The rate of a chemical reaction depends on certain factors. Some of these factors include; surface area, temperature, nature of reactants etc.
The trial that exhibits the slowest rate of dissolution of CuSO4 crystals is trial 2 because the crystals have a small surface area since the crystals were large. Also, the solution was not agitated or stirred to increase the rate of collision between the water and the CuSO4 crystals.
Increase in temperature, agitation of the reaction solution and high surface area increases the rate of collision between the water and CuSO4 crystals leading to a faster rate of dissolution. This occurs in trial 3.
