a student investigated the rate of reaction between sodium thiosulphate and dilute hydrochloric acid

the student placed a conical flask over a cross on a piece of paper

the student mixed the solutions in the flask

the solution went cloudy

the student timed how long it took until the cross could not be seen

the equation for the reaction is :

Na2 S2O3(aq) + 2HCl(aq) = 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

explain why the solution goes cloudy