AL

chemistry term 3 2025 science test

🧪 Chemical vs Physical Changes

  • Physical Change

    • No new substance is formed

    • Usually reversible

    • Change in state or form (e.g., melting, dissolving)

    • Example: Ice melting

  • Chemical Change

    • New substance is formed

    • Usually not reversible

    • Involves chemical reactions (e.g., rusting, burning)

    • Example: Baking a cake

🔍 Physical Change

  • No new substance

  • Can often be reversed

  • Involves mixtures or changes in state

  • Example: Dissolving salt in water

🧬 Chemical Change

  • A new substance is formed

  • Cannot be easily reversed

  • Involves chemical reactions

  • Example: Combustion (burning wood)

🌡 Changes in State

  • Solid → Liquid = Melting

  • Liquid → Gas = Boiling/Evaporation

  • Gas → Liquid = Condensation

  • Liquid → Solid = Freezing

  • Solid → Gas = Sublimation

  • Gas → Solid = Deposition

🌡 Temperatures for State Changes

  • Water melts at 0°C

  • Water boils at 100°C

  • Every substance has its own melting and boiling points

⚛ Particle Model (Solid, Liquid, Gas)

  • Solid: Tightly packed, vibrate in place

  • Liquid: Loosely packed, slide past each other

  • Gas: Spread out, move quickly in all directions

🔍 Properties of Solids, Liquids, Gases

State

Shape

Volume

Flow

Compressible?

Solid

Fixed

Fixed

No

No

Liquid

No

Fixed

Yes

Slightly

Gas

No

No

Yes

Yes

🔥 Melting & Boiling Point

  • Melting Point: Temp at which a solid becomes a liquid

  • Boiling Point: Temp at which a liquid becomes a gas

🧪 Identifying Physical or Chemical Change

  • New substance? → Chemical change

  • Reversible? → Physical change

  • Examples:

    • Tearing paper = Physical

    • Burning paper = Chemical

📈 Reading & Drawing Graphs (5 Marks)

  • X-axis = Independent variable (what you change)

  • Y-axis = Dependent variable (what you measure)

  • Label both axes clearly

  • Use even spacing & connect points correctly

  • Look for trends in the graph

⚠ 5 Signs of a Chemical Change

  1. Colour change

  2. Gas produced (bubbles)

  3. Temperature change

  4. Precipitate forms (solid from liquids)

  5. Light or smell released

📘 Big Chemistry Words

  • Diffusion: Particles spreading out

  • Expansion: Volume increases

  • Contraction: Volume decreases

  • Exothermic: Releases heat

  • Endothermic: Absorbs heat

  • Spontaneous: Happens on its own

  • Combustion: Burning with oxygen

  • Corrosion: Slow damage (e.g., rust)

  • Equation: Chemical reaction written in symbols

  • Catalyst: Speeds up reaction without being used

  • Reactants: Substances you start with

  • Products: New substances made

  • Precipitate: Solid formed in a liquid reaction

💨 Fast vs Slow Evaporation

  • Fast Evaporation: High temp, wind, big surface area

  • Slow Evaporation: Low temp, still air, small surface

  • Both are physical changes

⚗ Writing Chemical Equations

  • Word Equation:
    Hydrogen + Oxygen → Water

  • Symbol Equation:
    2H₂ + O₂ → 2H₂O

  • Reactants = Left side

  • Products = Right side

🔬 Identifying Variables in Experiments

  • Independent Variable: What you change

  • Dependent Variable: What you measure

  • Controlled Variables: What you keep the same