Pure Substances
Pure substances are made up of only one kind of particles and have a fixed composition. Examples include elements and compounds.
- Element: Cannot be broken down into simpler substances (e.g. gold, oxygen).
- Compound: Made up of two or more elements chemically combined (e.g. water, carbon dioxide).
Tip: Pure substances have consistent properties throughout the sample.
Mixtures
Mixtures contain two or more substances (elements or compounds) physically mixed together. Their composition can vary.
- Homogeneous mixture: Uniform composition (e.g. salt solution).
- Heterogeneous mixture: Non-uniform composition (e.g. sand in water).
Question: Is air a mixture or a compound?
Answer: Air is a mixture of gases (mainly nitrogen, oxygen, carbon dioxide, etc.).
Solutions
A solution is a homogeneous mixture of two or more substances. The substance present in larger amount is called solvent, and the one in smaller amount is solute.
- Example: Salt dissolved in water (water = solvent, salt = solute).
- Properties: Clear, does not scatter light, particles cannot be separated by filtration.
Concentration of solution: Amount of solute present in a given amount of solvent.
Suspensions
Suspensions are heterogeneous mixtures in which particles are large enough to settle down. Example: sand in water.
- Particles can be seen with naked eye.
- Can be separated by filtration.
- Scatter light (Tyndall effect).
Colloids
Colloids are mixtures where the particle size is intermediate between solutions and suspensions. Example: milk, fog.
- Particles do not settle down.
- Show Tyndall effect (scatter light).
- Cannot be separated by filtration.
Separation Techniques
Methods used to separate components of mixtures:
- Filtration: Separates insoluble solids from liquids.
- Evaporation: Removes liquid from solution to get solid.
- Centrifugation: Separates suspended particles from liquids.
- Distillation: Separates liquids based on boiling points.
- Chromatography: Separates components based on movement through a medium.
Question: Why do we use centrifugation?
Answer: To separate cream from milk or suspended particles from liquids.