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How did Joseph Black’s research explain why ice does not melt immediately above its freezing point?

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Joseph Black Joseph Black, detail of an engraving by J. Rogers after a portrait by Sir Henry Raeburn

Joseph Black’s research explained that ice does not melt immediately upon reaching its freezing point because it must absorb a specific quantity of heat—which he termed latent heat—to complete the phase transition from solid to liquid.1

Black demonstrated this through experiments comparing the warming rates of ice and water. He observed that while ice remained at its freezing point, it continued to absorb heat without an increase in temperature. By comparing this to the temperature rise of an equal quantity of water, he calculated that the heat absorbed by the ice during the melting process was sufficient to have raised the temperature of the same amount of liquid water by approximately 78 °C (140 °F).1

This discovery revealed that heat added during a phase change is consumed to overcome the forces holding the solid's structure together, rather than increasing the substance's temperature. This energy, known as the heat of fusion, is essential for the transition to the more disordered liquid state.213