# Determining Boiling Point and Melting Point by Graph

This is the plot of $$\Delta G^{^\circ}$$ vs $$T$$ for $$\ce{Pb-> PbO}$$

I understand that the points where there is a sudden change in the slope of the graph, there is a phase change but exactly what are the reactions going at all the four points shown and also how can we justify whether it is a point of melting or boiling and what further can we conclude from this graph about entropy? Specifically, I am more concerned about the main reaction and what are the individual reactions at every point: $$\ce{Pb(s) + \frac{1}{2}O_2(g) -> PbO(s)}$$

• Have you looked up the melting and boiling points ? Jul 6, 2021 at 16:14
• What is the context and origin of the graph shown? Cite the source to add detail to your question, please. For the points shown, there may be phase transitions solid -> solid, solid -> liquid ($\approx \pu{328 ^\circ{}C}$), liquid -> gaseous ($\approx \pu{1744 ^\circ{}C}$), the later two one may look up, e.g., in Wikipedia's properties box about the element. Yet that is not all yet, as there may be indeed onsets of oxidation reactions. Note, lead and oxygen may e.g., yield $\ce{PbO}$, or $\ce{PbO2}$, or mixed oxides (like minium, formally Pb II and Pb IV are present simultaneously). Jul 6, 2021 at 16:14
• What do you report in ordinate ? Is it a Gibbs energy of formation ? Of which product ? Why do the points seem equidistant on the graph ? Jul 6, 2021 at 16:35
• @Buttonwood, Yes, but specifically only about PbO
– UNAN
Jul 6, 2021 at 16:39
• @Maurice, gibbs energy of formation of PbO
– UNAN
Jul 6, 2021 at 16:39