Because the breakup of chemical bonds always absorbs energy, it cools the system down, so more sucrose molecules break apart and dissolve in the solution. So an increase in temperature causes the system to decrease energy, in an attempt to bring the temperature down. The crystallization process is explained by Le Châtelier’s principle, which states that a system that is shifted away from equilibrium acts to restore equilibrium by reacting in opposition to the shift. If we increase the temperature, we increase the dissolving process, and if we reduce the temperature, we decrease the dissolving process. In other words, the dynamic equilibrium is affected by a change in temperature. The only way to get all of that sugar to dissolve is to heat up the water, because increasing the temperature causes more sugar to dissolve in water. To make rock candy, we initially used more sugar than could dissolve in water at room temperature (three cups of sugar for one cup of water). This means that the size of the crystals stays the same, even though the sucrose molecules are constantly trading places between the solution and the crystals. In this case, the crystals and the solution are in dynamic equilibrium. So the sugar crystals cannot dissolve in the water anymore. This is what happens when the solution is saturated.Īs a result, past that point, if we add more sugar crystals, the process of dissolving will continue, but it will be exactly balanced by the process of recrystallization. In other words, the number of sucrose molecules leaving the crystals is the same as the number of sucrose molecules joining the crystals. However, the rate of dissolving is greater than the rate of crystallization-at least until the solution is saturated-so, overall, the sugar crystals remain dissolved in the water.Īs we add more granulated sugar to the solution, the rate of dissolving decreases and the rate of crystallization increases, so at some point, both rates are equal. The reason is that sucrose molecules are constantly moving in the solution, so nothing prevents some of them from binding again to sucrose molecules in the sugar crystals. You would also notice that some of the dissolved sucrose molecules are also crystallizing, that is, not only are sucrose molecules leaving the sugar crystals but other sucrose molecules are rejoining the sugar crystals, as well (Fig. Why is that so? If you were able to see the molecules of sucrose and water, you would notice that, in the beginning, when you add a small amount of granulated sugar to the water, most of the sucrose molecules are leaving the sugar crystals, pulled away by the water molecules. The additional solid just falls to the bottom of the container. At this stage, we say that the solution is saturated. If we add more than that amount, no more of that solid will dissolve. In general, only a certain amount of a solid can be dissolved in water at a given volume and temperature. The dissolving process involves two steps: First, the water molecules bind to the sucrose molecules and second, the water molecules pull the sucrose molecules away from the crystal and into the solution.
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