 Hi students, welcome to the HSC Chemistry and Module 5 Equilibrium and Acid Reactions. This is video number 17 and we're going to just review and extend our understanding of dissolution of ionic compounds. We've already looked at the fact that an equilibrium can actually be set up by substances, ionic substances which dissolve in water. We've looked at the fact that in any ionic crystal, in a crystal array, you have alternating cations and anions which are equally distributed around one another in direct proportion to the simplest ratio between those two ions. So in this case, we've got a KCl ionic lattice and the potassium chloride is dissolving in water and you can see the little water molecules surrounding each of these. This model kind of describes a little bit about what's happening so you can see the way that the polar water molecule is oriented is in such a way as to attract the individual ions. So where the ion is a cation or a positive ion such as it is here, you can see that this is the slightly negative end of the water molecules that are attracted to the cation. Alternatively, you can also see that the slightly positive end of the water molecules are attracted to the anion. So this is how a crystal of a ionic substance is actually pulled apart by water molecules. Water's effect as a solvent is a result of its polarity. So we know that the water molecule is a bent molecule and there is a polarity created by a slightly negative charge due to the high electronegativity of the oxygen in comparison with the hydrogens and therefore a slight positivity in the region of where the hydrogens are. This polarity is what is doing all the attracting so the fact that the water molecule has a positive and a negative side means that and the fact that it's in liquid means that the molecules can rotate means they can orient the cells in such a way as to attract those individual ions.