Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

FIGURE 11.11

Phase diagram for water (not to scale)

The curves AB, AC, and AD divide the diagram into regions that give combinations of temperature and pressure for which only one state is stable. Along any curve, the two states from the adjoining regions are in equilibrium.

The following is a dramatic demonstration of the effect of pressure on the melting of ice: Suspend a block of ice between two chairs. Then loop a length of wire over the block and place weights on the ends of the wire. The pressure of the wire will melt the ice under it. Liquid water will flow over the top of the wire and freeze again, because it is no longer under the pressure of the wire. As a result, the wire will cut through the ice and fall to the floor, but the block of ice will remain as one piece.

AB, dividing the solid region from the liquid region, represents the conditions under which the solid and liquid are in equilibrium.

S olid liquid

This curve gives the melting points of the solid at various pressures.

Usually, the melting point is only slightly affected by pressure. For this reason,

the melting-point curve AB is nearly vertical. If a liquid is more dense than the solid, as is the case for water, the melting point decreases with pressure. < The meltingpoint curve in such cases leans slightly to the left. In the case of ice, the decrease is indeed slight—only 1°C for a pressure increase of 133 atm. For most substances, the liquid state is less dense than the solid. In that case, the melting-point curve leans slightly to the right.