Final answer:
The freezing level at the surface on a phase diagram is depicted by the solid-liquid equilibrium line, representing the temperatures and pressures where ice and liquid water are in balance and illustrating the unique behavior of water's melting point decreasing with increased pressure.
Step-by-step explanation:
On a phase diagram, which is used in thermodynamics to show the states of matter of a substance at different temperatures and pressures, a freezing level at the surface would be depicted by the solid-liquid equilibrium line. This line, often labeled as BD on the diagram, represents the temperatures and pressures at which ice and liquid water are in equilibrium, signifying the melting/freezing points. The phase diagram of water on an expanded scale (Figure 10.6.2) illustrates that the melting point for water slightly decreases as pressure increases, shown by the boundary between ice and water having a negative slope. Contrastingly, most substances exhibit an increase in melting point with increased pressure. This property of water has important implications in natural phenomena like glacial movement, where the pressure at the bottom of a glacier can cause some of the ice to melt, allowing the glacier to slide over this layer of liquid water.