Materials Around Us
Materials and Their Properties
Everything around us — a chair, a spoon, a window, your clothes — is made of some substance. The substance that a thing is made of is called its material. A single object can be made of more than one material (a pencil has wood and graphite), and the same material can be used to make many different objects (glass makes windows, bottles and bulbs).
We choose a material for a job based on its properties — the features that describe how it looks and behaves. Studying properties helps us understand why each material is used where it is.
- Appearance — whether a material is shiny (lustrous) or dull. Metals like gold, silver and freshly cut iron are shiny; wood, chalk and paper are dull.
- Hardness — materials that cannot be easily pressed, scratched or dented are hard (iron, stone, diamond); those that can be are soft (cotton, sponge, clay).
- Rough or smooth — how the surface feels to the touch.
Because diamond is the hardest natural material, it is used to cut glass and other hard substances. Because iron is hard and strong, it is used to build gates and tools. Choosing the right property for the right use is the heart of this chapter.
Shiny (lustrous) materials reflect light; dull ones do not.
- (a) Shiny — a metal such as silver or freshly cut iron.
- (b) Dull — wood, chalk or paper.
To cut a hard material you need an even harder one.
- Diamond is the hardest natural material.
- So it can scratch and cut glass, which is softer than diamond.
A pencil combines materials with different useful properties.
- Wood — light and easy to hold and sharpen.
- Graphite (the ‘lead’) — soft and leaves a mark on paper.
Key Points
- The substance an object is made of is its material; one object may use several materials and one material may make many objects.
- We pick materials by their properties: appearance (shiny/dull), hardness (hard/soft), rough/smooth.
- Hard materials resist scratching and denting; soft ones do not.
- The property decides the use — e.g. hard diamond cuts glass.
Solubility, Transparency and Floating or Sinking
Some properties of materials are best seen when we put them in water or hold them up to light.
Soluble and insoluble: A material that dissolves in water and seems to disappear is soluble (sugar, salt). One that does not dissolve is insoluble (sand, chalk powder). Liquids too can be soluble — lemon juice mixes fully with water (miscible), while oil does not and forms a separate layer (immiscible).
Floating and sinking: Some materials float on water (a piece of wood, a plastic bottle, a dry leaf) while others sink (a stone, an iron nail, a coin). Whether something floats or sinks depends on the material and its shape, not just its size.
Transparency: Based on how much light passes through them, materials are:
- Transparent — you can see clearly through them; almost all light passes (clear glass, water, air).
- Translucent — only some light passes, so things look blurred (butter paper, frosted glass, oiled paper).
- Opaque — no light passes through, so you cannot see through them (wood, metal, cardboard, stone).
These properties decide everyday choices: windows use transparent glass to let in light, while a door uses opaque wood for privacy.
Soluble materials dissolve and seem to disappear; insoluble ones remain.
- Soluble: salt, sugar.
- Insoluble: sand, chalk powder.
Transparent lets you see clearly; translucent only lets light through in a blurred way.
- Window glass — you can see clearly → transparent.
- Butter paper — light passes but images are blurred → translucent.
Liquids that do not mix stay in separate layers.
- Oil and water do not mix together.
- Such liquids are called immiscible.
Key Points
- Soluble materials dissolve in water (sugar, salt); insoluble do not (sand, chalk).
- Liquids that mix are miscible; those that do not are immiscible (oil and water).
- Some materials float, others sink, depending on the material and shape.
- Transparent (clear), translucent (blurred), opaque (no light through) describe how much light passes.
Grouping and Classification of Materials
There are thousands of materials around us. To study and use them more easily, we group or classify them — that is, we put together materials that share a common property.
Grouping things is something we do all the time: a shopkeeper keeps all the soaps together and all the biscuits together; a library keeps science books on one shelf and story books on another. In science we group materials by a chosen property, for example:
- By appearance — shiny materials in one group, dull in another.
- By hardness — hard materials together, soft materials together.
- By solubility — soluble together, insoluble together.
- By transparency — transparent, translucent and opaque groups.
Why do we classify? Grouping makes it easier to find things, to study their properties together, and to choose the right material for a job. The same material can fall into different groups depending on the property we choose — glass is in the ‘hard’ group and also in the ‘transparent’ group. So the way we classify depends on the property we are interested in.
Classification organises a large number of materials.
- It makes it easier to find and store things.
- It helps us study similar materials together and pick the right one for a use.
The group an object goes into depends on the chosen property.
- By hardness, glass is hard.
- By transparency, glass is transparent.
Test which dissolve in water.
- Soluble: sugar.
- Insoluble: iron nail, sponge, sand, glass slab.
Key Points
- Classification means grouping materials that share a property.
- We can group by appearance, hardness, solubility or transparency.
- Grouping helps us find, study and choose materials easily.
- A material can belong to different groups depending on the property chosen.