2.0 SIMPLE CLASSIFICATION OF SUBSTANCES

How do we make sense of the millions of different materials around us? How can we separate a mixture back into its original parts? Why does ice melt, and why does salt make it melt faster? This chapter answers these questions by introducing the classification of matter—from mixtures to pure substances—and the kinetic theory that explains their behaviour. Mastering these ideas is essential before moving on to atoms and reactions.

2.1 SEPARATION OF MIXTURES

A mixture contains two or more substances not chemically combined. Because each component retains its properties, mixtures can be separated by physical methods. The method chosen depends on the physical properties of the components (particle size, magnetism, boiling point, solubility, etc.).

2.1.1 SEPARATING SOLID–SOLID MIXTURES

Both components are solids. We exploit differences in properties like particle size, density, magnetism, or ability to sublime.

• Sorting (handpicking): Physically picking out one component by hand or with forceps. Used when components are large enough and different in appearance (e.g., picking stones from rice).
• Sieving: Using a mesh (sieve) to separate particles of different sizes. The finer particles pass through the holes; the larger ones are retained.
  Example: Separating flour from bran, or pebbles from sand.
• Use of magnets: If one co