be the translucent one.

Some materials are soluble in water. Sugar and common salt disappear into it, not because they became nothing, but because they spread as particles you cannot see. The water tastes of them. That is the clue that they are still there. Some materials are insoluble. Sand, sawdust, and chalk powder stay as bits you can still point at. Stir, then let the glass stand. Insoluble bits often settle.

A few gases dissolve in water too. That quiet fact is why a fish has something to breathe in a river. You will meet oxygen more fully in a later journey. For now, solubility is not only a kitchen story about salt.

Matter, mass, and volume

Matter is anything that occupies space and has mass. The chair, the air in the chair's surroundings, the water in your bottle, the apple, the smoke's tiny particles: matter. A thought is not matter. A shadow is not matter. Light, as this class draws the line, is not put in the matter basket. If you can ask "how much space does it take, and how much stuff is in it?", you are talking about matter.

Mass is the amount of matter in an object. Volume is the space it occupies. A football and a cricket ball can teach the split if you are careful. The football takes more space, so its volume is larger, and it may still have less mass than a small stone. Big is not the same as heavy. Volume is not the same as mass.

Units come with both ideas. You already met metres for length. Volume of a box can be thought of as length times breadth times height, in cubic units, when the box is a neat rectangular one. Mass, in the kitchen, is what a balance compares. You do not need a new religion of units today. You need the two words kept apart.

Materials are chosen on purpose. A water bottle that is transparent lets you see what is left. A bottle that must block light, for a medicine that spoils in sun, is chosen opaque. A chair must be hard enough to hold a person, which is why cotton wool is a poor frame and a fine cushion. One object, many possible materials, and the properties vote.

PauseAir is invisible and easy to forget. What everyday event convinces you that air is matter, taking space, rather than empty nothing?