A researcher was examining rates of reaction. In which of the following scenarios would a colorimeter indicate a change in absorbance?

$Cu (s) + Na_2SO_4 (aq)$

$(CH_3)_3COH (aq) + K_2Cr_2O_7 (aq)$

$KBr (aq) + Cl_2 (aq)$

$HCl (aq) + NaOH (aq)$