A student investigated the effect of concentration on the rate of reaction between sodium thiosulfate, \( Na_2S_2O_3 \), and hydrochloric acid, \( HCl \).
\( Na_2S_2O_3(aq) + 2HCl(aq) \rightarrow S(s) + 2NaCl(aq) + SO_2(g) + H_2O(l) \)
Since the solid sulfur product is insoluble, the rate can be determined by measuring the time it takes for the clear solution to turn off-white or pale yellow until the X mark on a white tile below the flask can no longer be seen.