Materials Around Us · Lesson 3 of 7
Appearance, Lustre, Hardness, and Softness
“Use careful observation and simple tests to compare materials by shine, texture, hardness and softness.”
• Describe materials using visible features such as colour, texture and lustre. • Distinguish lustrous and non-lustrous materials while recognising that shine alone does not prove a material is a metal. • Explain why some metals may appear dull after exposure to air and moisture. • Distinguish hard and soft materials using compression and scratching. • Explain why hardness and softness are relative rather than absolute descriptions.
Appearance Gives Us Clues
Materials often look and feel different even before we perform any measurements. Colour, surface texture and shine are useful first observations. Freshly cut wood looks different from iron, while iron, copper and aluminium may share a metallic appearance that separates them from many non-metals.
A useful observation is specific. Instead of saying that a material 'looks nice', describe whether its surface is shiny or dull, smooth or rough, and note any colour or texture that helps distinguish it from other samples.
Lustrous and Non-Lustrous Materials
A material with a typically shiny surface is described as lustrous.
A material that does not have a shiny surface is described as non-lustrous.
Many metals, such as iron, copper, zinc, aluminium and gold, can show lustre. However, a metal surface may become dull after reacting with substances in air and moisture. This is why a freshly cut or freshly cleaned surface can sometimes look shinier than an old exposed surface.
Materials such as paper, wood, rubber and jute are commonly non-lustrous. Yet shine must be interpreted carefully. Polishing or coating can make a non-metallic surface look shiny. A thin layer of plastic, wax or another coating may reflect light even though the underlying material is not a metal.
Lustre is a useful observation, not a perfect test for identifying metals. A polished or coated non-metal can look shiny, and an old metal surface can look dull.
| Material or surface | Likely observation | Important reasoning |
|---|---|---|
| Freshly exposed metal surface | Often lustrous | Many metals reflect light strongly from a clean surface. |
| Old exposed metal surface | May appear dull | Air and moisture can change the surface. |
| Unpolished wood | Usually non-lustrous | Its surface does not usually show metallic shine. |
| Polished or coated non-metal | May look shiny | The shine may come from the coating rather than from metallic character. |
Investigating Appearance
A useful sorting investigation can use small, safe pieces of paper, cardboard, wood, chalk, copper wire, aluminium foil and objects made from common metals. Examine them under similar lighting. Record shine, texture and colour, then create groups. The important step is not simply making piles; it is stating the property used for each grouping.
Hard and Soft Materials
Appearance is only one way to compare materials. Touch and simple tests reveal another property: resistance to compression or scratching. A stone is difficult to compress with your hand, while an eraser or sponge changes shape more easily. A metal key can also scratch some surfaces more easily than others.
A material is described as soft when it can be compressed or scratched relatively easily compared with the material it is being compared against.
A material is described as hard when it is relatively difficult to compress or scratch.
The word 'relatively' matters. Hardness is not simply a permanent label with only two levels. Rubber is harder than a sponge but softer than iron. A comparison therefore needs a reference: harder than what? softer than what?
Problem
Material A is scratched by a metal key, material B is not. A sponge is compressed more easily than both. What can you safely conclude?
- 1.The key test shows that material A is easier to scratch than material B under the same test.
- 2.The sponge is easier to compress than both A and B.
- 3.You may describe A as softer than B for this comparison.
- 4.You should not claim that A is 'always soft' in every possible comparison because hardness is relative.
A Careful Hardness Investigation
To compare hardness fairly, use the same scratching object and similar pressure on each safe sample. Suitable classroom materials may include wood, aluminium, stone, iron, candle wax and chalk. Record whether each surface scratches easily rather than forcing the test so hard that every sample is damaged.
Another useful observation is whether an object can be compressed by hand. A pillow and sweater may deform easily, while a brick or rigid table does not. The investigation connects an object's feel with the material from which it is made.
| Object | Possible observation | Property focus |
|---|---|---|
| Brick | Difficult to compress by hand | Hard |
| Pillow | Compresses easily | Soft |
| Metal tumbler | Rigid under normal hand pressure | Hard |
| Sweater | Can be squeezed and deformed easily | Soft |
Quiz
A surface that normally appears shiny is described as what?
Why might an old metal object appear dull?
Which statement about a polished plastic surface is correct?
Rubber is harder than a sponge but softer than iron. What does this show?
Which test is most directly related to hardness?
Practice Problems
- Observe four safe materials and describe each using colour, texture and lustre.
- Explain why a dull-looking metal should not immediately be classified as a non-metal.
- Give an example of a non-metallic object that can look shiny because of polishing or coating.
- Arrange sponge, rubber and iron from softer to harder using the comparison given in the lesson.
- Design a fair scratch comparison using one scratching object and three samples. State what should be kept similar.
- Explain why 'hard' and 'soft' are better understood as relative descriptions.
Key Takeaways
• Appearance can be described using colour, texture and lustre. • Many metals are lustrous, but old metal surfaces may become dull and polished non-metals may also shine. • Shine alone is not enough to identify a metal. • Hard materials resist compression or scratching more than softer materials do. • Hardness and softness are relative properties that depend on what is being compared.