Thermodynamics of Solid-Liquid

If you have an ice cube in water and the ice cube does not melt after reaching equilibrium with the water, does the water have temperature 0 degrees Celsius?




Yes, when ice is present at equilibrium with liquid water (at 1 atm), the system is at 0 °C. 


If you continue to remove heat from the water (such as in your freezer), the temperature of the system will remain at 0 °C until all the liquid (water) becomes solid (ice) after which, continuing to remove heat will result in a drop of temperature below 0 °C. Conversely, if heat is added to the system, the temperature of the system will remain at 0 °C until all of the solid (ice) has melted into liquid, after which, the temperature will increase above 0 °C. The amount of (latent) heat required to melt ice is called its *enthalpy of fusion* (334 J/g). This energy is converted into intermolecular bonds (a change in internal or potential energy of the system) rather than a change in kinetic energy (that is, a temperature change).


Note that at a much lower pressure (0.006 atm) and slightly higher temperature (0.01 °C), pure water will be in three states or *phases* (gas, liquid and ice). This is known as the *triple point* of water, as illustrated in the phase diagram of water, shown below:


![phase diagram of water](https://i.stack.imgur.com/PgHVN.jpg)


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