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In school we had a lesson about chemical equilibrium. We performed an experiment: in two tubes we poured $\pu{5 mL}$ of $\ce{H2O}$ and placed $\ce{CuSO4}$, the concentration of the $\ce{CuSO4}$ should have been $\pu{0.5 mol/L}$. We homogenize the two solutions in the two tubes and both solutions I think became blue (might have been green, not sure). Then we were asked to put a gram of $\ce{NaCl}$ in one of the tubes and homogenize. The color of the solution changed (also we might have added another $\pu{5mL}$ to the first solution, the one without $\ce{NaCl}$). Can someone tell me what do they think was the purpose of this lesson and how does it correlate to chemical equilibrium?

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closed as off-topic by Jan, Todd Minehardt, Jon Custer, bon, Mithoron Oct 27 '17 at 13:53

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When you mix $\ce{CuSO4}$ and $\ce{NaCl}$ solutions, the following equilibrium forms: \begin{align} \ce{[Cu(H2O)6]^2+ + 4Cl^- &<=> [CuCl4]^2- + 6H2O},& \Delta Η &= \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 ($\ce{CuSO4 + NaCl}$)

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