Chemistry · 12.4 Melting & boiling points · Paper 5/6
Heating & Cooling Curves. Test the purity.
Heat or cool a substance and follow temperature against time. A pure substance changes state at a sharp, constant temperature (a flat plateau); an impure one melts lower and over a range. Use this to assess purity.
0620 Topic 12.4 — Purity by mp/bp
heating & cooling curves
Paper 5/6 — Practical
20 °C
state solid
Variables
—
0%
Live readouts
Temperature
20 °C
State
solid
Melting point
80 °C
Purity verdict
—
During a change of state the temperature stays constant — the energy breaks the forces between particles, not raising the temperature.
Temperature vs time
📋 Method & ideas
- Put the crushed solid and a thermometer in a tube; stand it in a water bath for even heating.
- Heat gently and record the temperature every 30 s while stirring.
- Plot temperature against time. The flat region (plateau) is the melting point.
- A pure substance gives a sharp, flat plateau at the exact melting point.
- An impurity lowers the melting point and makes it melt over a range (a sloping plateau).
⚠ Precautions
- Use a water bath (not a direct flame) for even heating and safety with flammable organics.
- Stir continuously and keep the thermometer bulb in the substance, not touching the glass.
- Measuring melting/boiling point is a quick test for purity and identity.
🎯 Syllabus reference (0620)
- 12.4 Separation & purification — identify substances and assess their purity using melting point and boiling point information; pure substances have sharp, fixed melting and boiling points.