Chemistry — Elements, Mixtures, Water & Air
1. Introduction to Chemistry
What Is Chemistry?
The branch of science that studies: what SUBSTANCES are made of, their PROPERTIES, and how they CHANGE and REACT.
Matter and Its Classification
- Pure Substance: Made of only ONE kind of particle. Cannot be separated by physical means.
- Mixture: Two or more substances PHYSICALLY mixed. Can be separated.
2. Elements, Compounds, and Mixtures
| Type | Definition | Examples | Separation |
|---|
| Element | Pure substance. Made of ONE kind of atom. Cannot be broken down. | Iron (Fe), Oxygen (O₂), Gold (Au), Carbon (C) | N/A |
| Compound | Pure substance. Made of TWO OR MORE elements CHEMICALLY combined in a FIXED RATIO. | Water (H₂O — always 2 hydrogen : 1 oxygen), Carbon dioxide (CO₂), Salt (NaCl) | Cannot be separated by physical means |
| Mixture | Two or more substances PHYSICALLY mixed. NOT in a fixed ratio. | Air, soil, salt water, salad | Can be separated by physical methods |
Separation Techniques
| Method | Used For | Example |
|---|
| Filtration | Separating insoluble solid from liquid | Sand from water |
| Evaporation | Separating dissolved solid from liquid | Salt from sea water |
| Sieving | Separating particles by size | Stones from flour |
| Magnetic separation | Separating magnetic from non-magnetic | Iron filings from sand |
| Sedimentation & Decantation | Settling heavier particles, pouring off liquid | Muddy water |
3. Water
Why Water Matters
- ~71% of Earth's surface is covered by water
- The human body is ~65-70% water
- ALL living things need water
States of Water
- Solid (ice), Liquid (water), Gas (water vapour/steam)
The Water Cycle (Hydrological Cycle)
- Evaporation: Water from oceans, rivers, lakes turns into WATER VAPOUR
- Condensation: Water vapour cools → forms CLOUDS
- Precipitation: Clouds release water as RAIN, SNOW, HAIL
- Collection: Water flows back to rivers, lakes, oceans
Potable Water
Water that is SAFE TO DRINK. Methods of purification:
- Boiling (kills germs)
- Filtration (removes solid impurities)
- Chlorination (adding chlorine to kill bacteria)
- Distillation (boiling and condensing — purest)
4. Air and Atmosphere
Composition of Air
| Gas | Approx. % |
|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon, CO₂, others | ~1% |
| Water vapour | Varies |
Properties of the Main Gases
- Oxygen: Supports COMBUSTION (burning) and RESPIRATION (breathing). Essential for LIFE.
- Nitrogen: Does NOT support combustion. Used by plants (through bacteria in soil). Dilutes oxygen — without nitrogen, everything would burn TOO FAST.
- Carbon Dioxide: Plants use it for PHOTOSYNTHESIS. Produced when we breathe out, when things burn. A greenhouse gas.
Air Pollution
- Causes: Factories, vehicles, burning fossil fuels, deforestation
- Consequences: Acid rain, respiratory diseases, global warming
