Heating curve (A → F): Shows internal energy vs. temperature as substance heated. Cooling curve (F → A): reverse process. Plateaus (flat lines) indicate phase changes where temperature remains constant.
| Segment | Process | Temperature Behavior |
|---|---|---|
| A → B | Solid heating | Temperature rises |
| B | Melting point | Start of melting |
| B → C | Solid → Liquid (melting) | Constant temperature (heat used to break bonds) |
| C → D | Liquid heating | Temperature rises |
| D | Boiling point | Start of boiling |
| D → E | Liquid → Gas (boiling/vaporization) | Constant temperature |
| E → F | Gas heating | Temperature rises |
| Segment | Process | Temperature Behavior |
|---|---|---|
| F → E | Gas cooling | Temperature falls |
| E | Condensation point (= boiling point) | Start of condensation |
| E → D | Gas → Liquid (condensation) | Constant temperature |
| D → C | Liquid cooling | Temperature falls |
| C | Freezing point (= melting point) | Start of freezing |
| C → B | Liquid → Solid (freezing) | Constant temperature |
| B → A | Solid cooling | Temperature falls |
Melting point: 0°C (ice → water) • Boiling point: 100°C (water → steam). Constant temperature during melting and boiling. Important: Steam at 100°C causes more severe burns than water at 100°C because steam releases latent heat of vaporization when condensing on skin.
• Phase change materials (PCMs) store/release thermal energy for clothing & building insulation.
• Phase changes can occur by changing pressure alone (e.g., ice melts under pressure in a snowball maker; ammonia gas liquefies at 1 MPa at 25°C).
1. Water vapor at 100°C causes worse burns than liquid water at same temperature because vapor releases additional latent heat when condensing into liquid on the skin.
2. Cotton clothes in summer – Cotton absorbs sweat and allows evaporation, which cools the body (evaporative cooling).
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