How does adding sodium chloride to a solution of copper sulfate change the equilibrium?
When you mix $\ce{CuSO4}$ and $\ce{NaCl}$ solutions, the following equilibrium forms:
\begin{align}
  \ce{[Cu(H2O)6]^2+ + 4Cl^- &amp;&lt;=&gt; [CuCl4]^2- + 6H2O},&amp;
  \Delta Η &amp;= \text{positive}\\
\end{align}

Upon heating the mixture, the equilibrium shifts forward (following Le Chateliers principle) to produce more of the yellowish coloured $\ce{[CuCl4]^2-}$ ions, also known as tetrachlorocuprate (II) ion. As the amount of yellow colour in the mixture increases the solution becomes greener due to subtractive mixing of colours, i.e. blue and yellow solutions mix to give a green coloured solution.

The experiment can be found on YouTube, for example: Chemical Equilibrium ($\ce{CuSO4 + NaCl}$)