I did a project for lab chemistry where my aim was to see which substance, from a selection of 4 chlorides, recrystallises from a solution to form the most crystals. The 4 chlorides were KCl, NaCl, MgCl2 and CaCl2.
KCl and NaCl didn't form any crystals, they formed a saturated solution whereas MgCl2 and CaCl2 formed crystals. It seems as if the charges on the positve ion of the chloride are important here; but why? How do I explain my results? Conclusion??
What procedure did you follow? To get recrystallization you must saturate (or nearly saturate) a solution at a high temperature, then let is cool down to a lower temperature. You get the most recrystallization if the solubility at high temperature is much higher than at low temperature. You need graphs of solubility v. temperature for you four salts. The ones for NaCl and KCl can be found in many textbooks and at:
http://Galileo.phys.Virginia.EDU/education/outreach/8thgradesol/TempSolubility.htm
I have not found graphs or tables for CaCl2 and MgCl2.
Recrystallization is about is about solubility at different temperatures, not about ionic charge.
To explain your results in terms of the charges on the positive ions of the chlorides, we need to understand the concept of solubility and ionic compounds.
Ionic compounds consist of positively charged ions (cations) and negatively charged ions (anions). When an ionic compound is dissolved in water, the water molecules surround the individual ions and separate them from each other. This process, called hydration, allows the compound to dissolve and form a solution.
The solubility of an ionic compound depends on the strength of the attraction between its ions and the water molecules. In general, compounds that have strong attractions to water molecules will be more soluble, while compounds that have weaker attractions will be less soluble.
Now let's analyze your results in terms of the charges on the positive ions of the chlorides:
1. KCl and NaCl: These compounds both contain monovalent cations (K+ and Na+) that have a single positive charge. These positively charged ions are relatively small and have less charge density. As a result, the attraction between the cations and water molecules is not strong enough to overcome the energy required to separate the cations and anions from each other. Therefore, KCl and NaCl remain as saturated solutions without forming crystals.
2. MgCl2 and CaCl2: These compounds contain divalent cations (Mg2+ and Ca2+) that have a double positive charge. These ions are larger and have a higher charge density compared to monovalent cations. The stronger positive charge density creates stronger attractions between the ions and water molecules, allowing the ions to be more easily separated from each other. As a result, MgCl2 and CaCl2 form crystals when the solution becomes saturated.
Based on your results, the formation of crystals is favored by higher charge density of the positive ions. The stronger the attraction between the positive ions and water molecules, the more likely the compound will recrystallize from the solution.
In conclusion, the results of your experiment suggest that the charges on the positive ions of the chlorides play a crucial role in the formation of crystals from a solution. Higher charge density enables stronger attractions to water molecules, resulting in better solubility and the ability to recrystallize from the solution.