Sorting Materials into Groups — Class 6 Physical Science
'Not all materials are the same — sorting them by properties is the first step to understanding the material world.'
1. Introduction
Everything around us is made of materials — wood, metal, plastic, glass, cloth, paper, and more. Different materials have different properties. By studying these properties, we can sort materials into groups and choose the right material for the right job.
Why This Chapter Matters
- Choose the right material for the right purpose
- Understand why objects are made of specific materials
- Foundation for chemistry and material science
- Practical: cooking, construction, daily life
2. Objects and Materials
An object is made from one or more materials.
| Object | Material(s) |
|---|---|
| Chair | Wood, plastic, metal |
| Spoon | Metal, plastic, wood |
| Window | Glass + metal/wood frame |
| Book | Paper (from wood pulp), cardboard |
| Tiffin box | Steel, plastic |
One material can make MANY objects:
- Wood: chair, table, door, pencil, bat
- Glass: window, mirror, tumbler, bulb
- Plastic: bottle, bucket, toy, bag, pipe
3. Properties of Materials
3.1 Appearance
-
Lustre (Shininess): Metals have a shiny surface (gold, silver, iron, copper)
- Non-metals: wood, plastic, paper — do not have lustre
- Exception: some plastics and stones can be polished to shine
-
Colour: Materials come in different colours
- Copper: reddish-brown
- Gold: yellow
- Iron: grey
- Wood, paper: brown/white
3.2 Hardness
| Property | Meaning | Examples |
|---|---|---|
| Hard | Difficult to scratch, does not bend easily | Iron, steel, stone, diamond |
| Soft | Easy to scratch, bends or compresses | Cotton, sponge, rubber, clay |
Testing: Try to scratch one material with another. If A scratches B, A is harder than B.
3.3 Solubility
Soluble: Dissolves in water — salt, sugar, alum Insoluble: Does NOT dissolve in water — sand, chalk, oil, wood
| Substance | Solubility in Water | Observation |
|---|---|---|
| Salt | Soluble | Disappears, water tastes salty |
| Sugar | Soluble | Disappears, water tastes sweet |
| Chalk powder | Insoluble | Settles at bottom |
| Sand | Insoluble | Settles at bottom |
| Oil | Insoluble | Floats on top |
| Alum | Soluble | Disappears |
'Not everything dissolves in water. Salt and sugar dissolve; sand and chalk do not.'
3.4 Flotation
Floats: Stays on the surface of water Sinks: Goes to the bottom
| Material | Float/Sink | Why? |
|---|---|---|
| Wood | Floats | Less dense than water |
| Thermocol | Floats | Very light, trapped air |
| Iron nail | Sinks | More dense than water |
| Stone | Sinks | More dense than water |
| Oil | Floats | Less dense (lighter) |
| Plastic bottle (empty) | Floats | Trapped air inside |
Surprising fact: A steel ship floats because of its shape! Its hollow body traps air, making the overall density less than water.
3.5 Transparency
| Type | Light Passes? | Can See Through? | Examples |
|---|---|---|---|
| Transparent | Completely | Clearly | Glass, water, air, clear plastic |
| Translucent | Partially | Blurred | Frosted glass, tissue paper, butter paper |
| Opaque | Not at all | No | Wood, metal, stone, cardboard |
Transparent: ██ → you can read clearly
Translucent: ██ → you can see light but blurry
Opaque: ██ → you can see NOTHING (blocked)
Example Applications
- Transparent: Windows, spectacles, car windscreens
- Translucent: Bathroom windows, lampshades
- Opaque: Walls, doors, curtains
4. Classification Table
| Material | Lustre | Hardness | Soluble in Water | Float/Sink | Transparency |
|---|---|---|---|---|---|
| Iron | Shiny | Hard | No | Sink | Opaque |
| Wood | Dull | Hard | No | Float | Opaque |
| Glass | Shiny | Hard | No | Sink | Transparent |
| Plastic | May be shiny | Soft-hard | No | Float* | Varies |
| Chalk | Dull | Soft | No | Sink | Opaque |
| Salt | Dull | Soft | YES | Sink | Opaque |
| Sponge | Dull | Soft | No | Float | Opaque |
*Depends on type of plastic
5. Why Sorting Matters
Sorting helps us choose materials for specific uses.
Examples
| Use | Why That Material |
|---|---|
| Windows made of glass | Transparent (lets light in), hard |
| Pans made of metal | Hard, conducts heat |
| Raincoat made of plastic | Waterproof, flexible |
| Furniture made of wood | Strong, can be shaped, looks nice |
| Electric wires covered with plastic | Plastic is an insulator (no electric shock) |
| Spoons for cooking made of metal | Hard, does not melt with heat |
Sorting Helps Reduce Waste
- Biodegradable vs non-biodegradable (for waste management)
- Recyclable vs non-recyclable
- In AP: waste segregation in Vijayawada and Visakhapatnam
6. Common Mistakes — Fix Them Now
| # | Mistake | Correction |
|---|---|---|
| 1 | Thinking all shiny things are metal | Glass and some plastics can also be shiny |
| 2 | Calling something 'soluble' when it just mixes | Soluble means it dissolves COMPLETELY, not just gets wet |
| 3 | Thinking 'heavy' means it sinks | A heavy ship floats; a small nail sinks. Density matters, not weight. |
| 4 | Calling chalk soluble because it forms a white mixture | Chalk does NOT dissolve — it forms a suspension that settles |
| 5 | Forgetting that materials can have multiple properties | A material is described by COMBINATION of all its properties |
7. Exam Focus
Marks Blueprint
| Question Type | Marks | Topic |
|---|---|---|
| MCQ | 1 | Property identification |
| Fill in blank | 1 | Example of transparent/opaque |
| Short answer | 2 | Difference between soluble and insoluble |
| Short answer | 2 | Why does a steel ship float? |
| Long answer | 3 | Classification of materials by properties |
Quick Self-Test (5 Questions)
Q1: Give an example of a transparent material.
<details><summary>Answer</summary>Glass, water, air, clear plastic.</details>Q2: Why does an iron nail sink but a ship float?
<details><summary>Answer</summary>The ship's hull traps air, reducing overall density. A nail is solid metal with no trapped air.</details>Q3: Name a material that is soluble in water.
<details><summary>Answer</summary>Salt, sugar, alum.</details>Q4: What property makes metals suitable for making jewellery?
<details><summary>Answer</summary>Lustre (shiny appearance) and malleability (can be shaped).</details>Q5: Classify butter paper as transparent, translucent, or opaque.
<details><summary>Answer</summary>Translucent — allows some light through but you cannot see clearly.</details>8. Chapter Summary
- Materials are classified by their properties
- Key properties: appearance (lustre, colour), hardness, solubility, flotation, transparency
- Soluble vs insoluble: dissolves in water or not
- Float vs sink: depends on density, not weight
- Transparent/translucent/opaque: based on light passage
- Sorting helps: choose right material, reduce waste
'By understanding the properties of materials, you can make smarter choices — from buying kitchen utensils to building a house.'
