I promised we’d talk about how dissolving something can be considered reaction, so here we go.

Remember what we were saying about metal ions in solution? How they aren’t stable on their own so they form complexes with anything that can donate electrons to them? Well, the same is true of almost everything. A molecule doesn’t just exist in a vacuum, unless it’s actually in a vacuum. If there are other things around, the total energy of the system is lower – that is, everything is happier and more stable – if they interact.

What happens is something called solvation. It’s easiest to visualise with water and something simple like salt, but the principle applies to all molecules in solution. Solvation is the term for the coordination of solvent molecules to something dissolved in them. No solvation, no solution – the molecule might be suspended in some water, but it isn’t dissolved in it without solvation.

Imagine a sodium ion. It’s a little round thing with a +1 charge, floating around in some water. It’s surrounded by water molecules, which are slightly negative at the oxygen end and slightly positive on the hydrogens. The water molecules are all moving around, but there’s a shell around the sodium ion where all the oxygen ends are pointing inwards. Those waters keep swapping places with the rest, they’re not static, but whichever waters find themselves around the ion will point towards it.

Likewise, imagine a chlorine ion in solution. It’s a big round thing with a -1 charge, so all the nearby water molecules have their hydrogens pointing towards it. The ions are solvated. When you drop a crystal of sodium chloride into water, the water molecules coordinate to the outermost ions in the crystal and pull them off into solution. No formal bonds are created and nothing officially changes, but there’s interaction between the electrons and that’s a reaction.Dece