A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Basic concepts and calculations
CSCA Chemistry · Lesson 1 of 21Matter and its changes
La matière et ses transformations · المادة وتحولاتها
▶ Watch the video lesson (A7LAM students)
Chemistry starts by sorting matter: pure substances or mixtures, elements or compounds, and changes that are physical or chemical.
Key ideas
- Pure substances are elements (one kind of atom) or compounds (fixed composition).
- Mixtures, like air or salt water, can be separated by physical methods.
- Physical change: no new substance (melting, dissolving). Chemical change: a new substance (burning, rusting).
- States: solids keep their shape, liquids keep their volume, gases fill their container.
Formulas to know
- Melting, boiling, dissolving: physical changes
- Burning, rusting, neutralisation: chemical changes
- Mass is conserved in every change
Worked example
Which is a chemical change: melting ice, dissolving sugar or burning paper?
- Ice: still H₂O after melting → physical
- Sugar: still sugar in the water → physical
- Paper: new substances CO₂, H₂O and ash → chemical
Answer: Burning paper
Practice questions
1. Which is a compound?
O₂ · Air · NaCl ✓ · Salt water
NaCl has two elements in a fixed ratio.
2. Which is a mixture?
Distilled water · Iron · Air ✓ · Carbon dioxide
Air is mainly N₂ and O₂ mixed.
3. Which change is physical?
Iron rusting · Water boiling ✓ · Wood burning · Milk souring
Steam is still water.