📌 internal energy & quick check
Internal energy = total energy of a substance: kinetic energy (particle motion) + potential energy (bonding between particles). Heat increases internal energy.
Translational motion: movement of particles from one place to another (entire molecule changes position). Gases & liquids exhibit this; solids do not (only vibration).
Quick check 14.1 Q2: Is the average kinetic energy of particles of a gas and a liquid the same at the same temperature? ✅ YES — at same Kelvin temperature, average KE is equal for all substances (gas/liquid/solid).
🔄 melting · boiling · condensation
Melting (solid → liquid)
Heating increases vibration; at melting point cohesive forces overcome → solid collapses. Temperature constant until fully melted.
Boiling (liquid → gas)
Vapour pressure = external pressure; bubbles rapid. Heat used to break forces (temperature constant). Evaporation occurs at all temps.
Condensation (gas → liquid)
Cooling reduces KE; molecules come closer, attractions dominate → liquid formed. Temperature constant during transition.
🔹 Evaporation: molecules escape from surface at any temperature.
🔹 Boiling point: temp at which vapour pressure equals external pressure.
🔹 Melting point: temp where solid turns to liquid; vibrational motion overcomes cohesive forces.
🔹 During phase change, temperature remains constant → all heat goes into breaking bonds (potential energy).
🧠 memorize easily
Internal energy = Kinetic + Potential.
Constant temperature during phase change (melting/boiling/condensation).
Evaporation (surface) vs Boiling (bulk + bubbles).
At melting point: solid collapses.
➡️ Also remember: “Translational motion = moving from A to B” – gases & liquids have it; solids only vibrational.
✨ “Same temp → same average KE” (for any state).
📝 10 MCQs · internal energy & phase changes
📚 teacher · no-tech planner
📐 lesson (40 min)
starter (5m): “what makes ice melt faster?” – elicit particle ideas.
input (12m): explain internal energy (kinetic + potential). Use water example.
activity (12m): students act phase changes: solid (vibrate close), liquid (move apart), gas (run). Add ‘heat’ clap → change.
plenary (6m): quick quiz with mini whiteboards + discuss constant temp.
🎲 class activity
• “phase transition race”: groups represent ice, water, steam; on signal they change motion.
• draw particle arrangements before/after heating.
• use water bottles & ice (realia) if available – observe melting/condensation on cold surface.
• discuss why temperature plateaus during melting.
📏 student guidelines
✓ Internal energy = movement energy + bond energy.
✓ Same T → same avg KE (any substance).
✓ Melting/boiling: energy goes into breaking forces, not raising T.
✓ Evaporation (surface) happens at all T; boiling at boiling point.
✓ Condensation releases energy (exothermic opposite).