Class 6 Science Chapter 6: Materials Around Us

Look at What Is on Your Desk

A notebook, a pen, a water bottle, a ruler. Now ask a different question about each one. Not what is it, but what is it made of. Paper. Plastic and metal. Plastic again. Wood, or plastic, or steel.

The Word for It

Whatever substance a thing has been created out of is called its material. So the notebook is the object and paper is the material. The two words are not rivals. They answer two different questions about the same thing on your desk.

A Short List You Already Know

Paper, wood, cloth, glass, metal, plastic, clay. You have handled every one of them today without giving it a thought. Everything around you is made out of some material or other, and usually you never notice which. The list is not long, and it covers a very great deal of what you own.

Rarely Just One

Look more closely and the neat answers break down. A pen is plastic and metal and ink together. A school bag is cloth, plastic zips and metal rings. Each part was chosen for something that material can do and the others cannot.

And the Same Material, Many Objects

It works the other way round too. Glass becomes a tumbler, a window, a bangle and a bulb. Clay becomes a pot, a lamp, a tile and a toy. One material, many objects, all of them different.

Now Try Sorting Them

Put a heap of everyday objects on a table and sort them into groups. You will find you can do it, and quite quickly. What is more interesting is that somebody else in the room will have sorted them differently and be just as right.

Three Ways to Start

You might group by shape, putting all the round things together. You might group by colour. Or you might group by the material they are made of, which puts a glass tumbler beside a window pane and a long way from a paper cup.

No Single Correct Answer

That is worth sitting with for a moment. The objects have not changed. What changes is which of their properties you decided to pay attention to. A grouping is a choice, and a different choice gives a different set of groups.

You Have Seen This Everywhere

A kitchen keeps similar utensils together. Spices occupy one corner of a grocer's shop and the pulses and grains another. Walk into a chemist shop and ask how the medicines are arranged. Nobody was told to do this.

So Why Bother?

Because grouping is not tidying. We find grouping useful because it helps us study things and observe patterns in their properties. Once similar things sit together, what they have in common stops being invisible. A single glass tumbler tells you almost nothing. A shelf of glass objects tells you a good deal.

The Pattern Is the Point

Put all the metals in one place and something appears: they shine, and the others mostly do not. You did not decide that. You uncovered it, and only because the grouping brought them side by side.

Not a New Idea

People have been doing this for a very long time. You have already met the classification of the living world in an earlier chapter. Classifying things that are not alive works on exactly the same principle, using properties. Rocks, plants, animals and materials have all been sorted this way.

Clay, Chosen Carefully

Take pottery. The earliest in the Indian subcontinent goes back seven to eight thousand years, to Lahuradewa in the Ganga plains and Mehrgarh in Baluchistan. From about 4000 BCE, Sindhu-Sarasvati potters were turning pots on a wheel.

What They Knew About Their Material

The clay was selected, cleaned, sieved, kneaded, turned on a wheel and finally baked in a kiln. Baked clay is called terracotta. Nobody does six steps to a material by accident. They knew exactly what that material would do.

And What They Made of It

Pots, dishes, bowls and very large storage jars, used for cooking and for keeping food grains, oil and ghee. During the Sindhu-Sarasvati Civilisation the work grew finer still, with bright red surfaces painted in black. The designs show geometric patterns and animals of the land and the water.

Scenario:

A class lists ten things in the room and the material each is made of. Wood, glass, metal, plastic, cloth and clay all appear. Then Arjun notices that two chairs on the list are made of different materials. Their teacher asks the class why the same object comes in more than one material. The children look at where each chair is kept. The plastic one stands in the yard and the wooden one indoors. Rain would spoil wood and does not spoil plastic. The class sees that the object says what it is for and the material says where it can go, and the two choices are made separately.

Analysis:

Object and material are two different questions about one thing. A chair names the use, and the material answers where that use has to survive.

Activity:

Find one object made in two different materials. Write where each version is kept and why that material suits the place.

Title:

Sort the same heap three times

Safety:

Choose nothing sharp, hot, breakable or valuable. Ask before taking anything from a kitchen or a cupboard, and put it all back afterwards.

What You Notice:

The three sortings disagree, and no two of them put the same things together. Objects with several materials are the awkward ones, because they could honestly go in more than one bin. Deciding what to do with them is the interesting part.

The earliest pottery in the Indian subcontinent goes back 7000 to 8000 years, to Lahuradewa in the Ganga plains and Mehrgarh in Baluchistan.

Baked clay has its own name: terracotta.

Ayurveda, an Indian system of medicine, sorts things by twenty qualities such as heavy or light and rough or smooth, so grouping by a property is an old idea here.

Object against material — What it is, and what it is made of — Chair is the object. Wood is the material. Two answers, one thing.

Ways to group — Shape, colour, material — Three starting points. Pick one, and the same objects fall differently.

Three Questions About Any Material

Pick up anything. Does it shine? Does it give way when you press or scratch it? Can you see through it? Three questions, three properties, and between them they will sort almost anything you can lay hands on.

Does It Shine?

Collect scraps of paper and cardboard, a bit of wood, a stick of chalk, some copper wire, a square of aluminium foil and anything made of brass or steel. Hold them where the light falls. Some throw the light back at you and the rest do not.

The Word Is Lustre

A surface that shines this way is described as having a lustrous appearance, and the shine itself is called lustre. Anything without it is non-lustrous, and paper, wood, rubber and jute are the usual examples.

Who Has It

Materials with lustre are usually metals. Iron, copper, zinc, aluminium and gold all belong in that group. So a shine is a genuine clue: it tells you which way to look, even before you know anything else about the material.

The Shine That Goes Away

But it is not a reliable clue, for two separate reasons. The first is that a metal can lose its lustre. Air and moisture work on the surface and it starts to look dull, which is why we often see the lustre only on a freshly cut face. The metal underneath has not changed at all.

And the Shine That Was Never Real

The second reason is the old saying: all that glitters is not gold. Surfaces are made shiny by polishing, or by coating them with thin layers of plastic or wax. Those materials may not be metals at all. The shine was borrowed.

So Treat Lustre as a Hint

Put the two together and lustre becomes a hint rather than a test. A dull thing may still be a metal, and a shiny thing may not be one. Neither answer follows from the shine alone.

Now Press It

On to the second question. Press different objects with your hands. A stone barely gives at all. An eraser squashes easily under your thumb. There is a clear difference here and it has nothing to do with shine.

Or Scratch It

Try it another way. Take a metal key and drag it across wood, aluminium, stone, iron, a candle and a piece of chalk. Some surfaces mark at once. Others you could scrape all day and leave nothing behind. Wood and chalk mark; stone and iron mostly do not.

Hard and Soft

That gives the two words. Anything that squashes or marks without much effort is soft, and anything that holds out against both is hard. The key and your thumb are testing the same property in two different ways.

The Catch With Hardness

Here is the important part, and it is easy to miss. These properties are relative in nature. Set rubber beside a sponge and it is the harder of the two; set it beside iron and it is the softer. Rubber has not changed between those two sentences. Only its companion has.

So Always Ask, Harder Than What?

That means hard and soft are comparisons pretending to be descriptions. Calling a material hard without saying hard compared to what is only half a statement, much like a number with no unit. The second half of the sentence is where the information is.

Now Look Through It

The third question. A game of hide and seek is under way, with Ghulan, Sheeta and Sara in it. Ghulan goes behind a wall, Sheeta behind a big tree, and Sara behind a frosted glass door with a hazy surface. From his house, Sheeta's younger brother can watch the lot through a glass window.

Three Answers, Not Two

Something you can see clearly through is transparent. Glass, water, air and cellophane paper are examples, and the brother's window is one. Anything you cannot see through at all is opaque, like wood, cardboard and metals.

And the One in Between

Sara is the interesting case. Something you can see through, but not clearly, is translucent, and butter paper and frosted glass are the examples. She is neither fully hidden nor fully visible, which is why the door was a good choice.

Scenario:

A class is asked to put four materials in order of hardness: a sponge, an eraser, a wooden ruler and an iron nail. The children test each pair by pressing and scratching. The order comes out sponge, eraser, wood, nail. Then Meera asks where glass would go. Nobody knows until they test it against the nail, and the nail scratches it. Their teacher asks what the list really says. It does not give each material a number. It only says that each one is harder than the one before and softer than the one after, so hardness here is an order rather than a measurement.

Analysis:

The class produced a ranking, not a scale. Each place in the list is fixed only by its neighbours, which is why a new material has to be tested against them.

Activity:

Rank five materials from your home by hardness using a scratch test. Then add a sixth and say which pair you tested it against.

Title:

Test the same six things three ways

Safety:

Do not shine a torch into anybody's eyes. Use a blunt spoon rather than a key for any scratching, and do not scratch furniture or anything you did not choose yourself.

What You Notice:

The three columns do not line up. Shiny things are not all hard, and hard things are not all opaque, which shows that the three properties are genuinely separate questions. The ranking by hardness is the one you can only do by comparing, never by judging one object alone.

A metal may lose its shine in air and moisture, so the lustre often appears only where the metal has been freshly cut.

Frosted glass has a hazy surface, which makes it translucent rather than transparent.

Hardness is not fixed: the same rubber is the harder material beside a sponge and the softer one beside iron.

The three seeing-through words — Clear, hazy, blocked — Transparent is clear, translucent is hazy, opaque blocks the view completely.

The lustre warning — All that glitters is not gold — Polish and coatings shine too. A shine is a clue to a metal, never a proof.

A Glass of Shikanji

Ghulan comes home tired and thirsty after playing. His mother stirs a spoonful of sugar into a tumbler of water, adds a pinch of salt and a little lemon juice, and hands it to him. He notices something on the way: the sugar and the salt disappeared while she was mixing.

Where Did They Go?

They have not been destroyed. Nothing has been thrown away and nothing has leaked out of the glass. The drink tastes sweet and slightly salty, so both are plainly still in there. They have simply stopped being visible.

Five Glasses, Five Materials

This is worth testing properly. Take five glass tumblers and fill each about two thirds with water. Into the first put a teaspoon of sugar, into the second salt, into the third chalk powder, into the fourth sand, and into the fifth sawdust.

Predict Before You Stir

Before touching a spoon, write down what you think will happen in each glass. This step feels unnecessary and is not. A prediction you have written down cannot be quietly adjusted afterwards to match what you saw.

Now Stir and Wait

Stir the contents of each tumbler well and leave them for a few minutes. Then look. Two of the five glasses now look very much like plain water. The other three quite plainly do not.

The First Group

Certain materials vanish altogether once they are mixed into water. We say that such a material dissolves, or, putting the same thing another way, that it is soluble in water. Sugar and salt behaved this way, which is why the shikanji looked ordinary.

The Second Group

Other materials refuse to mix, and they are still there however long the stirring goes on. Such a material is insoluble in water. Chalk powder, sand and sawdust are all visibly present at the end, settled or floating.

Stirring Longer Will Not Help

That is the part worth being clear about. An insoluble material is not one that needs more effort. You can stir sand for an hour and it remains sand in water. The difference is in the materials, not in how hard you worked. Effort is not what is being tested here.

Why Water Matters So Much

Water matters enormously to the way our body works, and this is the reason why. A great many materials will dissolve in it. That single property is what lets water carry things about inside a living body. Nothing much would move if water dissolved nothing.

Putting It to Use

ORS, or Oral Rehydration Solution, treats the dehydration that comes with diarrhoea and other illnesses. Ready packets are sold at primary health centres and in the market, and a packet goes into a litre of water before it is drunk.

And If You Have No Packet

It can be made at home. Take one litre of boiled and cooled water and stir into it six spoons of sugar and half a spoon of common salt. Both of those are soluble, which is what makes a simple stirred drink possible at all.

What About Liquids?

Solids are only half the story. Add a liquid to water and one of two things happens. Some liquids get completely mixed with water and cannot afterwards be told apart from it.

Or a Layer of Their Own

Others do not mix at all. Left undisturbed for a while, they settle into a separate layer sitting above or below the water, with a visible boundary between the two. Oil in water is the example you have most likely seen.

And Gases Too

Gases behave in the same divided way: some of them dissolve in water and some do not. This is easier to overlook, because a dissolved gas leaves nothing whatever to look at. It is no less real for that. A glass of clear water may be carrying a gas you cannot see at all.

The Gas That Everything Depends On

Oxygen gas dissolves in water, and everything living there depends on it. That dissolved oxygen is what animals and plants in the water actually breathe. A fish does not come up for air. It does not need to.

Scenario:

A class stirs sugar into a glass of water until it disappears. Rohit says the sugar is gone. Sana disagrees and tastes the water, which is sweet. Their teacher asks the class to settle it by weight instead. They weigh the glass of water, add a spoon of sugar, stir until nothing can be seen, and weigh again. The reading has gone up by the weight of the sugar. The class works out what disappearing means here. The sugar has spread through the water too finely to see, and the balance shows it is all still there. Rohit rewrites his sentence to say dissolved rather than gone.

Analysis:

Not visible and not present are different claims. The balance settles which one is true, because the weight accounts for every grain.

Activity:

Weigh a glass of water, dissolve a spoon of salt, and weigh it again. Record both readings and explain the difference.

Title:

Predict first, then stir

Safety:

Do not taste anything except the sugar and salt glasses, and only if an adult says you may. Use plastic cups rather than glass if you can, and clean up any spills.

What You Notice:

Two glasses end up looking like plain water and three plainly do not. The three insoluble materials fail in three different ways: one clouds the water, one sinks and one floats. Insoluble does not mean they all behave alike, only that none of them dissolved.

Oxygen gas dissolves in water, and everything living there survives by breathing it.

Water matters enormously to the way our body works, because so many materials will dissolve in it.

A litre of boiled and cooled water, with six sugar spoons and half a salt spoon stirred in, gives you ORS at home.

The two words — Gone means soluble, still there means insoluble — Stir well, wait, and look. Sugar is gone; sand is still there.

What can dissolve — Solids, liquids and gases alike — Sugar, some liquids, and oxygen. Dissolving is not a solids-only idea.

Three Cups and a Balance

Take three identical paper cups and half fill them. Water in the first, marked A. Sand in the second, marked B. Pebbles in the third, marked C. Predict which will be heaviest and which lightest, then weigh each on a balance and see.

The Property You Measured

Whatever came out on top, you were measuring something real. Being heavier or lighter is itself a measurable property, and its name is mass. The heavier cup has more mass and the lighter cup has less.

Units for Mass

Mass is measured in grams, written g, and kilograms, written kg. A kilogram is the SI unit, and in everyday speech people often say weight when they mean mass, because mass is what a weighing machine gives you.

Bags on the Seats

Now a different idea. Madam Vidya asks the class to put their bags on their seats and then sit down. Nobody can. When she asks why, the answer is obvious. Each chair has a bag sitting in the place a pupil would have used.

Two Tumblers, Same Size

Two pupils are each handed a glass tumbler, the two tumblers being identical, and asked to empty whatever is left in their water bottles into them. One tumbler ends up half full. The other is almost completely full.

Same Capacity, Different Amounts

Both tumblers are the same size, so both hold the same when full. The water levels differ because the amount of water in each is different. The tumblers were never the question; what they held is. Capacity belongs to the tumbler, and the amount inside is a separate matter.

The Word Is Volume

Less space is taken up by the water in the first tumbler, so its volume is the smaller of the two. The room that something fills is what we mean by its volume. That is the whole definition.

You Have Read It on a Label

Look at a drinking water bottle in a shop. You will find 1 L, 500 mL or 200 mL printed on it as net quantity. Those figures are volumes, telling you how much space the contents take up. Milk bottles carry the same figures for the same reason.

Units for Volume

So volume is measured in litres, written L, and millilitres, written mL. The SI unit of volume is the cubic metre, abbreviated m3 with a small raised three, and one cubic metre is 1000 litres.

Writing These Down

The rules follow the ones you already know. Kilogram is written kg in small letters, with no space between the k and the g, and no full stop after it in mid-sentence. Seven kilograms is written 7 kg and never 7 kgs.

One Odd Capital

Volume has a quirk. Litre is written as a capital L, and millilitre as mL, with a small m and a capital L and no space between them. Five hundred millilitres is written 500 mL. As always, a space goes before the unit.

Now Look Back

You have met several properties in this chapter. Lustre, hardness, whether you can see through it, whether it dissolves. Useful as they are, not one of them belongs to every material. Glass has no lustre; sand does not dissolve. Each one sorts materials into those that have it and those that do not.

The Two That Never Fail

Mass and volume are different. They are the two properties possessed by all materials. Water has both. Sand has both. So does a pebble, a paper cup, and the air in the room, which you cannot see at all.

So Give It a Name

Two properties that everything shares deserve a name for the things that share them. Whatever takes up space and has mass is given the name matter. Its mass tells you the quantity of matter present, and the room it fills is its volume.

Materials Are Kinds of Matter

That ties the chapter together. Water counts as matter, so do sand and pebbles, and so does the cup holding them. Materials are simply the kinds of matter that objects get made out of. So every material you have met is matter, seen from another angle.

Scenario:

A school orders a plastic water drum for the corridor. The shopkeeper says it holds 200 L. When it arrives, Sara notices the label has been rubbed and reads only 200. Ghulan says it must be 200 kg, since that is how drums are sold. The caretaker fills it from a tap using a 2 L jug and stops when the water reaches the brim, having counted ninety-four jugs. Sheeta then points out that nobody can lift the full drum to check anything at all, and that the empty drum by itself weighs something too. Madam Vidya writes three figures on the board, 200, 188 and 94, and asks the class what each one is a measurement of, and which of them can be trusted.

Analysis:

Two different properties have been confused because one of them was written down without its unit. The counting of jugs is the only measurement anybody in the story actually made, and it measures just one of the two properties.

Activity:

Take a 1 L bottle and a smaller cup whose volume you do not know. Fill the bottle from the cup, counting the cupfuls, and work out the volume of the cup. Then weigh the empty bottle, fill it with water and weigh it again. Write both results down with their units and say which property each one measures.

Title:

Same space, different mass

Safety:

Wash the pebbles and keep sand away from a sink drain. Do not overfill anything you have to carry, and wipe up spills before somebody slips.

What You Notice:

The three cups held the same volume and gave three different masses, which shows the two properties are independent. Topping up the water changed its volume without changing anything about the cup. Every reading you wrote needed a unit to mean anything.

One cubic metre holds 1000 litres, which is why m3 suits a reservoir and mL suits a medicine spoon.

The label on a bottle reading 500 mL is telling you the volume of what is inside.

Weight is often used in everyday speech for mass, because mass is found by weighing.

The two universal properties — Takes up space, and has mass — Every material does both. Anything that does both is matter.

The volume symbols — Small m, capital L — Millilitre is mL. No space inside the symbol, one space before it.