Experimental setup for measuring the approach to equilibrium for a saturated solution of CuSO4 in water.     Time-lapse video of solid CuSO4 dissolving in water (elapsed time = 8 hours)

Approaching Equilibrium:

Saturated solutions

When an ionic solid such as CuSO₄ dissolves in water, it dissociates into ions, as represented by this equation:

CuSO₄ (s) Cu²⁺ (aq) + SO₄^2- (aq)

Solutions of copper sulfate are blue, so the change in concentration of the ions can be detected visually -- as the concentration of the ions increases the solution becomes more blue.  When an unchanging color is reached, the solution has reached equilibrium.  The electrical conductivity of the ions can be measured at the same time.  As the amount of ions in solution increases, so does the conductivity of the solution.

In this time lapse video, copper sulfate is dissolved in water¹.  At first, the color changes rapidly as the solid copper sulfate dissolves.  Initially only the forward reaction takes place.  However, as the concentration of the ions begins to increase, the reverse reaction starts to increase in speed.  The result is that the net rate of solution drops.  This can be observed as the color change is very rapid at the start of the reaction, but it soon slows down.   The electrical conductivity shows the same effect.

As the reverse rate becomes faster², the net reaction becomes very slow, so that the color and conductivity no longer change much.  It takes several hours for the solution to reach an equilibrium³, but as shown in this graph, after about 6 hours there is almost no change in the conductivity of the solution.  At this point, since the observable properties are no longer changing, the reaction has reached equilibrium.

• A small amount of solid I₂ dissolves to become aqueous I₂ molecules.   

I₂(s) I₂(aq)

• These molecules then react with the water, to form iodate (IO₃^-) and iodide (I^-) ions.

3 I₂(aq) + 3 H₂O(l) IO₃^-(aq) + 5 I^-(aq) + 6H⁺(aq)

• The iodide ions then react with the I₂, to make the brown colored I₃^- ion.  It is actually this brown ion that gives the solution of iodine its color.

I₂(aq) + I^-(aq) I₃^-(aq)

However, the net result is the same, as there is an initial rapid rise in color, followed by a gradual leveling off and reaching a constant fixed color.  The conductivity, though much lower than for an ionic substance like copper sulfate, shows the same behavior.

¹The solubility curve was measured by adding a fixed quantity of solid copper sulfate (CuSO₄^.5H₂O) to 100 mL of solvent.  The crystals were approximately 4 to 5 mm in size (this size was selected to allow a reasonable time for equilibrium to be achieved).  The system was stirred at a constant rate, using a magnetic stirrer.  The electrical conductivity, in units of µS/cm, was measured using a Vernier conductivity probe attached to a CBL/TI-82 calculator.  In order to keep the conductivity at a measurable level, the solvent used was a 25% (v/v) solution of isopropanol in water.  This decreases the solubility of the copper sulfate somewhat from its value in pure water.

²The amount of solid present in the system was large enough that there should be only a minimal effect on the rate of the forward reaction because of a decrease in reactants.  Since very little of the solid dissolves, the surface area does not change very much.  Therefore, the forward reaction rate will not slow down with time.  In this case, the reason the two rates become equal is because of the increase in the reverse rate, rather than any decrease in the forward rate.

³The time required to reach equilibrium will depend on the rate of stirring, the temperature, and the surface area of the solid copper sulfate.