Heat curve for water ABCD
A heat curve for water ABCD is a graph of temperature versus heat added (or heat removed) at approximately constant pressure. The letters A–B–C–D label characteristic points where the physical state or the dominant energy process changes. A common ABCD convention shows ice warming below 0 °C, melting at 0 °C, and liquid water warming up to 100 °C.
Assumed ABCD labeling used in many general-chemistry diagrams
Point A: ice below 0 °C; point B: ice at 0 °C (start of melting); point C: liquid water at 0 °C (end of melting); point D: liquid water at 100 °C (boiling point at 1 atm).
Segment meanings and heat expressions
Sloped segments correspond to temperature change within a single phase and use the calorimetry form \(q = m\,c\,\Delta T\). Flat segments (plateaus) correspond to phase change at (nearly) constant temperature and use latent heat forms \(q = m\,\Delta H\).
| Segment | Physical process | Heat expression | Typical temperature range |
|---|---|---|---|
| A → B | Ice warming (solid water heated) | \(q_{AB} = m\,c_{\text{ice}}\,\Delta T\) | \(T_A \lt 0\,^\circ\mathrm{C}\) up to \(0\,^\circ\mathrm{C}\) |
| B → C | Melting at 0 °C (solid ⇄ liquid coexistence) | \(q_{BC} = m\,\Delta H_{\text{fus}}\) | \(0\,^\circ\mathrm{C}\) plateau |
| C → D | Liquid water warming | \(q_{CD} = m\,c_{\text{water}}\,\Delta T\) | \(0\,^\circ\mathrm{C}\) up to \(100\,^\circ\mathrm{C}\) |
Energy interpretation of slopes and plateaus
The slope on a heat curve for water reflects the specific heat capacity of the phase: a steeper rise in temperature per unit heat corresponds to a smaller \(c\), while a gentler rise corresponds to a larger \(c\). The melting plateau (B→C) stays near 0 °C because the added energy mainly disrupts the solid lattice and hydrogen-bond network rather than increasing the average kinetic energy of the molecules.
If the heat curve were extended beyond point D under the same pressure, a second plateau near 100 °C would represent vaporization with heat \(q = m\,\Delta H_{\text{vap}}\), followed by a sloped steam-heating segment.
Visualization of the ABCD heating curve
Reference quantities commonly paired with the curve
Numerical heat values depend on mass and the constants adopted for the temperature range, with widely used approximations including \(c_{\text{ice}} \approx 2.09\ \mathrm{J\cdot g^{-1}\cdot ^\circ C^{-1}}\), \(c_{\text{water}} \approx 4.184\ \mathrm{J\cdot g^{-1}\cdot ^\circ C^{-1}}\), and \(\Delta H_{\text{fus}} \approx 333\ \mathrm{J\cdot g^{-1}}\). Dimensional consistency is captured by \(q\) in joules when \(m\) is in grams.
Common pitfalls
- Axis interpretation: temperature is on the vertical axis, while heat added is on the horizontal axis; a plateau indicates heat input without temperature increase.
- Coefficient placement in formulas: the latent-heat region uses \(q = m\,\Delta H_{\text{fus}}\) rather than \(m\,c\,\Delta T\).
- Unit mismatch: grams versus kilograms and \(^\circ\mathrm{C}\) versus kelvin differences in \(\Delta T\) handling; temperature differences satisfy \(\Delta T(\mathrm{K})=\Delta T(^\circ\mathrm{C})\).