The phase diagram for substance A has a solid-liquid line with a positive slope, and that for substance B has a solid-liquid line with a negative slope. What macroscopic property can distinguish A from B?

As per the phase diagram, the positive slope between the pressure and temperature denotes that with an increase in temperature, the pressure increases for the co-existence of solid and liquid.

The effect of the pressure and temperature on the volume (or density) can be understood by this equation:

$\mathbf{V}\mathbf{=}\frac{\mathrm{nRT}}{P}$

Now, for a positive slope, with a more increase in temperature, there is less increase in pressure. It means that the volume of the liquid will be more than solid during the fusion of the compound. This means that the liquid will have less density than the solid.

Now, for a negative slope, with a more increase in temperature, there is less decrease in pressure. It means that the volume of the liquid will be less than solid during the fusion of the compound. This means that the liquid will have more density than the solid.

As during the phase transition, compound A have more solid density than that of the liquid whereas, for the compound B, the density of liquid is more than solid, the density (macroscopic property) can be used to distinguish between the two