Hi,

If you think about pressure, it is an unyielding force that wants to crush everything to as small/dense as possible.

Normally this means that if pressure had its way (ie high pressure), it would crush down all the way to the most dense state - a solid.

However, this assumes that solids are more dense than liquids.

For CO2, this is a reasonable assumption, as solid CO2 is more dense than liquid CO2. This way, solid CO2 is favoured at high pressure and you get that line with a positive slope.

For H2O, this is not actually the case. H2O is more dense in its liquid form than its solid form (ice). As a result, higher pressures can actually favour the formation of liquid water - that's why you get this negative slope, the high pressure allows the liquid water to overcome these lower temperatures that it should freeze at otherwise.

Hope this helps!

Katt