Sorting materials by their properties and how they change
Objects are made from many materials, and we group materials by their properties to make them easier to study and use. Some materials have lustre, a shine seen in metals like copper and gold. Some are hard and cannot be scratched easily, like iron, while others are soft, like a sponge. Materials may be transparent, like glass, translucent, like frosted glass, or opaque, like wood. Some are soluble in water, like salt and sugar, while others, like sand, are insoluble. Oil does not mix with water and floats on top.
Materials can also change. Some changes are reversible: ice melts into water and freezes again, and a stretched spring returns to shape. Other changes are irreversible: burning paper, cooking rice and making curd from milk cannot be undone. Heating can cause both kinds of change; melting wax is reversible, but burning it is not. Grouping materials and understanding their changes helps us decide how to use them.
Why it matters
Engineers and designers choose materials by their properties, using glass for windows, metal for utensils and plastic for containers. Knowing which changes are irreversible explains cooking and why burnt food cannot be restored. Sorting waste into recyclable materials saves resources. These ideas lead to the study of physical and chemical changes and states of matter.
Key terms
- Lustre
- The shiny appearance of a material, especially metals.
- Soluble
- Describes a substance that dissolves completely in a liquid such as water.
- Reversible change
- A change that can be undone so the original material is obtained again.
- Irreversible change
- A change that cannot be undone, such as burning or cooking.