📌 heating & cooling curves (fig 14.2)
Heating curve (A → F): Solid (A) heated → temperature rises to melting point (B). During melting (B→C) temperature constant → energy used to overcome forces. Liquid heats (C→D) to boiling point (D). During boiling (D→E) temperature constant → energy breaks liquid→gas. After E, gas temperature rises.
Cooling curve (F → A): Gas cools to condensation point (E→D constant T), liquid cools to freezing point (C→B constant T), then solid cools.
Melting point = freezing point (same temp). Boiling point = condensation point.
14.2 Quick Check!
1️⃣ Why do water vapour at 100°C cause more severe burns than liquid water at same temp? Vapour has extra latent heat (condensation releases energy).
2️⃣ Why do we feel comfortable wearing cotton in summer? Cotton absorbs sweat, evaporation causes cooling.
Did you know? Phase change materials used in clothing/insulation. Ice melts under pressure (snowball maker). Ammonia liquefied at -34°C (atm pressure) or 1MPa at 25°C.
🧠 memorize the curve
➡️ On heating: solid heats (slope), melting plateau (B→C), liquid heats, boiling plateau (D→E), gas heats.
➡️ Plateaus = phase change, T constant, energy breaks bonds (potential energy ↑, KE constant).
➡️ Freezing point = melting point; condensation point = boiling point.
➡️ Water vapour at 100°C has extra latent heat (more burns).
📝 10 MCQs · heating curves & phase changes
📚 teacher planner · no-tech
📐 40-min lesson
starter (5m): show ice cube melting — why temperature constant?
input (12m): draw heating curve on board, label A→F. Explain plateaus.
activity (13m): students act particles: “solid” close & vibrate; “heat” → melt (move more); “boil” → run. mark plateaus.
plenary (10m): quick check questions + cotton & burns discussion.
🎲 class activity
• “human heating curve”: 7 students hold signs (A,B,C,D,E,F) and arrange. Class shouts temperature constant segments.
• using water, ice, thermometer (if available) observe melting plateau.
• discuss “why steam burns worse” – latent heat idea.
📏 student guidelines
✓ Phase change = constant T (energy breaks/forms bonds).
✓ Melting/freezing same temp; boiling/condensation same temp.
✓ Cotton helps sweat evaporate → cooling.
✓ Pressure can change melting point (ice under skates).
✓ Memorize curve shape: slope-plateau-slope-plateau-slope.