When there are weak interactions in the solid state, there is a low temperature miscibility gap.

Sketch free energy curves to find phase diagram

Look at how curves move with respect to each other when raising temperature. A liquid curve moves down. There may be a eutectic temperature.

Eutectic point features

Ag-Cu

Mixture melts at lower temperature

Water and salt

Add salt to water to melt at lower temperature

Lead tin

Water and air

Peritectic

Energy curves and phase diagram

Solidification

Generalize reactions

Peritectic upon cooling is a eutectic upon heating. Two phases turn into one or one turns into two. In a eutectic reaction, <math>\alpha</math> and <math>\beta</math> are formed, and <math>\gamma</math> is formed in a peritectic reaction. It is the same reaction, but the temperature axis is inverted. Eutectics involve liquids, and only solids are invloved in the eutectoid reactions.

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All pieces of the phase diagram have been discussed.

Recommendations

Get good at sketching graphs. Know what the anchor points are and whether lines go down. Consider solubility issues. It works systematically.