Materials Around Us: Class 6 Science · Chapter 6

6.1 Observing Objects Around Us

Tip your school bag out onto your desk. Out tumble a water bottle, a notebook, a pencil, a lunch box and maybe a crumpled hanky. Each of these is an object: a thing you can see, hold and use. Now ask a sharper question. What is each one made of?

The bottle may be steel. The notebook is paper, the lunch box is plastic and the hanky is cotton. Steel, paper, plastic and cotton are materials (पदार्थ). Wood, glass, clay, rubber, leather and jute are materials too. Every object around you is made of one or more of them.

Scientists start by observing: using their senses carefully and noticing details. Try a blindfold game. Close your eyes while a friend puts a steel spoon in one hand and a wooden spoon in the other. Can you tell them apart without looking? The steel one feels cool and smooth, and heavy for its size. Open your eyes, and it shines too. Every clue comes from the material.

6.2 How to Group Materials?

Imagine a shop where everything is dumped in one big heap. The bangles are under the potatoes and the soap sits on the rice. You would never find anything! That is why shops, kitchens and libraries all keep things in groups.

Grouping is also how scientists make sense of the world. The trick is to pick one property (गुण), a feature that describes a thing. It could be its shape, perhaps, or how shiny it is, or how easily it bends. Then you put together everything that shares it. Change the property, and the groups change too.

Scientists have a name for this kind of sorting, where every member of a group shares the property you picked: classification (वर्गीकरण). A good grouping saves time, because you know exactly where to look. It can also reveal surprising patterns, as you will see later in this chapter.

Look back at your heap and you will notice two things. One object can be made of several materials: a school bag may be cloth with plastic zips and metal buckles. And one material can make many objects: metal alone gives us coins, keys, spoons and bicycle chains.

Choosing the right material

Every object has a job to do. A tumbler must hold a drink without leaking, and it should be easy to hold and wash. So the job comes first. Then we pick a material whose properties suit that job.

Often, one object needs several materials, each chosen for its own job. A kadhai has a metal body that carries heat to the food. Its handle is often wood or hard plastic, which stays cool enough to hold.

Balls show the same idea. Squeeze a rubber ball, a ball of wool and a ball of wet clay. All three are round and about the same size, yet each feels different in your hand. Will they behave differently when they hit the floor? The bounce test will tell you.

6.3 What are the Different Properties of Materials?

You have already sorted things by a few properties. Now let us look at them one by one, like a detective with a magnifying glass. For each property we will test some materials and sort them.

6.3.1 Observe and identify appearance of materials

Shine a torch across a kitchen shelf at night. Some things flash back at you: a steel glass, a brass diya, the foil around a chocolate. Others, like a wooden spoon or a cloth bag, stay dull. How a material looks is its appearance. Its colour, its shine and whether it is rough or smooth are all part of it.

In the shine hunt, every object that flashed back the light was a metal. This special shine is called lustre, and a material that has it is lustrous (चमकीला). A gold chain, a silver payal and a new coin all show it: lustre runs in the metal family. The clay, cork, cloth and wood that stayed dull are non-lustrous.

So why does an old copper vessel or an iron gate look dull? Many metals slowly react with damp air, and a dull skin forms on the surface. Copper turns brown or green, silver turns black and iron grows reddish rust. Clean away that skin, and the lustre is still there underneath.

Does everything that shines have to be a metal? No! A varnished wooden table, a glossy magazine cover and a new plastic toy can all gleam. Their shine sits only on the surface: a coat of varnish on the table, a glossy layer on the magazine page. Scratch through that coat and the dull material shows underneath.

Lustre puts all the metals into one group. But metals are not all alike. In Exploring Magnets you met another property that splits them up. Try it below.

6.3.2 Which materials are hard?

Press your thumb into a ball of atta dough. It dents at once. Now press a pebble just as hard. Nothing happens! The pebble is hard (कठोर): it refuses to dent, and it is tough to scratch. The dough is soft (मुलायम): it gives way under your thumb, and a fingernail marks it easily.

A second test is the scratch test. Drag the point of an iron nail across different materials. A deep groove means the material is soft. A faint line means it is harder. No mark at all means it is harder still.

Is wood hard? Next to butter, yes. Next to a stone, not really. Hard and soft are comparing words, so it often helps to say "harder than" or "softer than".

6.3.3 Explore materials through which one can see or cannot see

At a sweet shop, the laddoos sit behind a clear glass counter, so you can see every one of them. A bathroom window often has bumpy glass: it lets light in but hides who is inside. A steel almirah hides everything. The difference is how much you can see through each material.

A material you can see through clearly is transparent (पारदर्शी). A material that lets some light through, but shows only blurry shapes, is translucent (पारभासी). A material you cannot see through at all is opaque (अपारदर्शी).

6.3.4 What is soluble in water; what is not?

When you have a sore throat, an elder may stir a spoonful of salt into warm water for you to gargle. Watch the salt closely. The white grains shrink, then vanish completely. Yet the water tastes salty! The salt has not gone away. It has spread out through the water in bits far too small to see.

The salt in your gargle dissolved: it broke up into bits too small to see and spread all through the water. Salt, sugar, glucose and jaggery are soluble (घुलनशील) in water, which means they can dissolve in it. Sand, rava and pencil shavings are insoluble (अघुलनशील) in water. Stirring can whirl them around, but it can never make them vanish.

Your body runs on this dissolving power. Blood is mostly water, and it carries dissolved sugar and salts to every part of you. Plants need it too: roots can take in minerals only after they have dissolved in the water in the soil.

Try liquids next. Squeeze lemon juice into water and it spreads through the whole glass; soon you cannot tell where the juice ends. Coconut oil behaves quite differently. Stirring breaks it into blobs, but within a minute the blobs rise and join into one oily sheet on top of the water.

Why does a fish tank need a bubbling air pump? Fish breathe too, but they take their oxygen from the water around them. A little oxygen from the air dissolves in water, and the pump helps more of it to dissolve. So gases can be soluble as well. Soda is another example: it holds carbon dioxide gas squeezed into it. Open the bottle, and the gas fizzes out as bubbles.

6.3.5 How heavy or light?

Here is an old riddle. Which is heavier, a kilogram of cotton or a kilogram of iron? Think before you answer. They are just as heavy, because each is one kilogram! The riddle shows that we cannot judge heaviness just by looking. We need to measure.

The amount of matter in something is its mass (द्रव्यमान). The more mass something has, the heavier it feels in your hand. We compare masses with a balance and measure them in grams (g) and kilograms (kg).

At the doctor's, you may hear "Your weight is 35 kg." Strictly, that number is your mass. In science, weight and mass mean different things, and that difference is a story for higher classes.

6.3.6 Space and volume

Fill a bowl with water right up to the brim, and stand it in a plate. Now gently lower a stone into it. Splash! Water spills over the edge. Why? The stone needs space of its own, so it pushes some water out of the way.

Everything takes up some space. The amount of space something takes up is its volume (आयतन). The stone pushed out an amount of water equal to its own volume. Pour that spilled water into a measuring cup, and you have measured the stone's volume!

Pour a glass of milk into a wide bowl. Its shape changes, but its volume stays the same: it still fills just one glass when you pour it back. Packets and bottles tell you the volume of what is inside. Look at a 5 L can of cooking oil, a 1 L carton of juice or a 180 mL pack of lassi.

6.4 What is Matter?

Think back over every material in this chapter: steel, cotton, clay, salt, water and murmura. Some shine and some do not. Some dissolve, some are hard, some let light through. But two things are true of all of them. Each one has mass, and each one takes up space.

Anything that has mass and takes up space is called matter (द्रव्य). A grain of sand is matter. So are a mountain, a glass of lassi and the air inside a tyre. So where do materials fit in? A material is matter put to work: wood becomes a door, clay becomes a kulhad and cotton becomes a kurta.

Is air matter?

You cannot see air, so is it matter? Let us test both parts of the rule. Does air take up space? Does it have mass?

The tissue stays dry because the glass was already full of something: air. The air takes up space and keeps the water out. Only when you tilt the glass can the air escape as bubbles, and then the water rushes in.

So air has mass and takes up space. Air is matter, even though it is invisible. Light, sound, shadows and your thoughts are different. They have no mass and fill no space, so they are not matter.

Why bother sorting? Because groups show up patterns. When we lined up the things that flashed in torchlight, a pattern jumped out: every one was a metal. Then the magnet split the metals again. Living things are sorted in just the same way, as you saw in Chapter 2, Diversity in the Living World.

Time to put every property together. The sorting table has six rounds: shine, hard or soft, see-through, in water, float or sink, and matter. Watch how the same things land in different boxes. A wooden block is dull, hard, opaque and floats. A steel spoon is lustrous, hard and sinks.