The miscibility of a pair of liquids refers to the degree to which they mix spontaneously. A pair of partially miscible liquids mixes under some conditions but not at others. The phase diagram for a pair of partially miscible liquids
illustrates their behavior as a function of temperature and overall composition. For conditions corresponding to a point inside the curve, two phases are present (i.e., two layers are observed in the container). One phase is mostly substance
dissolved in it. The other phase is mostly substance
dissolved in it. Inside the two phase regions, the compositions of the two phases are determined by drawing horizontal lines (here shown dashed) to the red line (for the composition of the "
-rich" phase) and the blue line (for the "
-rich" phase). These are called "tie lines" or "levers". The lever rule is used to calculate the relative amounts of the two phases, which are represented on the bar graph on the right. The compositions of the two phases are indicated by dotted lines on the phase diagram and numerically at the top of the bar graph. Outside the curve,
mix spontaneously to form one layer. The highest temperature at which two phases form is called the upper consolute temperature and is indicated by a black dot on the phase diagram.