3. Heating & Cooling Curves

Heat a solid steadily and something surprising happens to its temperature. It rises — but then pauses while the substance changes state, even though you keep adding heat.

On a heating curve (temperature vs. heat added):

• The slanted parts are where the temperature rises (the solid, liquid, or gas warming up).
• The flat (horizontal) parts are where the substance is changing state (melting or boiling). The temperature stays constant because the added energy is breaking the bonds between particles instead of speeding them up.

The temperature where a solid melts is its melting point; where a liquid boils is its boiling point. For water these are 0°C and 100°C.

⚠️ Common misconception: temperature does not keep climbing while ice melts or water boils. It plateaus until the change of state is complete.

💡 Tip: the flat parts of a heating curve are changes of state; the slanted parts are warming.