# What is the upper pressure limit for solid to liquid phase border of water on 0.0 °C?

On phase diagram of water, solid to state border is a straight vertical line. Someone can argue it is only an approximation and nature cannot have vertical lines. For me however, it is clearly vertical. There exist no mention of this limit and it is not given a name. What is the name of this point, if such name exists, and what is the pressure value of it? If it is not measured, why? It seems like a very important point. Is there a measured, exact value of that point, not calculated or approximated?

In response to comments below (website does not reacts to my attempts to send a comment): I wrote 0.0 °C which is one-decimal digit. Anyone can see the line is absolutely straight in range that stretches at least from 10 mbar to approximately 10 bar which is a very big range. The upper limit of this range is not defined or named which is what my question is about.

• It is not clear to me what you are asking here. You have a link to the phase diagram of water, which clearly shows various key triple points. BTW, the ice Ih (ordinary ice) / liquid water fusion curve is not quite vertical. It slopes slightly to the left with increasing pressure. This sloping to the left with increasing pressure is highly unusual but is not unique. The fusion curve for most substances slopes to the right with increasing pressure. This sloping to the left is a consequence of the fact that ice floats. Jan 10 at 11:37
• "[...] and nature cannot have vertical lines." Where does that come from?
– noah
Jan 10 at 20:00

$$\frac{\Delta P}{\Delta T}=\frac{L}{T_\mathrm{m}\Delta v},$$
where $$\Delta P/\Delta T$$ is the change in pressure per unit temperature, $$L=334\,\mathrm{J}\,\mathrm{kg}^{-1}\,^\circ\mathrm{C}^{-1}$$ is water's latent heat of fusion, $$T_\mathrm{m}=273\,\mathrm{K}$$ is its melting temperature, and $$\Delta v=9.05\times10^{-5}\,\mathrm{m}^3\,\mathrm{kg}^{-1}$$ is its change in specific volume upon melting. The volume change $$\Delta v$$ is small, so a great deal of pressure is required to shift the melting temperature.