Why Do We Sort Materials?
Look around your classroom right now. The chalk is separate from the duster, books are stacked apart from bottles, and the metal tap is nowhere near the wooden desk. This sorting isn’t random — we group things because materials that share properties behave similarly, and that makes life easier.
In science, sorting materials into groups (called classification) is one of the most fundamental skills. A doctor sorts medicines by their use. A builder sorts bricks from glass. And in Class 6, we sort everyday objects by their physical properties. Once you understand why we sort, the chapter becomes obvious.
The central idea: every material has a set of properties — appearance, hardness, solubility, transparency, whether it floats or sinks. These properties are what we use to group things. A single material can belong to multiple groups depending on which property we’re looking at.
Key Terms and Definitions
Material — any substance that objects are made from. Wood, plastic, iron, glass, cotton — these are all materials.
Property — a characteristic or quality of a material that we can observe or measure. Hardness, colour, lustre, transparency — these are all properties.
Lustrous — materials that have a shiny surface, like metals (gold, silver, copper). Most metals are lustrous. Non-metals like wood and plastic are generally non-lustrous.
Hard vs Soft — hardness is how much a material resists being scratched or compressed. Diamond is the hardest natural material. Sponge is very soft.
Soluble vs Insoluble — a material is soluble in water if it dissolves completely (salt, sugar). It’s insoluble if it doesn’t dissolve (sand, oil, chalk powder).
Transparent, Translucent, Opaque — three categories based on how light passes through:
- Transparent: light passes through completely; you can see clearly through it. Example: glass, clear water.
- Translucent: some light passes through but you can’t see clearly. Example: butter paper, frosted glass.
- Opaque: no light passes through. Example: wood, metals, cardboard.
Floats vs Sinks — whether an object stays on the surface of water or goes to the bottom. A piece of thermocol floats; an iron nail sinks.
How We Sort: The Main Properties
1. Appearance and Lustre
Hold a steel spoon next to a wooden stick. The spoon shines; the stick doesn’t. That shine is lustre, and it’s a property most metals have.
We can sort materials based on:
- Colour — red brick vs white marble vs black coal
- Texture — rough sandpaper vs smooth glass
- Lustre — shiny (copper wire) vs dull (rubber)
Lustre and colour can change over time — iron rusts and turns dull, silver tarnishes. But when we sort materials by lustre, we consider their natural state.
2. Hardness
Try this: can you scratch it with your fingernail? If yes, it’s soft. If not, try a coin. If the coin scratches it, it’s moderately hard. If nothing scratches it easily, it’s hard.
| Material | Category |
|---|---|
| Sponge, cotton, wax | Soft |
| Wood, plastic | Moderately hard |
| Iron, stone, glass | Hard |
Students often confuse heavy with hard. A big block of sponge can be heavy, but it’s still soft. Hardness is about resistance to scratching, not about weight.
3. Solubility in Water
Stir a spoonful of salt into water — it disappears. Now try the same with sand — it settles at the bottom. This is the difference between soluble and insoluble materials.
A few important points:
- “Dissolving” means the material mixes completely so you can’t see separate particles anymore.
- Some materials are partially soluble — they dissolve a bit, but not fully.
- Solubility depends on the liquid too. Mustard oil doesn’t dissolve in water, but it dissolves in some other solvents.
Add the material to water → stir well → observe after 5 minutes
- Clear solution = Soluble
- Cloudy or settled particles = Insoluble
- Slightly cloudy = Partially soluble
4. Transparency
This is about how light travels through the material.
Imagine holding different materials in front of a lamp:
- Glass sheet — you see the lamp clearly → Transparent
- Butter paper — you see a glow but not the lamp’s filament → Translucent
- Cardboard — no light at all → Opaque
We use this property practically all the time. Windows are transparent (so light enters). Bathroom windows are often frosted (translucent — light enters but privacy is maintained). Walls are opaque.
CBSE exams often give you a list of materials and ask you to sort them into transparent, translucent, and opaque. The tricky ones are water (transparent), oiled paper (translucent), and thin cloth (translucent to semi-opaque depending on weave).
5. Floating and Sinking
Place a wooden block and a metal spoon in a bucket of water. The wood floats; the spoon sinks. This depends on the density of the material relative to water (though at Class 6, we just say “floats” or “sinks” without using the word density formally).
Key observations:
- Most metals sink in water
- Most wood floats
- Wax floats
- Stones sink
- Rubber (small piece) sinks; rubber ball with air inside floats
A heavy object can still float if it’s shaped right — think of a ship. But in Class 6 sorting exercises, we deal with solid lumps of material, so shape effects are not asked about.
Solved Examples
Example 1 — Easy (CBSE Class 6)
Sort the following into lustrous and non-lustrous: gold, rubber, iron, wood, copper, coal.
Solution:
First, recall that metals are generally lustrous.
- Lustrous: Gold, Iron, Copper (all metals with shiny surfaces)
- Non-lustrous: Rubber, Wood, Coal (non-metals, dull surface)
Example 2 — Easy (CBSE Class 6)
A student tests four materials in water: sugar, pebbles, chalk powder, salt. Which are soluble?
Solution:
Test each mentally:
- Sugar → dissolves completely → Soluble
- Pebbles → settle at bottom → Insoluble
- Chalk powder → settles after some time → Insoluble (it appears to mix but doesn’t truly dissolve)
- Salt → dissolves completely → Soluble
Answer: Sugar and salt are soluble in water.
Chalk powder mixed in water looks white and milky, so students think it dissolved. It didn’t — if you leave it undisturbed, the chalk settles. A truly dissolved substance never settles.
Example 3 — Moderate (CBSE Class 6)
Classify these materials by transparency: clear glass, wood, butter paper, water, thick cardboard, oiled paper.
Solution:
Go through each:
- Clear glass → see through completely → Transparent
- Wood → no light passes → Opaque
- Butter paper → partial glow, can’t read through → Translucent
- Water → see through completely → Transparent
- Thick cardboard → no light → Opaque
- Oiled paper → partial light, slightly clearer than butter paper → Translucent
Example 4 — Moderate (CBSE Class 6)
A material is hard, non-lustrous, opaque, and insoluble in water. It sinks. Name a material that fits all these properties.
Solution:
Let’s match each clue:
- Hard → not soft like cotton or sponge
- Non-lustrous → not a shiny metal
- Opaque → no light passes through
- Insoluble → doesn’t dissolve in water
- Sinks → denser than water
Answer: Stone (or pebble). Stone is hard, has no lustre, is opaque, doesn’t dissolve in water, and sinks.
Example 5 — Higher Order (CBSE Class 6 Application)
Riya says “oil floats on water, so oil is lighter than water.” Is her reasoning correct? Can we test this in a solubility experiment?
Solution:
Riya’s observation is correct — oil does float on water. Her reasoning is informally correct at Class 6 level (oil is less dense, meaning a fixed volume of oil weighs less than the same volume of water).
In a solubility experiment: add a few drops of oil to water and stir. The oil forms droplets and doesn’t mix — it separates and floats. This shows oil is insoluble in water.
So two properties can be observed together: oil floats (doesn’t sink) and is insoluble (doesn’t dissolve).
Exam-Specific Tips
CBSE Class 6 Marking Pattern
This chapter is worth 3-5 marks in most school tests. Questions usually come as:
- “Sort the following list into Group A and Group B” (2 marks)
- “Give two examples of…” (1 mark)
- Short answer: “Why is glass used for windows?” (expecting: transparent, so light enters) (2 marks)
- Activity-based: describe what happens when you add X to water (1 mark)
Always give the reason in descriptive answers, not just the classification. Writing “Glass is transparent” scores 1 mark; “Glass is transparent, so light passes through it, which is why it is used for windowpanes” scores full marks.
Key pairs to memorise for MCQs:
- Lustrous ↔ Most metals
- Transparent ↔ Glass, water, clear plastic
- Soluble ↔ Salt, sugar, copper sulphate
- Floats ↔ Wood, wax, thermocol, cork
- Hard ↔ Iron, stone, diamond
Common Mistakes to Avoid
Mistake 1: Calling chalk powder “soluble” Chalk makes water look cloudy when stirred. Students mistake this for dissolving. True solubility means particles fully mix — no settling. Chalk settles. It’s insoluble.
Mistake 2: Mixing up translucent and transparent Translucent = light passes, but image is blurry. Transparent = light passes, image is clear. Butter paper is translucent. Window glass is transparent. Don’t swap them in answers.
Mistake 3: Saying iron is non-lustrous because it rusts Rusted iron is dull, yes. But iron before rusting is lustrous. The property refers to the natural state of the pure material.
Mistake 4: Thinking all heavy objects sink A ship is incredibly heavy but floats. At Class 6, we say “a solid lump of this material” sinks or floats. A solid lump of iron sinks; a solid block of wood floats.
Mistake 5: Giving only one example when the question says “two” In a 2-mark question asking “give two examples of transparent materials,” writing only “glass” gets 1 mark. Always count how many examples are needed and give exactly that many.
Practice Questions
Q1. Name one material that is: (a) transparent, (b) translucent, (c) opaque.
(a) Transparent: Glass / clear water (b) Translucent: Butter paper / frosted glass (c) Opaque: Wood / cardboard / metal sheet
Q2. Which of these float in water: wood, iron nail, wax, pebble, cork, copper coin?
Float: Wood, Wax, Cork (these are less dense than water) Sink: Iron nail, Pebble, Copper coin (denser than water)
Q3. Classify as soluble or insoluble in water: sugar, sand, salt, chalk, mustard oil, copper sulphate.
Soluble: Sugar, Salt, Copper sulphate Insoluble: Sand, Chalk, Mustard oil
Q4. A material is soft, non-lustrous, and floats on water. Give two examples.
Sponge (soft, non-lustrous, floats) and Cork (soft, non-lustrous, floats) are good answers. Cotton wool also works — it’s soft, non-lustrous, and floats briefly before absorbing water.
Q5. Why do we group materials? Give two practical reasons from daily life.
- To choose the right material for a purpose — e.g., we use transparent glass for windows because we need light to pass through.
- To store and organise things safely — e.g., medicines are kept separate from food; flammable materials away from heat sources.
Q6. A student claims: “All shiny things are metals.” Give one example that proves this wrong.
Mica is a non-metal mineral that has a shiny, lustrous appearance. So not all lustrous materials are metals. Similarly, freshly cut phosphorus has some lustre and is a non-metal. At Class 6 level, mica is the best example to give.
Q7. Kerosene is poured into water. What do you observe? Is kerosene soluble in water? Does it float or sink?
Kerosene does not mix with water — it forms a separate layer on top. So kerosene is insoluble in water. Since it stays on top, kerosene floats on water.
Q8. Fill in the blanks: Gold is ___ (lustrous/non-lustrous). Cotton is ___ (hard/soft). Salt is ___ (soluble/insoluble) in water. Cardboard is ___ (transparent/opaque).
Gold is lustrous. Cotton is soft. Salt is soluble in water. Cardboard is opaque.
FAQs
Q: Why is glass used for making windows and not wood?
Glass is transparent — light passes through it completely, allowing natural light into rooms. Wood is opaque; no light would enter. Some bathroom windows use frosted (translucent) glass to allow light but maintain privacy.
Q: Can a material be both soluble and float on water?
These are two separate properties, so yes, in theory — but the common example is tricky. If something dissolves, it becomes part of the water and can’t “float.” But a material like wax is insoluble and floats. For soluble materials that haven’t dissolved yet (like a sugar crystal just placed on water), the crystal may briefly float before dissolving. In practice, at Class 6, we test these separately.
Q: Is water transparent or translucent?
Clean, clear water is transparent — you can see through it clearly. Muddy water becomes translucent or opaque because of the particles in it. The property of the material (water) is transparency; the mud changes the mixture’s property.
Q: Why does oil float on water if oil is a liquid?
Floating and sinking applies to any material — solid, liquid, or gas — placed in water. Oil is less dense than water, so it floats. This is why oil spills spread on ocean surfaces.
Q: Are all metals hard?
No — sodium and potassium are metals that are soft enough to cut with a knife. But for Class 6, we say metals are generally hard. The exceptions (sodium, potassium) come in higher classes.
Q: What’s the difference between colour and lustre?
Colour is the hue we see (red, blue, white). Lustre is the shine — how much light reflects off the surface. A metal can be grey in colour but highly lustrous (like silver). Coal is black in colour and non-lustrous (dull surface).
Q: Why do some objects sink even though they’re hollow inside?
At Class 6, we sort solid materials, not hollow objects. A solid iron block sinks. A hollow steel ship floats because of the large amount of water it displaces relative to its weight — this is explained in higher classes using Archimedes’ Principle.
Q: How many groups can one material belong to?
As many as there are properties. Iron, for example, is: hard, lustrous, opaque, insoluble in water, sinks, and non-soluble. So depending on the grouping criterion, iron fits into multiple categories simultaneously. This is why we always ask “sorted by which property” before answering.