Chlorides of Period 3 in water — and reading bonding off the evidence

Same period, wildly different behaviour

Drop five chlorides into water and you get five different stories. Sodium chloride just sits there and dissolves. Silicon tetrachloride hisses, gives off choking fumes and leaves a white solid.

The difference is how the chloride is bonded. Ionic chlorides simply come apart into ions. Covalent chlorides are attacked by water and give off hydrogen chloride.

chloride	what you see in water	equation	likely pH
$\text{NaCl}$	dissolves, no reaction	$\text{NaCl(s)} \rightarrow \text{Na}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)}$	about 7, neutral
$\text{MgCl}_2$	dissolves, very slight reaction	$\text{MgCl}_2\text{(s)} \rightarrow \text{Mg}^{2+}\text{(aq)} + 2\text{Cl}^{-}\text{(aq)}$	about 6–7, very slightly acidic
$\text{AlCl}_3$	white precipitate, fumes	$\text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl}$	about 2–3
$\text{SiCl}_4$	white solid, choking fumes	$\text{SiCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 4\text{HCl}$	about 1–2
$\text{PCl}_5$	steamy fumes, clear solution	$\text{PCl}_5 + 4\text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_4 + 5\text{HCl}$	about 1–2

Read the last column: the pH falls as you cross the period. Something is releasing more and more $\text{H}^{+}$ ions.