For the straight-line graphs in Figure 23-8, explain why 

(a) Breaks occur at the melting points and boiling points of the metals; 

(b) The slopes of the lines become more positive at these breaks; 

(c) The break at the boiling point is sharper than at the melting point.

Figure 23-8

A,G, kJ mol-1 -200 - 400 -600 -800 - 1000 - 1200 - 1400 | 0 2C+0 2 CO mp mp 500 bp bp 2 Zn + 02 ZnO 2 Mg + O 2 MgO 1000 1500 Temperature, C 2000 2500