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??
Since you've posted this numerous times, I'm responding but I don't have a real answer for you.
First, I don't think four sample runs are enough to make profound conclusions. Second, I think the NaCl and KCl would have crystallized if they stood long enough because as the water evaporates some of the salt MUST come out of solution.
Third, crystal formation often depends upon the nature of the material as well as a seed to get it started.
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/DOWNSTREAM/form.htm
To explain the results of your experiment and understand why some substances formed crystals while others did not, you need to consider the solubility of the chlorides in water and the charges on the positive ions.
Solubility is a measure of how much of a substance can dissolve in a given solvent, in this case, water. Some substances have high solubility and readily dissolve, while others have low solubility and form crystals when the solution becomes saturated.
In your experiment, KCl and NaCl formed a saturated solution, meaning they reached their limit of solubility in water and could not dissolve any further. This indicates that these chlorides have high solubility and are more likely to remain in the solution rather than crystallizing.
On the other hand, MgCl2 and CaCl2 formed crystals because they have lower solubility in water. These chlorides reached their limit of solubility and were no longer able to stay dissolved, resulting in the formation of crystals. The lower solubility of these substances is likely due to the charges on their positive ions.
In a solution, ions separate from each other and become surrounded by water molecules. The attraction between the positive ions and the negative ends of water molecules helps keep the ions dispersed and dissolved. However, if the positive ions have higher charges, they attract more water molecules, reducing the availability of water molecules to dissolve additional ions. This leads to a decrease in solubility and an increased likelihood of crystal formation.
In your case, K+ and Na+ have only a single positive charge, while Mg2+ and Ca2+ have two positive charges. The increased charge of Mg2+ and Ca2+ ions means they attract more water molecules and have a lower solubility than K+ and Na+ ions, which leads to the formation of crystals.
Based on your observations, you can conclude that the chlorides with higher charges on the positive ions (MgCl2 and CaCl2) have lower solubility and are more likely to form crystals when the solution becomes saturated. Conversely, the chlorides with lower charges on the positive ions (KCl and NaCl) have higher solubility and are less likely to form crystals.
In summary, the charges on the positive ions of the chlorides influence their solubility in water, with higher charges leading to lower solubility and increased crystal formation.