The same substance can be useful in one place and dangerous in another. Whether you can tell the difference depends on knowing what things are made of.
Melamine is light, strong and can be moulded into any shape, so it is used for bowls, cups and spoons. Nothing wrong with that.
In 2008, some manufacturers mixed melamine into milk powder. They did it because the chemical composition of melamine is similar to that of the proteins in milk — so a poor-quality powder tested as if it were high in protein, and health inspectors were fooled. Many babies who drank it suffered kidney problems, and some died.
Some recycling bins take paper, glass, plastics and metals separately. The blue bins in housing estates and schools take a mixture of them. Either way, sorting only works if you know what the item is made of — a coated paper cup is not the same as a sheet of paper, even if it looks similar.
E-waste — televisions, phones, batteries, hard disks, refrigerators — gets its own bin. These contain metals and other substances that are hazardous to people and the environment, so they must be disposed of properly and never simply thrown into a rubbish bin.
In Chapter 2 you classified matter by its physical properties — melting point, density, conductivity. This chapter classifies it by chemical composition: what it is built from. There are exactly three answers.
| Type | What it is | Lesson |
| Element | One kind of atom only. Cannot be broken down into anything simpler. | Lesson 2 |
| Compound | Two or more elements chemically combined, in a fixed proportion. | Lesson 3 |
| Mixture | Two or more elements and/or compounds not chemically combined, in any proportion. | Lessons 4–6 |