tags: #publish links: [[Chemistry]] created: 2021-10-30 Sat --- # Things we don't know about water http://m.nautil.us/issue/25/water/five-things-we-still-dont-know-about-water There are a surprising number of things that science is still trying to understand about water. Water is interesting complex, because of the intermolecular *hydrogen bonds* (polar attractions between positively and negatively changed ends of adjacent molecules) that give it its polar solvent properties, high boiling and freezing points for the molecule size, etc. This means that the bulk behaviour is different from the behaviour at the surface or at very localised scales involving individual molecules and hydrogen bonds, where you can't just average it all out. Even as a liquid, it makes something approaching a semi-regular tetrahedral lattice, at least close to freezing point. - Ice - there are 17 different known regular and irregular crystalline arrangements, e.g. a different temperatures and pressures, so far - is that all of them? - Liquid phase transitions - if any! There are definitely ice phase transitions but is there a liquid one too, at unusual pressures or temperatures near the ice transition? It's unclear whether there's a second liquid phase, and hard to study. - Evaporation mechanism and kinetics - how exactly do the molecules get free of the hydrogen bond given its strength, how does it relate to the surface behaviour, computing this at molecular level. - Surface has different acid/base properties because there are more "free" hydrogen bonds at the surface. So is it more acidic or more basic? This affects a lot of reactions, solvent effects, gas diffusion, enzyme kinetics, but we don't fully understand it yet. - Properties at nanoscale and when closely confined. Being about individual molecules, this gets into quantim mechanical territory so behaves differently from bulk water. (Note: there's also a lot of questionable fringe pseudo-science about water - the above is not that; these are all, rather obviously, legit unresolved mainstream questions in search of deeper understanding - rather than questionable beliefs in search of "proof" :D )