Solubility and Dissolution
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In this segment we’ll talk about DISSOLUTION and Solubility
We have two tasks in this segment. One task is to present a novel solubility guide in the shape of a logical decision tree, and the second task is to clarify the manner in which ionic compounds dissolve.
Compounds are considered soluble in water if they form a homogeneous mixture with water (solution). In this case, water is called the solvent, while the compound is called the solute.
The process of formation of the solution is called dissolution. Due to the ubiquity of water, most of the time water is not explicitly mentioned, i.e. the terms “solubility” and “dissolution” imply “in water”, unless other solvents are specified.
Based on the IUPAC definition, Ionic compounds are described as Insoluble, Slightly Soluble, or Soluble, according to the amount that can be mixed with water. Quantitative data for specific compounds can be gleaned from specialized tables. To reckon whether an ionic compound is expected to be soluble, people use general trends presented as “solubility charts” or “solubility guides”.
Along these lines I created a visual “decision tree solubility guide” to help with deciding whether an ionic compound is expected to be soluble, slightly soluble, or insoluble.
Starting with the formula of the ionic compound as input (green box), then we’ll decide whether the compound is going to be soluble or not. For example if it contains nitrate, or acetate ion or ammonium ion or group one ions, the compound is going to be soluble. If not, we go to the right, and see whether the compound contains one of the halogen anions chloride, bromide, or iodide, again, unless it contains the cations silver, lead two plus, or mercury one plus, then it is going to be soluble. If the compound contains those cations, it will be insoluble. If the compound does NOT contain the mentioned halogenide anions, we proceed to the right, to continue along the tree until we reach a conclusion. There are many common ions in the KembloX™ chart of ions not present in this solubility guide, and their absence means that their ionic compounds are probably insoluble.
I’ll illustrate the use of the decision tree solubility guide through two examples: Lead two iodide, and iron two carbonate.
For lead two iodide we see that it does not contain any of the always-soluble ions, so the next stop will be the halogen ions decision block and the answer is positive: the compound contains the ion iodide. However, lead two iodide contains the lead two ion, and therefore will be insoluble.
For iron two carbonate, on the other hand, the answer is negative for all the decision blocks, which leads us to an insoluble ionic compound.
The dissolution of the ionic compounds occurs through a simple process. When an ionic compound dissolves, the ions that make up the compound come apart in the process.
We’ll use KembloXTM (https://kemblox.org/description-of-the-kemblox-system/) to illustrate this process and try to help avoid common misconceptions encountered in the classroom.
When dealing with two-ion compounds, there is little room for confusion. A good example is table salt, sodium chloride, NaCl, whose KembloX™ model is shown.
As we see, the two ions, sodium ion and chloride ion, come apart as ions, i.e. they keep their charge. After coming apart, they move independently in the solution, and this process can be symbolically represented by a formula in which sodium chloride dissociates into two ions
NaCl -> Na+ + Cl–
For more complex ionic compounds, the number of imaginable dissociation patterns increases. For example, even for the three-ion magnesium chloride, whose formula is MgCl2, and whose KembloXTM representation is shown
, there are several possible dissociation patterns, each of them deemed acceptable by certain students, with only one pattern being the correct one.
In the first clip we see a pattern, in which only one of the chloride ions separates from the formula unit and there will be a magnesium chloride one plus combination and a chloride minus ion moving freely in solution. In symbolic form, the process looks like this:
MgCl2 -> MgCl+ + Cl–
In the second clip we see a pattern, in which the magnesium two plus ion and the two chloride minus ions separate from each other. However, in this pattern, the two chloride ions stay together and behave as one entity that moves through the solution.
The process can be symbolically represented as a dissociation resulting in one magnesium two plus ion and one chloride two, minus two ion:
MgCl2 -> Mg+2 + Cl2-2
Finally, in the third clip, we see a pattern in which magnesium chloride separates into three ions, each of them moving independently through the solution.
This can be symbolically represented as magnesium chloride dissociating into one magnesium two plus ion and two chloride minus ions
MgCl2 -> Mg+2 + 2 Cl–
Of all the patterns, only one is correct. The first and the second ones are not correct, and the third one IS correct and it is the only way in which dissolution occurs.
In summary we tried to make two points. One is that in order to assess the solubility of an ionic compound, one can use the KembloX™ chart and the solubility guide presented in this segment.
The second, more important, is that the dissolution of ionic compounds occurs such as to result in the maximum number of ions in solution.
This concludes another segment regarding the use of KembloX™ for modeling ionic compounds.