Materials Around Us

A water bottle could, in principle, be shaped out of almost anything: paper, cloth, wood, glass, metal, plastic. Try it out with paper or cloth, though, and it fails immediately, water soaks straight through both. A cooking pan made of paper would burn through the moment it touched a flame. A cricket ball built like a soft hand-exercise ball would barely bounce off the bat, while a tennis ball built as hard as a cricket ball would be painful to play with and impossible to control. None of this is an accident: every object is quietly making a decision about which material to use, based on what that material can and cannot do, its properties, matched against what the object actually needs to do, its purpose. A pen even mixes materials within a single object: plastic for the body, metal for the tip, ink for the writing, each part built from whatever material suits that part's own job best.

Take any handful of objects, say a stone, a sponge, a coin and a rubber band, and you can sort them in more than one way: by shape, by colour, by how they feel, or by what material they are made from. The method of arranging objects or materials into groups based on a property they share is called classification. It is not a special scientific trick invented for a classroom; a grocer classifying spices into one corner and grains into another, or a family keeping similar utensils stacked together in a kitchen, is doing exactly the same thing. Classification matters because it turns a confusing pile of many different things into a small number of understandable groups, and once something is grouped correctly, its behaviour becomes far easier to predict.

Hold up a freshly cut piece of copper wire next to a piece of chalk, and the difference jumps out immediately: the copper gleams, catching and reflecting light, while the chalk simply looks flat and dull no matter how it is turned. Materials with that shiny, reflective surface are called lustrous, and most lustrous materials are metals, iron, copper, gold, aluminium among them. Materials without a shiny surface, like paper, wood, rubber or jute, are non-lustrous. There is a catch worth remembering: some metals lose their shine over time as air and moisture react with their surface, which is why lustre often shows up best on a freshly cut or freshly polished surface rather than an old one. There is a second, sneakier catch too: not everything that shines is a metal. A material can be coated with a thin layer of plastic, wax or paint specifically to make it look shiny, which is exactly why the old saying warns that all that glitters is not gold.

Press a stone and it does not give way at all; press a foam eraser and it compresses easily under your thumb. Materials that resist being compressed or scratched are called hard; materials that give way easily are called soft. Try scratching a piece of wood, aluminium, chalk and iron with the tip of a metal key, and some scratch far more easily than others, another way of testing the very same property. What makes hardness tricky is that it is not a fixed, absolute label: rubber is harder than a sponge, yet the very same rubber is softer than iron. Whether something counts as hard or soft always depends on what it is being compared with, which is exactly why the more useful question is not "is this hard?" but "is this harder or softer than that?"

Imagine three friends hiding during a game: one behind a solid wall, one behind a sheet of frosted glass, and one behind a clear glass window. The friend behind the wall is completely hidden, nobody can see through wood or brick at all: this makes wood and brick opaque. The friend behind the clear window is spotted instantly, since glass lets you see straight through: this makes glass transparent. The friend behind the frosted glass is the interesting case, a searcher can tell someone is there, a blurred shape, but not clearly who it is: frosted glass is translucent, letting light and a hazy shape through without a clear, sharp image. Butter paper behaves the same way as frosted glass. The three categories, transparent, translucent and opaque, describe exactly how completely you can see through a material, and choosing the wrong one for the wrong job, say opaque paper for a lampshade meant to glow, or transparent glass for a changing-room wall, would obviously defeat the object's purpose.

Stir a spoonful of sugar into a glass of water and, within a minute, it is gone, not destroyed, just spread invisibly through the water so evenly that no trace of it can be seen. Salt does the very same disappearing act. Try the same test with sand or sawdust, though, and neither one vanishes, no matter how long or how hard the mixture is stirred, they simply settle back down once stirring stops. Materials that dissolve completely this way are soluble in water; materials that do not are insoluble. This is not just true of solids: some liquids, like the lemon juice in a glass of shikanji (Indian lemonade), mix completely into water, while an oily liquid poured into water instead separates back out into its own layer once left undisturbed. Even gases dissolve in water to some extent: oxygen gas dissolves into rivers, lakes and oceans, and that dissolved oxygen is exactly what fish and other water-dwelling creatures depend on to breathe. Homemade ORS (Oral Rehydration Solution), used to treat dehydration, relies on this same everyday chemistry: six teaspoons of sugar and half a teaspoon of salt, both soluble, stirred into a litre of boiled and cooled water.

Fill one paper cup with water, a second with sand and a third with pebbles, all to the same level, then place each on a balance. Even though all three looked like they held about the same amount, the balance shows real differences: some cups pull the balance down more than others. That pulling-down effect is a property called mass, the quantity of matter an object contains, and it is measured with a balance in grams (g) or kilograms (kg). An object with more matter packed into it has more mass and is heavier; an object with less matter has less mass and is lighter. Mass is often loosely called "weight" in everyday conversation, which is close enough for daily use, though mass and weight are technically different ideas that get properly separated in later classes.

Pour identical bottles of water into two identical glass tumblers and something odd can happen: one tumbler ends up only half full, the other almost completely full, even though both tumblers themselves have exactly the same capacity. The water levels differ because the amount of water poured into each was different, and the space that water actually occupies inside its tumbler is called its volume. Look at any bottle of drinking water or milk sold in a shop and a number like 500 mL or 1 L is printed on it, this is exactly the same idea: the volume of liquid the bottle contains, measured in litres (L) and millilitres (mL). Together, mass and volume turn out to be special: unlike lustre, hardness or transparency, which only some materials have in an interesting way, every single material, without exception, has both a mass and a volume.

Lustre, hardness and transparency are each interesting, but none of them belongs to every material, plenty of materials are non-lustrous, or soft, or opaque. Mass and volume are different: absolutely every material has both, without exception. Anything that has mass and occupies space is given a general name: matter. The mass of an object tells you how much matter it contains, and the space that matter occupies is its volume. Water, sand, pebbles, air, even a single sheet of paper, are all examples of matter for exactly the same underlying reason: each one has mass, and each one takes up space. This single, universal idea, matter, is the thread that quietly connects everything explored in this chapter: every material examined for its lustre, hardness, transparency or solubility was matter all along, just being looked at from a different angle each time.

Grouping materials by their properties is not a modern invention. Pottery found in the Ganga plains and in Baluchistan dates back seven to eight thousand years, and by around 4000 BCE the Sindhu-Sarasvati (Harappan) civilisation had developed real technical skill in it: clay carefully selected, cleaned, sieved and kneaded, shaped on a wheel, and baked into terracotta in kilns, with some pieces decorated in bright red with black geometric patterns. Choosing clay over other materials, and preparing it in exactly this way, was itself an act of classification, understanding which material and which treatment produced pots capable of storing grain, oil or ghee without failing. India's classification of matter went further still: Ayurveda describes twenty properties, ten opposite pairs such as heavy and light, hot and cold, hard and soft, smooth and rough, used to describe not just non-living matter but plants, animals, humans and even food. Several of these pairs will already sound familiar, hard and soft, hot and cold, are exactly the kind of property-based thinking this very chapter has been building, just applied by scholars thousands of years earlier than any classroom.