Materials and classification of objects
A school bag holds a steel box, a plastic bottle, a paper book and a cloth cap. Everything we use is made of some material.
The material is what an object is made of: wood, metal, glass, plastic, cotton, paper. One object can be made of several materials, and one material can make many objects. We group objects by a property they share, such as how they look, feel or behave with water or light.
In the figure: A chair can be wood, metal or plastic; wood makes chairs, doors and pencils.
Rule 1: Object and material — Ask: what is it made of?
- Easy: A spoon can be steel, wood or plastic.
- Medium: Glass makes windows, bottles and spectacles.
- Exam-type: A bicycle uses metal, rubber and plastic together.
Rule 2: Grouping by a property — Choose one property and sort by it.
- Easy: Group by shine: shiny metals and dull wood.
- Medium: Group by water: things that float and things that sink.
- Exam-type: A different property gives a different grouping.
Name the property you are using to group, every time.
Thinking one object is made of only one material.
"Object, material, property, group."
Practice questions
- 1Any substance used to make an object is called a —
- 2Which of these is a material from which objects are made?
- 3A pen may be made of plastic, metal and ink. This shows that —
- 4Baked clay is called —
- 5In Activity 6.4, each ball is dropped from a fixed height to —
- 6The method of arranging objects into groups is called —
- 7Objects can be classified on the basis of —
- 8What decides the material chosen to make an object?
Show answers (try them yourself first)
- 1material
- 2Clay
- 3different parts of an object can use different materials
- 4terracotta
- 5compare the height of its bounce
- 6classification
- 7a common property
- 8Its properties and the purpose of the object
Materials and classification of objects
In the app you can move this figure with your finger.
Open in the appAppearance and hardness of materials
A steel tumbler gleams, while a clay pot looks dull; a nail scratches wood but wood cannot scratch a nail.
Metals look shiny and are said to have lustre; most other materials look dull. Materials may be rough or smooth, and hard or soft. Hardness is tested by scratching: the harder material scratches the softer one.
In the figure: Metals are shiny and hard; cotton and sponge are dull and soft.
Rule 1: Lustre and texture — Look and touch before you decide.
- Easy: A new coin and a copper vessel are shiny.
- Medium: Sandpaper is rough and a glass sheet is smooth.
- Exam-type: Dull does not mean weak: wood can be very hard.
Rule 2: Scratch test for hardness — Rub one material on another and see which gets a mark.
- Easy: An iron nail scratches wood, so iron is harder.
- Medium: A fingernail scratches chalk, so chalk is softer than the nail.
- Exam-type: Soft materials like cotton and sponge squeeze easily.
Test hardness gently on a hidden spot so the object is not spoiled.
Saying that every shiny thing is a metal.
"Metals shine; the harder scratches the softer."
Practice questions
- 1Why do some metals look dull?
- 2Which group contains only metals?
- 3Which material is softer than iron?
- 4In the sorting challenge, which material would shine when light falls on it?
- 5Materials that typically have shiny surfaces are called —
- 6Materials with lustre are usually —
- 7Materials that can be compressed or scratched easily are —
- 8Which of these is easily compressed?
Show answers (try them yourself first)
- 1Air and moisture affect them
- 2Iron, copper, aluminium
- 3Rubber
- 4Copper wire
- 5lustrous
- 6metals
- 7soft
- 8An eraser
Transparent, translucent and opaque materials
We see the road clearly through a clean window, see only a blur through a bathroom glass, and see nothing through a wooden door.
A transparent material lets almost all light pass, so we see clearly through it. A translucent material lets some light pass, so things look blurred. An opaque material lets no light pass and makes a shadow.
In the figure: Light passes fully through glass, partly through butter paper and not at all through wood.
Rule 1: Test with a torch — Shine a torch through each material and look from behind.
- Easy: Clear glass and clean water are transparent.
- Medium: Butter paper and frosted glass are translucent.
- Exam-type: Wood, metal and cardboard are opaque.
Rule 2: Uses and shadows — Choose the material to match the job.
- Easy: Windows use transparent glass; bathroom windows use translucent glass for privacy.
- Medium: Only opaque objects make a dark shadow.
- Exam-type: A translucent object makes a faint shadow.
Decide by what you see through the material, not by its colour.
Believing that a coloured glass is not transparent.
"See clearly, see blurred, see nothing."
Practice questions
- 1Could Sheeta's brother see the children through a closed wooden window?
- 2Which material is the best choice for a window through which you want to see outside clearly?
- 3Materials through which things can be seen clearly are —
- 4Which of these is transparent?
- 5Materials through which things can be seen, but not clearly, are —
- 6Sheeta's brother saw the children through a glass window. The glass is —
- 7Butter paper is —
- 8Materials through which nothing can be seen are —
Show answers (try them yourself first)
- 1No, wood is opaque
- 2Transparent glass
- 3transparent
- 4Glass
- 5translucent
- 6transparent
- 7translucent
- 8opaque
Solubility in water
Sugar disappears in tea when stirred, but sand stirred in water settles at the bottom again.
A material that mixes completely with water is soluble and forms a solution; one that does not is insoluble. Salt and sugar are soluble; sand and chalk powder are not. Stirring and warm water help a solid dissolve faster. A given amount of water can dissolve only a limited amount of a substance.
In the figure: Salt dissolves and disappears; sand stays visible and settles.
Rule 1: Soluble or insoluble — Stir a spoonful in a glass of water and wait.
- Easy: Salt, sugar and lemon juice are soluble.
- Medium: Sand, chalk powder and sawdust are insoluble.
- Exam-type: A cloudy liquid that clears on standing contains an insoluble solid.
Rule 2: What speeds up dissolving — Change one thing at a time and compare.
- Easy: Stirring spreads the solid and speeds dissolving.
- Medium: Warm water dissolves more sugar than cold water.
- Exam-type: Fine powder dissolves faster than big crystals.
Wait a few minutes after stirring before you judge soluble or insoluble.
Thinking a dissolved solid has vanished; it is still in the water.
"Mixes and vanishes: soluble; settles: insoluble."
Practice questions
- 1Which gas dissolves in water and is important for aquatic animals and plants?
- 2In Activity 6.7, how many glass tumblers are used?
- 3Materials that completely disappear when mixed in water are —
- 4Materials that do not disappear even after long stirring in water are —
- 5Chalk powder in water is —
- 6Water plays an important role in our body because it —
- 7Each ready-made ORS packet is dissolved in —
- 8Which of these is soluble in water?
Show answers (try them yourself first)
- 1Oxygen
- 2Five
- 3soluble in water
- 4insoluble in water
- 5insoluble
- 6can dissolve a large number of materials
- 7a litre of water
- 8Sugar
Mass, volume and matter
A balloon full of air is harder to lift than the same balloon flat, and it fills space: air too has mass and takes up room.
Matter is anything that has mass and takes up space (volume). Solids, liquids and gases are all matter. Mass is measured with a balance in kilograms, and the volume of a liquid is measured in litres and millilitres with a measuring jar.
In the figure: A balance gives mass in kg; a measuring jar gives volume in mL.
Rule 1: Mass and volume are different — Heavy does not mean big.
- Easy: A stone is small but heavy; a pillow is large but light.
- Medium: Mass: how much matter; volume: how much space.
- Exam-type: Mass is in kg and g; liquid volume is in L and mL.
Rule 2: Is air matter? — Check both parts of the definition.
- Easy: A filled balloon takes up space, so air has volume.
- Medium: A filled ball weighs more than a flat one, so air has mass.
- Exam-type: Light, sound and heat are not matter: they have no mass.
To call something matter, check for both mass and volume.
Mixing up mass (kg) and volume (L).
"Matter: has mass, takes space."
Practice questions
- 1Which two properties are possessed by all materials?
- 2How many properties (guna) does Ayurveda use to describe matter?
- 3How heavy or light an object is can be measured in terms of its —
- 4The object that is heavier has —
- 5The correct way to write seven kilograms is —
- 6Two cubic metres is written as —
- 7The space occupied by water represents its —
- 8Anything that occupies space and has mass is called —
Show answers (try them yourself first)
- 1Mass and volume
- 220, in ten opposite pairs
- 3mass
- 4more mass
- 57 kg
- 62 m³
- 7volume
- 8matter