Points to Ponder

The equation $T = t_c + 273.15$ and the assignment $T = 273.16\ \text{K}$ for the triple point of water are exact by definition. Because the triple point is now the single fixed point (it has a unique, reproducible temperature), the melting and boiling points of water at 1 atm are no longer exactly $0\ ^\circ\text{C}$ and $100\ ^\circ\text{C}$ — they are extremely close to those values, but not exact. In the original Celsius scale, those two points were the exact fixed points.

When a liquid and its vapour are in equilibrium, the pressure and temperature are uniform throughout the entire system. The two phases differ only in their molar volume (and therefore in density). This same principle holds for any system with any number of phases in equilibrium.

Heat transfer always requires a temperature difference between two systems or between two parts of the same system. If energy is transferred without any temperature difference playing a role, that transfer is not classified as heat. …