Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds — FAQ

It's a complex balance between two enthalpy changes: lattice enthalpy (energy to break the solid) and hydration enthalpy (energy released when ions are solvated). Both decrease down the group as the cation gets larger. For sulfates, the lattice enthalpy decreases more slowly than the hydration enthalpy, making the process less favourable down the group (less soluble). For hydroxides, the opposite is true, and the process becomes more favourable (more soluble). You are mainly required to know the trends rather than explain them in full detail at AS Level.

Polarising power is the ability of a cation to distort the electron cloud of an anion. It's highest for small, highly charged cations. Think of a small, dense positive charge (like $Mg^{2+}$) pulling the large, 'fluffy' electron cloud of an anion (like $CO_3^{2-}$) towards itself, distorting its shape.

The reaction of magnesium with water is exothermic. With cold water, the reaction is very slow and any heat generated dissipates. The product is magnesium hydroxide, $Mg(OH)_2$. With steam, the reaction is much faster and occurs at a high temperature. At this high temperature, any magnesium hydroxide that might form would thermally decompose into magnesium oxide and water: $Mg(OH)_2(s) \rightarrow MgO(s) + H_2O(g)$. So, the final product is the more thermally stable magnesium oxide, MgO.