Interconversion of States

• Melting (fusion) — solid → liquid; freezing — liquid → solid.
• Evaporation/vaporisation — liquid → gas; condensation — gas → liquid.
• Sublimation — solid → gas directly (camphor, naphthalene, dry ice).
• Role of temperature and pressure; latent heat concept (introductory).

1. Melting point — temperature at which solid melts (ice: 0 °C at 1 atm).
2. Boiling point — liquid boils (water: 100 °C at 1 atm).
3. Latent heat of fusion — heat for melting without temperature rise.
4. Latent heat of vaporisation — heat for boiling/evaporation at constant T.
5. Pressure effect — higher pressure raises boiling point (pressure cooker).
6. Diagram (text) — heating curve: plateaus at melting and boiling points.
7. NCERT Ch. 1 — dry ice sublimes; wax melts on heating.
8. Real world — refrigeration cycle; snow sublimes in cold dry weather.

Why does temperature stay at 100 °C while water boils?

Step 1 — Heat supplied breaks intermolecular bonds (phase change).
Step 2 — Energy used as **latent heat of vaporisation**, not to raise temperature.
Step 3 — Thermometer stays at 100 °C until all liquid vaporised (in open container).
Step 4 — After boiling completes, temperature of steam can rise above 100 °C.
Conclusion: phase change absorbs heat at constant temperature.

• Thinking boiling and evaporation are identical (boiling at fixed T throughout liquid; evaporation at surface any T).
• Misconception: cold objects cannot sublime (sublimation depends on substance, not only heat).
• Confusing condensation with freezing.
• Forgetting deposition (gas → solid directly).
• Assuming melting always needs high temperature (ice melts at 0 °C).

• Define melting point and boiling point.
• What is sublimation? Give two examples.
• Why does a pressure cooker cook food faster?
• What is latent heat?

Open the Practice tab for graded questions on Interconversion of States.