Chapter 6: Materials Around Us

Step 1 – Look carefully at the surroundings
Standing (or sitting) in any ordinary room gives us plenty of day-to-day objects: a wooden chair, a glass window-pane, the steel rim of a water-bottle, an exercise-book, a plastic pen, a cotton handkerchief, etc.

Step 2 – Decide the main substance for each object
An object can be made from several substances (a school-bag, for example, has cloth, zip-metal and plastic buckles). For this introductory exercise, pick the single material that forms the major part or the most important working part of the object.

Step 3 – Write the observations in Table 6.1

Why do we call these “materials”?
Because each of them (paper, steel, plastic, glass, wood, cotton) can be obtained, processed and then shaped into useful articles. Any substance that can be turned into objects of utility is called a material.

Conclusion
This activity shows that different objects around us are made from different materials, and sometimes the same material (for example, glass) can be used to make many different objects (bottles, window-panes, spectacle lenses, etc.).

Step 1 — Observe the picture (Fig. 6.1)
The figure shows ten everyday articles:

• steel spoon
• iron nail
• aluminium cooking pot
• glass tumbler
• mirror
• plastic water bottle
• plastic comb
• wooden ruler
• wooden pencil
• newspaper

In order to compare them sensibly we first note the material of each article:

Step 2 — Choose one common property
Many properties are possible (colour, lustre, hardness, etc.). Here we select “material they are made of” because it is clear and unambiguous.

Step 3 — Group the objects

Step 4 — Answer the sub-questions

1. Which property did you use?
     We used “material of construction” (metal, glass, plastic, wood, paper).
2. Did your friends choose the same basis?
     Some did, but others preferred colour (shiny/dull) or hardness (hard/soft). Hence answers differed.
3. What did you learn?
     • The same set of objects can be grouped in many correct ways.
     • Selecting one clear property makes classification easier.
     • Grouping helps us study similarities and differences quickly.

Conclusion
By grouping the objects according to the materials they are made from, we discovered that systematic classification is a useful scientific tool for understanding the world around us.

Step 1 – Recall the requirements for a tumbler
A tumbler is used to hold water or other drinks. Therefore, the material chosen must have all of the following properties:

• It should be water-proof – the liquid must not seep through.
• It must be hard and strong so that the tumbler keeps its shape.
• It should not react with water or the drink kept in it.
• Ideally, it should be easy to clean and safe for use with food.

Step 2 – Think of materials that satisfy the above
We examine some common materials one by one:

• Glass – Non-porous, does not react with most drinks, keeps shape.
• Plastic – Light, waterproof, inexpensive, keeps shape.
• Stainless steel – Very strong, does not rust, food-safe.
• Aluminium – Light metal, waterproof, commonly used for utensils.
• Copper / Brass – Traditional metals for vessels, strong and waterproof.

Step 3 – Fill the blanks in Fig. 6.2
If six names are to be written around the picture of the tumbler, we can fill them as follows (order is not important):

These six materials all fulfil the conditions listed in Step 1, so any one of them can be used to make a tumbler.

Step 1 – Recall the properties of paper-like materials

• They are light in weight.
• They are usually porous (they allow air and water to pass through tiny gaps).
• They are not waterproof; when water, oil or any liquid spills on ordinary paper, it soaks through and the paper becomes weak or tears.
• Most importantly, paper is made mainly from cellulose fibres. Cellulose begins to burn (catch fire) at about $$225\text{--}230\,{}^{\circ}\mathrm{C}$$, which is lower than the temperature reached while cooking on a gas stove or in a microwave oven.

Step 2 – Recall the requirements for a safe cooking utensil

• It must withstand high temperatures—much more than $$100\,{}^{\circ}\mathrm{C}$$ (boiling point of water) and even up to $$300\,{}^{\circ}\mathrm{C}$$ or higher when frying.
• It should be strong and rigid so that it can hold the weight of food and liquids without bending or tearing.
• It has to be water- and oil-resistant because food often contains water, oil, spices, acids, salt, etc.
• It must be safe and non-flammable when kept directly over a flame or an electric heater.

Step 3 – Compare paper with the above requirements

Step 4 – Conclusion

Since paper fails all the essential tests for a cooking utensil, using paper-like materials for such a purpose is not a good idea. Metals (such as aluminium, stainless steel, iron) or heat-resistant ceramics and glass are far more suitable because they tolerate high heat, do not burn, and do not soak up liquids.

Step 1 ‒ Recall what a tumbler is used for
A tumbler is a container meant to hold liquids such as water without letting them escape.

Step 2 ‒ List the properties a suitable material must have

• It should not allow water to pass through it; that is, it must be non-porous and water-proof.
• It should be hard enough to retain its own shape when it is filled; that is, it must be rigid.

Step 3 ‒ Check whether cloth has these properties

• Porosity: Cloth is made of woven fibres that leave many tiny gaps. Water can easily seep through these gaps. Therefore cloth is porous and not water-proof.
• Rigidity: Cloth is soft and flexible. It cannot stand upright on its own in the form of a cup or tumbler.

Step 4 ‒ Conclude
Because cloth is porous (lets water escape) and non-rigid (cannot keep the required shape), a tumbler made of cloth cannot store water.

Step 1 – Recall what a “ball” has to do in a game
Every game fixes a few measurable requirements for its ball, for example

• diameter and mass (given in the rule-book)
• how high it must bounce when dropped from a certain height
• how easily players can grip or strike it
• whether it must fly very fast/far or move slowly
• whether it is used indoors, on grass, on a hard pitch, or in water

Step 2 – Relate each requirement to a property of matter studied in the chapter

Step 3 – Put the idea together
Because no single substance shows all the required properties at once, manufacturers select the material (or combination of materials) that gives exactly the right balance for that particular sport.

Step 4 – State the conclusion clearly
Different sports demand balls of different size, weight, bounce, hardness and safety. Only by choosing different materials—rubber, leather, cork, plastic, synthetic composites, etc.—can these varied requirements be satisfied. Hence, balls for various sports are made from different materials.

Step 1 ‒ Recall the purpose of choosing a particular material
Every game uses equipment whose properties (how hard, heavy, rough, bouncy, etc.) suit the rules and the safety of the players. A ball that is good for one game is not automatically good for another.

Step 2 ‒ Note the properties of the two balls

Step 3 ‒ Test the consequences of using the wrong ball

• Safety: Force on the racket or a player is $$F = m a$$. Because $$m_\text{cricket}$$ is nearly three times $$m_\text{tennis}$$, the impact force would be about three times larger, risking injury.
• Equipment damage: A cricket ball can break the strings of a tennis racket or crack the frame because the strings are designed to stretch under the smaller force of a tennis ball.
• Game quality: Tennis rules demand a certain bounce height. A hard leather ball absorbs little energy and therefore skids instead of rising, making rallies impossible.

Step 4 ‒ Conclusion
Because the cricket ball is much harder, heavier and less bouncy than a tennis ball, it does not meet the properties required for tennis. Therefore, a cricket ball cannot be used to play tennis.

Step 1 − Setting up the test
• Place a metre-scale (or stick marked in centimetres) vertically against a wall.
• Mark the release height at $$100\,\text{cm}$$ (exactly 1 metre) on the scale.
• Stand so that the bottom of each ball is exactly at the $$100\,\text{cm}$$ mark and release it without giving any push.
• A friend watches the scale and notes the top point to which the ball rises after the first rebound.

Step 2 − Performing three trials for each ball
We drop every ball three times so that small errors average out. Write every reading in rough and then find the average.

Typical readings (all heights in cm)

Step 3 − Converting average height into "high", "medium" or "low"
We compare the average rebound height with the release height $$100\,\text{cm}$$.
• If rebound is more than $$60\,\text{cm}$$  → high.
• $$30\,\text{cm}$$ to $$60\,\text{cm}$$  → medium.
• less than $$30\,\text{cm}$$  → low.

Step 4 − Filling Table 6.2

Step 5 − Identifying the highest bounce
The rubber super-ball (or, if that is not available, the tennis ball) shows the greatest rebound height, so it is the ball with the highest bounce.

Why do different balls behave differently?
• A tennis ball and super-ball are made chiefly of elastic rubber. When they hit the ground they are compressed and quickly spring back, returning most of their energy as kinetic energy, hence a high bounce.
• A cricket ball has a hard cork core wrapped in leather. It is less elastic than pure rubber, so it loses more energy as heat and sound; the bounce is medium.
• A hand-exercise (stress) ball is filled with a soft gel. The gel flows rather than springs back, so much of the energy is lost internally; therefore it bounces only a little (low).

Thus, by simple observation and measurement we learn that materials—rubber, cork, leather or gel—decide how much energy is conserved in a collision and, therefore, how well an object bounces.

Step 1 — Studying different sports balls

*Bounce is the height the ball’s centre reaches after being dropped onto a hard floor from 1 m.

Step 2 — Relating properties to use

• Size: Must fit the playing equipment and the players’ ability to handle it. A basketball is large so it is easy to catch with two hands; a golf ball is small so the club can strike it cleanly.
• Colour: Bright colours (tennis, cricket white-ball games) improve visibility; brown leather basketball contrasts with the wooden court.
• Texture: Pebbles, felt or seams give grip or control spin; smooth surfaces (table-tennis) reduce unpredictable bounce.
• Bounce: Different games need different rebound energy. High bounce is vital for dribbling (basketball). A cricket ball must not bounce too high so bowlers can control pitch behaviour.
• Material choice: Materials are selected for the right strength-to-weight ratio, elasticity, weather resistance and safety. Rubber bladders hold pressurised air to give lightness; leather survives strong impacts.

Step 3 — Grouping the objects in Fig. 6.4
(If the class textbook is opened, Fig. 6.4 shows a mix of household items such as a glass tumbler, steel spoon, plastic bucket, cotton shirt, wooden chair, iron nail, rubber band, woollen sock, paper sheet, stone and a balloon.)

Here are eight different ways to sort them. Each grouping rule forms two or more sets.

1. By state of matter
   
   • Solids – chair, spoon, glass, stone, nail, etc.
   • Liquids – if any liquid bottle is present.
   • Gases – air inside the balloon.
2. By raw material
   
   • Metallic – steel spoon, iron nail.
   • Wooden – chair.
   • Plastic – bucket, comb, toy car.
   • Glass – tumbler.
   • Rubber – band, balloon.
   • Fabric – cotton shirt, woollen sock.
   • Paper – sheet, book.
3. Natural versus man-made
   
   • Natural – stone, wood, cotton fibre, wool, rubber (from latex).
   • Man-made – glass, steel, plastic, paper.
4. Hard and soft
   
   • Hard – stone, glass, spoon, nail, chair.
   • Soft – cotton shirt, woollen sock, rubber band, balloon.
5. Heavy and light (feel in hand)
   
   • Heavy – stone, glass tumbler, wooden chair.
   • Light – paper sheet, balloon, rubber band, cotton shirt.
6. Transparent, translucent, opaque
   
   • Transparent – glass tumbler.
   • Translucent – thin plastic sheet (if present), balloon (partly).
   • Opaque – wood, metal, paper, stone.
7. Float or sink in water
   
   • Float – plastic toy, wooden chair leg, balloon (air-filled), rubber band.
   • Sink – glass tumbler, iron nail, stone, steel spoon.
8. Magnetic and non-magnetic
   
   • Magnetic – iron nail, steel spoon (if stainless steel attracts magnet).
   • Non-magnetic – wood, plastic, glass, stone, cotton, rubber.

Thus, depending on which property we choose, the same set of objects can be arranged again and again into different useful groups. Scientists and engineers pick the grouping that helps them select the right material for a job — just as sports engineers choose a special material to make every kind of ball behave exactly as that game demands.

Concept of lustre
When we say a substance is lustrous we simply mean that its surface reflects light well and therefore appears shiny.

Most metals are lustrous: for example, a freshly cut piece of sodium or a polished strip of copper shines brightly because the smooth metallic surface reflects almost all the incident light.

Observation of non-metals
Yet the shiny appearance is not restricted to metals. A few common non-metals also possess visible lustre, as shown below.

Because these examples are not metals yet still look shiny, lustre clearly cannot be used as a test that identifies metals alone.

Conclusion
The property of being lustrous does not belong exclusively to metals. Hence, the statement “all lustrous materials are metals” is incorrect.

Therefore,

Understanding the activity

We have to touch each listed object, press it gently between our fingers and decide whether it resists our pressure (hard) or gives way easily (soft). Next, we identify the substance(s) from which the object has been manufactured. Because classmates may own slightly different articles (plastic bottle versus metal bottle, wooden table versus metal table, etc.) our answers can vary; below is one possible complete set of observations.

How to test hardness or softness

• Keep the object on the bench or hold it firmly in one hand.
• Press it with a finger or thumb of the other hand.
• If it does not change its shape at all, call it hard.
• If it can be pressed and changes shape easily, call it soft.

Filled-in Table 6.3

Discussion with friends

• If a friend owns a steel water bottle, it is still hard but the material changes to stainless steel.
• A plastic tumbler found in the canteen is again hard; only the material column changes to plastic (polypropylene).
• Thus, while the property (hard/soft) often remains the same, the material can vary from one child’s object to another.

By comparing, we learn that hardness/softness is a property of the object, whereas the material tells us what the object is made of.

Step 1 – Recall the property of the materials around us
All objects can be grouped, on the basis of how they interact with light, into three broad classes:

• Transparent: transmit almost the whole light that falls on them; we can see clearly through them.
• Translucent: allow light to pass only partly; we see a blurred image.
• Opaque: do not allow any light to pass; things kept behind them cannot be seen.

Symbolically, if the intensity of incident light is $$I_0$$, then for an opaque material the transmitted intensity is $$I = 0$$.

Step 2 – Link the property to the game of hide-and-seek
When one is hiding, one naturally wishes not to be seen. Hence the safest places are those made of opaque materials because these materials stop light completely, making the person behind them invisible to the seeker.

Step 3 – Identify the nature of each hiding place

Step 4 – Reason for their choice
Wood, thick cardboard and heavy cloth either do not transmit light at all or transmit it so poorly that anything behind them cannot be seen. Therefore:

• They guaranteed that the seeker would receive practically no light $$\big(I \approx 0\big)$$ coming from the children’s bodies.
• With no light reaching the seeker’s eyes, the children remain invisible.

Conclusion
Ghulan, Sheeta and Sara chose these particular spots because the materials present there are opaque (or almost opaque) and do not let light pass through, ensuring that they could not be seen by the seeker.

Step 1 – Recall the types of materials with respect to light.
We have three broad groups:

• Transparent materials  →  allow almost all the light that falls on them to pass through. Example: clean glass.
• Translucent materials  →  allow only a part of the light to pass. Example: oiled paper.
• Opaque materials  →  do not allow light to pass through them at all. Example: wood.

We can write this idea symbolically as the amount of transmitted light $$I_T$$ compared to the incident light $$I_0$$:

$$ \text{Transparent:}\; I_T \approx I_0, \qquad \text{Translucent:}\; 0 < I_T < I_0, \qquad \text{Opaque:}\; I_T = 0 $$

Step 2 – Identify the material of the window.
The window mentioned in the question is made of wood and it is closed. A closed wooden shutter is therefore a continuous sheet of wood.

Step 3 – Apply the property of wood.
Wood is an opaque material, so for it $$I_T = 0$$, i.e. light cannot pass through.

Step 4 – Conclude about seeing through the window.
Because no light from the room where Sheeta and her friends are can reach her brother’s eyes through the closed wooden window, he will not obtain any image of them. In other words, he cannot see them.

Therefore, if the wooden window is properly shut (with no holes or openings), Sheeta's brother will not be able to see her or her friends through it.

Step 1 – Observe each part of Fig. 6.5 very carefully
In the textbook picture four children are doing different day-to-day activities. The parts of the figure are marked A, B, C and D.

• Part A (Ghulan) – He is shown cooking/serving food in a shiny silvery utensil. Such utensils are always made of a metal (generally aluminium or stainless steel) because metals conduct heat well and do not break on heating.
• Part B (Sheeta) – She is standing behind a clear window-pane through which we can see her face. A clear window-pane is made of glass, the common transparent, hard and brittle material.
• Part C (Sara) – Sara is holding or wearing a soft white cloth piece. The soft, fluffy appearance tells us it is a cotton fabric (a plant fibre).
• Part D (Sheeta’s brother) – He is carrying a thin, flexible shopping bag. The only material that gives this smooth, water-proof, flexible look is plastic (polythene).

Step 2 – Write the nature of the four materials
Summarising our observations:

Step 3 – Fill the blanks exactly as asked
(A) Metal   (B) Glass   (C) Cotton   (D) Plastic

Step 1 – Recall the three groups
Light can behave differently with different materials.

• Transparent: Materials that let almost all the light pass straight through them, so we can see clearly through the object.
• Translucent: Materials that let light pass but scatter it. Objects seen through them look blurred or hazy.
• Opaque: Materials that do not let light pass through at all; we cannot see through them.

Step 2 – Test each object in turn

1. Glass tumbler: When you look through an ordinary glass tumbler you can see objects on the other side clearly ⇒ it lets almost all light pass ⇒ Transparent.
2. Butter paper (grease-proof paper): Hold it up to a torch; you see a glow but not a clear image ⇒ light passes but is scattered ⇒ Translucent.
3. Eraser: No light comes through; you cannot see anything through it ⇒ Opaque.
4. Frosted glass: Commonly used in bathrooms; it glows when light is behind it but images are not clear ⇒ Translucent.
5. Wooden board: Blocks light completely ⇒ Opaque.
6. Window glass (plain, not frosted): Allows a clear view outside ⇒ Transparent.

Step 3 – Fill in Table 6.4

Hence, each object is correctly classified according to the way it interacts with light.

Step 1 — Recall the definitions of the three kinds of materials

• Transparent: materials that let light pass straight through them, so objects on the other side can be seen clearly. We write this property symbolically as $$\text{Light passes completely} \;\Rightarrow\; \text{transparent}.$$
• Translucent: materials that allow only some light to pass; objects appear blurred.
• Opaque: materials that do not allow light to pass through at all: $$\text{Light blocked} \;\Rightarrow\; \text{opaque}.$$

Step 2 — Observe ordinary (pure) water

Hold a clear glass of clean tap water in front of a printed page. You can read the letters through the water easily. Hence light is passing straight through without noticeable scattering, so water in this state is transparent.

Step 3 — Making the same water opaque — a simple activity

1. Take the same glass of water.
2. Add one teaspoon of an insoluble substance such as chalk powder, fine sand, or soil.
3. Stir the mixture well. Tiny solid particles remain suspended in the liquid and scatter or block the light.
4. Now try to read the printed page through the glass again. You will not be able to see the letters because almost no light comes straight through; the water appears cloudy or even completely non-see-through.

Because the suspended particles prevent light from passing, the mixture behaves like an opaque medium. We therefore say that the same water can be made opaque by adding a suitable insoluble material.

Step 4 — Conclusion

• Pure water is transparent.
• By mixing substances that do not dissolve (and hence remain as tiny particles), we can scatter or block light, turning the water opaque.

Concept required

• If a substance mixes uniformly with water and cannot be seen separately even after stirring and standing, we say it dissolves in water. It is then called soluble.
• If a substance remains as separate particles – either settling down or floating – it is said to be insoluble in water.

Materials collected: sugar crystals, common salt (table salt, chemically $$\mathrm{NaCl}$$), finely powdered chalk, sand and sawdust.

Step–by–step work

1. Five identical glass tumblers are filled two-third with tap-water so that the amount of water is same in every case.
2. A level teaspoon of each material is added to a separate tumbler as instructed.
3. Each tumbler is stirred 15–20 s with the same spoon to give every substance an equal chance to dissolve.
4. All tumblers are left undisturbed for about 5 min and then examined from the side and from the top.

Prediction before the test

• Sugar and common salt are everyday examples that disappear when we prepare sweet syrup or saline water, so they are expected to dissolve.
• Chalk powder (mostly $$\mathrm{CaCO_3}$$) does not dissolve when we make blackboard-cleaning slurry; it is predicted to remain undissolved.
• Sand is known to stay separate in rivers and does not dissolve, so it should remain as grains.
• Sawdust is lighter than water and is not observed to dissolve in carpentry work; it should neither dissolve nor sink – it will probably float.

Actual observations

• Sugar: Completely disappears; water turns clear and sweet. No residue at the bottom.
• Salt: Completely disappears; water remains clear but tastes salty. No residue.
• Chalk powder: Water looks milky while stirring; after a few minutes the white solid settles at the bottom, leaving a clear layer on top. Hence chalk does not dissolve.
• Sand: Grains remain visible and settle quickly at the bottom; the water above is perfectly clear. Sand is insoluble.
• Sawdust: Light brown flakes remain floating on the surface; a few may get wetted and sink slowly, but they are still visible. Sawdust is insoluble.

Completed Table 6.5

Conclusion

• Sugar and salt are soluble substances.
• Chalk powder, sand and sawdust are insoluble; among them, sawdust also shows the effect of being lighter than water and therefore floats.
• Whether a material dissolves or not helps us decide the methods for its separation from water (filtration, decantation, evaporation, etc.).

Step 1 : What does “disappear” mean in this context?
When we say that a substance put into water “disappears”, we really mean it dissolves. The particles of the substance (called the solute) spread out so finely between the particles of water (the solvent) that we can no longer see them with our eyes. The resulting uniform mixture is called a solution.

Step 2 : Setting up a simple test
Take five transparent glasses, each half-filled with tap water $$\mathrm{(H_2O)}$$. Label the glasses A to E and add one spoonful of a different substance to each:

Stir each glass with a clean spoon for about a minute and observe.

Step 3 : Recording the observations

Step 4 : Interpreting the results
Salt and sugar are soluble in water; hence they really do “disappear”. Sand, sawdust and oil are insoluble; their particles are still visible, either settled at the bottom or floating at the top. Water is often called a “universal solvent” because it can dissolve many substances, but not every substance.

Step 5 : Conclusion
Therefore, everything we put in water does not disappear. Only substances that are soluble in water, such as common salt or sugar, dissolve and seem to vanish. Substances that are insoluble—like sand, sawdust, plastic bits, or cooking oil—remain undissolved and can still be seen.

Objective  — To find out whether three common liquids (cooking oil, vinegar and honey) dissolve (are miscible) in water.

Materials needed

• 3 clean, dry test-tubes (or transparent glasses) each marked at the 20 mL level
• Measuring cylinder / dropper / teaspoon
• Tap water
• Cooking oil (any vegetable oil)
• Vinegar (contains about $$4\,\text{–}\,5\%$$ acetic acid, $$\mathrm{CH_3COOH}$$)
• Honey
• Glass rod (or spoon) for stirring
• Permanent marker / sticky labels

Procedure

1. Label the three tubes “Oil”, “Vinegar” and “Honey”.
2. Pour water up to the 20 mL mark in every tube.
3. Add exactly 5 mL of cooking oil to the tube labelled “Oil”. Do not stir yet.
4. Add exactly 5 mL of vinegar to the second tube.
5. Add 5 mL (a level teaspoon) of honey to the third tube.
6. Observe each tube without shaking and note what you see.
7. Now stir each tube with a clean glass rod for about 30 seconds and observe again.
8. Record all observations in a table.

Observations

Results and explanation

• Oil is immiscible with water. Oil particles do not interact strongly with water particles, so the two liquids stay separate. Because oil is less dense than water, it floats.
• Vinegar is an aqueous solution of acetic acid and water; it is therefore already mostly water. It mixes in every proportion, so it is completely miscible.
• Honey contains mainly sugars such as glucose and fructose. These sugars dissolve in water, so after stirring honey gives a clear sugar solution. Hence honey is soluble in water.

Conclusion 

Among the three liquids tested:

• Cooking oil does not dissolve in water; it remains as a separate layer.
• Vinegar dissolves (is miscible) in water in any amount.
• Honey dissolves after stirring, forming a uniform solution with water.

Step 1 – Sort the words into two kinds

• Properties / characteristics   → Transparent, Opaque, Lustrous, Solid
• Substances / articles   → Iron, Copper, Wood, Glass, Plastic, Bottle

A sensible arrow, therefore, will usually point

• from a property to some substance that shows that property, or
• from one article to the material it is commonly made of.

Step 2 – Recall the everyday facts

• Glass lets light pass through it → it is transparent.
• Wood does not allow light to pass → it is opaque.
• Metals such as Iron and Copper shine when polished → they are lustrous.
• Anything that keeps its own shape at room-temperature is a solid; plastic objects, wooden blocks, iron nails, copper wires and glass pieces are all solids.
• Most bottles meant for water or soft-drinks are made of glass or plastic.

Step 3 – Draw the arrows one by one

• Transparent → Glass
• Opaque → Wood
• Lustrous → Iron
• Lustrous → Copper (two different metals share the same property)
• Solid → Plastic
• Solid → Wood (wood is also a solid)
• Solid → Glass
• Bottle → Glass (glass bottles)
• Bottle → Plastic (plastic bottles)

Step 4 – Present the complete chain

The final network of companions is therefore:

Transparent → Glass ← Bottle → Plastic ← Solid → Wood ← Opaque
↑
| Bottle (again)

Lustrous → Iron ← Solid and Lustrous → Copper ← Solid

Any layout that keeps exactly these arrows (property → example, article → material) will earn full marks.

Step 1 – List all 12 words from the chapter
Lustrous, non-lustrous, soluble, insoluble, hard, soft, matter, mass, transparent, opaque, volume, translucent.

Step 2 – Choose any nine words at random
Suppose we pull the following slips out of a bowl without looking:

• lustrous
• mass
• opaque
• soft
• volume
• matter
• soluble
• hard
• transparent

Step 3 – Write the nine words in the 3 × 3 grid

Tip: Do not repeat a word and do not leave any square blank, otherwise you will have fewer than nine chances to tick.

Step 4 – Understand every word so you recognise it quickly

Step 5 – Play

1. The facilitator shuffles two decks of slips – one deck with the 12 words, the other with 12 definitions. He/she now calls out either a word or a definition.
2. Each time you hear something, scan your grid.
   If it is there, put a small tick (✓) in that square.
   If not, wait for the next call.
3. Keep listening. The moment you have ticks in all nine boxes, shout "Hurray!"

Worked example of ticking

Step 6 – Declare the winner
Because you ticked the ninth word first and shouted "HURRAY!", the facilitator quickly cross-checks. All nine words really were called or defined by call 10, so you are the winner.

Why this activity is useful

• Revises the meanings of key scientific terms.
• Trains the ear to pick out definitions accurately.
• Encourages fair competition and attention.

Extra practice
After the game, switch roles: let a student read out the words/definitions; everyone else prepares a fresh 3 × 3 grid with a new random selection of nine words. Play again!

Step 1 : Actual observation in the kitchen

• Rice, wheat flour and pulses stored together in large tins kept on the floor.
• Sugar and salt packets placed on an open shelf above the cooking platform.
• Spices (turmeric, chilli powder, whole garam-masala) mixed with tea leaves on the same rack.
• Cooking oil cans and ghee tins lying beside phenyl and dish-washing liquid under the sink.
• Biscuits, chocolates and dry fruits kept in the refrigerator although they do not need cooling.

What problems did I notice?

1. Difficult to find an item quickly because similar looking packets are scattered.
2. Chance of cross-contamination (e.g. strong spice smell entering biscuits).
3. Some things (tea, biscuits) absorb moisture because they are kept near the sink.
4. Heavy tins on the floor need bending every time.

Step 2 : Choosing sensible properties for grouping

• Physical state → solid / liquid.
• Perishability → needs refrigeration or not.
• Frequency of use → daily / weekly / occasionally.
• Nature → cereals & pulses / spices / snacks / cleaning agents.
• Weight → light packets / heavy sacks.

Step 3 : New shelf-wise plan

Step 4 : Why is this better?

• Similar items sit together → very less hunting time.
• No strong-smelling spice near snacks → taste remains unchanged.
• Cleaning chemicals completely separated from food → safe.
• Heavy sacks at bottom avoid accidents; light jars stay at a comfortable height.
• Refrigerator space is saved for real perishables.

Step 5 : Neat labelling for quick learning

1. Write category names on small stickers: “SPICES”, “PULSES”, “SNACKS”, etc.
2. Paste them on the shelves and on the containers’ lids.
3. Review once a month and shift any item that has migrated.

You can draw a simple diagram of your kitchen cupboards showing the new layout and colour each shelf according to the table above.

Remark for the notebook: I used properties such as state, perishability, frequency of use and weight to form logical groups. This is exactly what scientists do when they classify materials in chemistry.

Step 1 – Unscramble the jumbled words (Column I)

Step 2 – Recall the property each word describes

• MATTER → Anything that occupies space and has mass.
• SOLUBLE → A substance that mixes completely in water.
• TRANSPARENT → A material through which objects can be seen clearly.
• LUSTRE → A shiny surface.

Step 3 – Match Column I with Column II

Thus each word from Column I is correctly paired with its property in Column II.

Step 1 — Meaning of a transparent container
When a material is transparent, light passes through it, so the things kept inside can be seen from outside.

Step 2 — What the user of a container wants to do

• Know what is inside without opening the lid.
• See how much of the material is left.
• Notice any change in colour or other quality quickly, in case the material is getting spoiled.
• (For shopkeepers) Show the goods clearly to customers so that the articles look attractive on the shelf.

Step 3 — Why transparency helps
Because the walls can be looked through, all the points listed in Step 2 can be checked at a glance, with no need to touch or transfer the contents. This prevents waste, saves time and keeps the material clean.

Step 4 — Conclusion
Therefore containers used in shops and at home are usually made of transparent substances such as glass or clear plastic.

Step 1: List the three kinds of waste and note their special features

• (i) Food waste – wet, can leak liquids, starts rotting and smelling, attracts insects.
• (ii) Broken glass – has sharp edges, heavy for its size, can cut through thin material.
• (iii) Waste-paper – light, dry, not sharp, occupies a lot of volume.

Step 2: Recall what a good container must do

• Hold the waste safely without tearing, cracking or leaking.
• Be easy to lift, empty and clean (where necessary).
• Not react with or be damaged by the waste kept inside.

Step 3: Decide the properties needed for each case

Step 4: Match suitable materials with the required properties

1. Plastic bucket or bin for food waste
   Plastic (e.g. polyethylene) is water-proof, does not rust, and is easy to wash. A tight-fitting plastic lid keeps odours and insects away.
2. Metal (steel) box or thick plastic crate for broken glass
   Steel is very hard and will not be pierced by glass. It also resists heat. If steel is not available, a thick rigid plastic crate can work, provided its walls are strong enough not to split.
3. Cardboard carton or cloth bag for waste-paper
   Cardboard is itself paper, so it is light, cheap, and already the right size and shape. An old cloth bag or gunny sack can also be used because only dry paper is stored.

Step 5: Summarise the answer

Thus, by thinking about hardness, strength, water-proof nature, ability to resist corrosion, ease of cleaning and weight, we can select the right material for each waste container.

Step 1 : Recall the definitions

• Transparent substance  – allows almost all incident light to pass through it. Objects on the other side can be seen clearly.
• Translucent substance  – allows some light to pass, but scatters it. Objects on the other side look blurred or faint.
• Opaque substance  – allows no light to pass through it. Objects on the other side cannot be seen at all.

Step 2 : Classify the given materials

• Air lets light travel through without noticeable scattering, so objects remain clearly visible. Hence, air is transparent.
• A wooden door blocks light completely; nothing can be seen through it. Hence, wood is opaque.

Step 3 : Match with the options

The pair “transparent” (for air) and “opaque” (for wooden door) appears only in option (i).

Therefore, the correct choice is option (i).

Step 1 · Read the description carefully
We are told two independent things about each mysterious material:

• What happens when we press it (this tells us about hardness or softness).
• What happens when we put it in water (this tells us about solubility).

Step 2 · Relate “pressing” to hardness/softness
A material that does not change shape on pressing is called hard or rigid.
A material that changes shape easily on pressing is called soft.

Step 3 · Relate “mixing with water” to solubility
If a material disappears completely in water, we say it is soluble in water.
If it remains as it is, we say it is insoluble in water.

Step 4 · Match the given behaviour with common substances

Step 5 · Choose any one familiar pair
The most familiar pair for a Class-6 learner is:

• Material X: Table salt, $$\mathrm{NaCl}$$.
• Material Y: Wet clay (or kneaded wheat dough).

Step 6 · Answer each part explicitly

1. What can X and Y be?
      X can be common salt; Y can be clay.
2. Is X hard or soft?
      Because it does not change shape on pressing, X is hard.
3. Is Y hard or soft?
      Because it changes shape easily, Y is soft.

Step 7 · Justification in one sentence
Hardness-softness is judged by resistance to shape change; solubility is judged by ability to disappear in water. Salt is hard and soluble, while clay is soft and insoluble, matching the observations, so the identification is correct.

Step 1 : Listing the given materials

• Vinegar (mainly $$\mathrm{CH_3COOH}$$ in water)
• Honey
• Mustard oil
• Water
• Glucose (solid powder, $$\mathrm{C_6H_{12}O_6}$$)
• Wheat flour

Step 2 : Recall of the idea of solubility

A substance is said to be soluble in a liquid if, after stirring, it completely disappears and forms a uniform solution. If it does not disappear even after stirring, it is insoluble.

Step 3 : Trying possible combinations

Step 4 : Writing the required pairs

1. Pairs in which one material is soluble in the other
   • Glucose (solute) + Water (solvent)
   • Vinegar (solute) + Water (solvent)
2. Pairs in which one material is insoluble in the other
   • Mustard oil + Water
   • Wheat flour + Water

Thus the two required soluble pairs and the two required insoluble pairs have been identified.

Step 1 : Understand what “recycling” means

Recycling is the process in which a waste material is collected, cleaned and converted back into a usable raw material so that it can be made into new things instead of being thrown away or burnt.

Step 2 : Decide how you will collect information

1. Read the date-wise pages of an old newspaper and cut out any article that talks about waste management or recycling.
2. Look at magazines for advertisements that carry the recycle symbol (three chasing arrows).
3. Talk to elders such as parents, shopkeepers, rag-pickers or the person who collects garbage from your home; ask which things they separate for sale to the kabadiwala (scrap dealer).
4. Use the Internet (with an elder’s help) to check government or NGO websites that list recyclable materials.

Step 3 : Make a rough list in your notebook

Write the material first, then give one or two examples of objects made of that material. Also note the recycling code number if it is printed on the object. For plastics the code is often inside a small triangle, e.g. the code “1” stands for $$\mathrm{PET}$$.

Step 4 : Arrange the data neatly in a table

Step 5 : Verify with local practice

Ask your neighbourhood scrap dealer which of the above materials he actually buys. Note any differences—for example, in many towns mixed or dirty plastic bags (code 7) are not accepted for recycling.

Step 6 : Present your findings

You can submit your work as a small project report. Attach photographs of the recycle symbols you found, paste the newspaper clippings and include the above table. End the report with two or three sentences on why recycling saves energy and reduces pollution.

Step 1 – Planning the survey

• I visited three local scrap-dealers (kabadiwala A, B and C).
• I carried a prepared list of common household wastes: newspapers, magazines, iron utensils, aluminium cans, copper wire, plastic bottles, plastic toys, broken glass bottles, ceramic cups, earthen pots, thermocol plates and used batteries.
• I asked two questions to every dealer:
  “What property do you look for before buying this item?” and “Will you take it? If not, why?”

Step 2 – Recording the observation

The table below records what each recycler checked. The middle three columns show whether the dealer judged the item by its appearance/lustre, by a magnet test, or by weight on a balance. Prices are noted in Indian Rupees per kilogram (₹/kg).

Step 3 – Analysing the properties they value

1. Ability to be recycled easily – Metals such as iron, aluminium and copper melt and can be re-shaped, so dealers pay good rates.
2. High mass compared to volume – Heavier scrap means more material for the smelter and better profit.
3. Lustre / colour – Copper’s reddish colour and aluminium’s silvery shine help them identify the metal quickly.
4. Magnetic property – A simple magnet separates iron/steel (magnetic) from non-magnetic metals; this saves sorting time.
5. Clean and dry condition – Wet or food-stained paper/plastic reduces quality and increases weight artificially; hence they refuse or pay less.
6. Recycling code on plastics – Codes 1 ($$\mathrm{PET}$$), 2 ($$\mathrm{HDPE}$$) and 5 ($$\mathrm{PP}$$) are accepted; codes 3, 6 and 7 are often rejected.

Step 4 – Materials all three recyclers refused

• Thermosetting or foamed plastics – e.g. thermocol (foamed $$\mathrm{PS}$$) and Bakelite parts – cannot be remelted.
• Ceramics and earthenware – very hard, already fired once; remelting them is impossible under normal recycling conditions.
• Hazardous / toxic items – used batteries, tube-lights, CFL bulbs; they contain lead, mercury or acids that need special treatment.
• Contaminated or wet paper/plastic – fungus growth spoils fibre quality; moisture adds useless mass.

Step 5 – Conclusion

Recyclers mainly look for useful physical properties such as metallic lustre, magnetism, weight and the recyclability symbol rather than the shape or condition of the object. Materials that cannot be re-melted, have no resale value or are hazardous are not bought. Thus the decision to buy is guided entirely by the intrinsic properties of the material, not by whether the object is broken or whole.

Step 1 – Making the list

Twenty-four common objects were collected from home and noted one below the other.

1. Steel spoon
2. Iron nail
3. Copper electric wire
4. Aluminium kitchen foil
5. Brass door handle
6. Glass tumbler
7. Window-pane glass sheet
8. Ceramic teacup
9. Earthen (clay) flower pot
10. Plastic water bottle (PET)
11. Plastic lunch-box (PP)
12. Polythene carry bag
13. PVC garden pipe
14. Wooden ruler
15. Wooden chair leg
16. Card-board packing box
17. Cotton hand-towel
18. Woollen scarf
19. Silk ribbon
20. Rubber band
21. Bicycle tyre tube
22. Marble pestle (stone)
23. Piece of charcoal
24. Bar of bath-soap

Step 2 – Listing the important properties of the materials

For a Class 6 comparison we note six simple properties.

• Lustre (shiny / dull)
• Hardness (hard / soft)
• Transparency (transparent / translucent / opaque)
• Ability to conduct electricity (good / poor)
• Water absorption (absorbs / does not absorb)
• Elasticity (stretchable / not stretchable)

These are the same properties introduced in NCERT Chapter 4 and recalled in Chapter 6.

Step 3 – Recording observations

Step 4 – Sorting them into groups

When the above list is examined we notice that objects made of the same broad kind of material share similar properties. They can therefore be grouped as follows:

Step 5 – Did the objects fall neatly into separate groups?

Yes. Every article could be placed in one and only one of the six groups above because the properties of the material are quite different from the properties of the other families. For example no plastic item showed metallic lustre or electrical conductivity; every metal item did so.

Step 6 – Relating a property with the use of an object

• The steel spoon must touch hot curry, so we need a material that does not melt easily and is strong; metal fits.
• Window glass must let light in; only a transparent material works, hence glass.
• Copper wire must allow electrons to flow; copper is a very good conductor.
• Polythene bag must survive rain; plastic is waterproof and flexible.
• Cotton towel must soak water; cotton fibres are highly absorbent.
• Rubber band must stretch and come back; rubber is elastic.
• Marble pestle must not wear away quickly; stone is very hard.

Thus the choice of material for every household object is closely linked with the special property required for that job. In short:

• Properties → Suitability → Use.

Conclusion

The exercise shows that classification by material properties is possible and useful. It helps us understand why a particular substance, and not some other, has been chosen by makers of the object.

Chapter 6 — Materials Around Us
Activity : Making and evaluating a useful article from waste

Remember : Your teacher will actually ask you to bring the object to school. This written solution only shows how you can do it step by step and how you may hold the discussion.

A. My object – A "3-in-1 Desk Organiser" (phone-holder + pen-stand + mini-notice-board)

1. Collecting discarded materials
   • 1 empty rectangular cardboard box (the box in which a mobile phone came)
   • 1 clean but dented aluminium soft-drink can (diameter ≈ 6 cm, height ≈ 10 cm)
   • 1 scrap piece of thick corrugated cardboard (≈ 15 cm × 10 cm)
   • Waste gift-wrap paper, small pieces of jute thread and two used ice-cream sticks
   • Fevicol / white glue, an old brush, scissors, old magazine sheet to protect the table
2. Categorising the materials (link to the lesson)
   • Cardboard → made mainly of cellulose fibres ⇒ $$\text{Natural origin (plant)}$$, opaque, rough
   • Aluminium can → metal $$\mathrm{Al}$$, shiny, opaque, good conductor
   • Gift-wrap (poly-coated) → plastic + paper composite, flexible, colourful
   • Jute thread → natural fibre, biodegradable
3. Tools required – old geometry-box ruler, pencil, compass (for circles), cello-tape.
4. Step-by-step construction
   1. Preparing the phone slot
      • Hold the mobile phone against the short side of the cardboard box.
      • Mark its width and depth with pencil.
      • Cut along the marked U-shaped outline so that a slot $$5\,\text{cm}$$ deep is formed.
      • The slot should be slightly tilted (≈ 15°) so that the phone’s screen faces you.
   2. Fixing the pen-stand
      • Rinse the aluminium can, dry it thoroughly.
      • With a geometry compass, punch two tiny holes $$2\,\text{cm}$$ below the rim for drainage (in case pens leak).
      • Apply glue on the outside of the can and wrap it with waste gift-paper.
      • Glue the can firmly to the right-hand rear corner inside the box using Fevicol and two support pieces cut from the ice-cream sticks.
   3. Making the mini-notice-board
      • Cut the corrugated cardboard into a neat rectangle $$12\,\text{cm}\times8\,\text{cm}$$.
      • Cover the front with another piece of coloured magazine paper.
      • Stick the board vertically on the back panel of the box.
      • Tie a loop of jute thread at the top so that tiny reminder-notes can be hung with a paper-clip.
   4. Finishing touches
      • Cover the entire visible surface of the outer box with the same gift-wrap so the organiser looks uniform.
      • Twist two strands of jute thread around the rim of the can for rustic decoration.
      • Let the glue dry overnight.
5. Functionality checklist

   Feature	Requirement	Test result
   Stability	No toppling when phone + 5 pens are placed	Pass
   Durability	Joints hold when organiser is lifted	Pass
   Finish	No sharp metal edges exposed	Pass

6. Safety note – File or fold the cut edge of the aluminium can; otherwise it can cut skin.

B. Classroom discussion and peer-feedback

General lesson – All projects showed that what we usually call "waste" can still be a useful resource. By classifying materials (plastic, metal, fibre, glass, etc.) we know their properties and can decide how best to reuse them.

C. Points to remember for improvement in future

• Plan the load-bearing parts in metal or thick cardboard; decorative parts can be in lighter material.
• Use water-proof coating (varnish, plastic-sheet) if the object will face moisture.
• Think of multi-function; for example the pen-stand also holds scissors, ruler and USB cable.
• Check every cut edge with your finger – if it hurts, sand or cover it.
• Keep a record of the time taken and the cost saved; this persuades others to up-cycle too.

Carry your organiser proudly to class — it cost almost nothing, saved several materials from the dustbin and proves the science you learnt in Chapter 6!