Can you give an explanation of a sodium thiosulfate and hydrochloric acid solution?

Answer 1

When sodium thiosulfate (Na2S2O3) reacts with HCl the colorless solution turn opaque (yellow) because a precipitate of sulfur (S) forms.

Na₂S₂O₃ (aq) + 2HCl (aq) ==> 2NaCl (aq) + H₂O (l) + S(s) + SO₂(g)

This reaction is often times used to illustrate how the kinetics of a chemical reaction can be studied because you can time how long it takes for the yellow precipitate to form as a function of the concentration of the reactants.