- General features
- Rules of solubility
- Rule 1
- Rule 2
- Rule 3
- Rule 4
- Rule 5
- Rule 6
- Rule 7
- Rule 8
- Final comment
- References
The solubility rules are a set of observations collected from multiple experiments that predict how sales will be or not soluble in water. Therefore, these apply only to ionic compounds, regardless of whether they are monatomic or polyatomic ions.
The solubility rules are very diverse, as they are based on the individual experience of those who develop them. That is why they are not always approached in the same way. However, some are so general and reliable that they can never be missing; for example, the high solubility of alkali metal and ammonium compounds or salts.
The solubility of sodium chloride in water can be predicted by knowing some simple solubility rules. Source: Katie175 via Pixabay.
These rules are only valid in water at 25ºC, under ambient pressure, and with a neutral pH. With experience, these rules can be dispensed with, as it is known in advance which salts are soluble in water.
For example, sodium chloride, NaCl, is the quintessential water-soluble salt. It is not necessary to consult the rules to know this fact, as daily experience proves it by itself.
General features
There is no fixed number for solubility rules, but it is a personal matter how they are broken down one by one. However, there are some generalities that help to superficially understand the reason for such observations, and may be useful to understand the rules even more. Some of them are the following:
- Monovalent anions or anions with a negative charge, and which are also bulky, give rise to soluble compounds.
- Polyvalent anions, that is, with more than one negative charge, tend to give rise to insoluble compounds.
- Bulky cations tend to be part of insoluble compounds.
As the rules are cited, it will be possible to see how well some of these three generalities are fulfilled.
Rules of solubility
Rule 1
Of the solubility rules, this is the most important, and it means that all the salts of the metals of group 1 (alkaline) and of the ammonium (NH 4 +) are soluble. NaCl obeys this rule, as does NaNO 3, KNO 3, (NH 4) 2 CO 3, Li 2 SO 4, and other salts. Note that here it is the cations that mark the solubility and not the anions.
There are no exceptions to this rule, so you can be sure that no salt of ammonium or these metals will precipitate in a chemical reaction, or will dissolve if added to a volume of water.
Rule 2
The second most important and infallible solubility rule indicates that all salts of nitrate (NO 3 -), permanganate (MnO 4 -), chlorate (ClO 3 -), perchlorate (ClO 4 -) and acetates (CH 3 COO -) they are soluble. From this it is predicted that Cu (NO 3) 2 is soluble in water, as well as KMnO 4 and Ca (CH 3 COO) 2. Again, this rule has no exceptions.
In this rule, the first generality mentioned is fulfilled: all these anions are monovalent, bulky and integrate soluble ionic compounds.
By memorizing the first two solubility rules, exceptions can be made for the ones that follow.
Rule 3
The salts of chlorides (Cl -), bromides (Br -), iodides (I -), cyanides (CN -) and thiocyanates (SCN -), are soluble in water. However, this rule does present several exceptions, which are due to the metals silver (Ag +), mercury (Hg 2 2+) and lead (Pb 2+). Copper (I) (Cu +) salts also make up these exceptions to a lesser degree.
Thus, for example, silver chloride, AgCl, is insoluble in water, as are PbCl 2 and Hg 2 Br 2. Note that here another of the aforementioned generalities begins to be seen: bulky cations tend to form insoluble compounds.
And what about fluorides (F -)? Unless they are alkali metal or ammonium fluorides, they tend to be insoluble or slightly soluble. A curious exception is silver fluoride, AgF, which is very soluble in water.
Rule 4
Most sulfates are soluble. However, there are several sulphates which are insoluble or sparingly soluble, and some of them are: BaSO 4, SrSO 4, CaSO 4, PbSO 4, Ag 2 SO 4 and Hg 2 SO 4. Here again the generality is observed that bulky cations tend to form insoluble compounds; except rubidium, as it is an alkali metal.
Rule 5
Hydroxides (OH -) are insoluble in water. But according to rule 1, all alkali metal hydroxides (LiOH, NaOH, KOH, etc.) are soluble, so they are an exception to rule 5. Likewise, the hydroxides Ca (OH) 2, Ba (OH) 2, Sr (OH) 2 and Al (OH) 3 are slightly soluble.
Rule 6
Leaving momentarily compounds derived from metals, all inorganic acids and hydrogen halides (HX, X = F, Cl, Br and I) are soluble in water.
Rule 7
In rule 7 several anions are brought together that agree with the third generality: polyvalent anions tend to give rise to insoluble compounds. This applies to carbonates (CO 3 2-), chromates (CrO 4 2-), phosphates (PO 4 3-), oxalates (C 2 O 4 2-), thiosulfates (S 2 O 3 2-) and arsenates (AsO 4 3-).
However, it is no longer surprising that its salts with alkali metals and ammonium are exceptions to this rule, as they are soluble in water. Likewise, Li 3 PO 4, which is poorly soluble, and MgCO 3 can be mentioned.
Rule 8
The last rule is almost as important as the first, and that is that most oxides (O 2-) and sulfides (S 2-) are insoluble in water. This is observed when trying to polish metals using only water.
Again, alkali metal oxides and sulfides are soluble in water. For example, Na 2 S and (NH 4) 2 S are one of those two exceptions. When it comes to sulfides, they are one of the most insoluble compounds of all.
On the other hand, some alkaline earth metal oxides are also soluble in water. For example, CaO, SrO and BaO. These metal oxides, together with Na 2 O and K 2 O, do not dissolve in water, but instead react with it to give rise to its soluble hydroxides.
Final comment
The solubility rules can be extended to other compounds such as bicarbonates (HCO 3 -) or diacid phosphates (H 2 PO 4 -). Some rules can be easily memorized, while others are often forgotten. When this happens, one must go directly to the solubility values at 25 ºC for the given compound.
If this solubility value is higher or close to that of a solution with a concentration of 0.1 M, then the salt or compound in question will be highly soluble.
Meanwhile, if said concentration has a value below 0.001 M, in that case it is said that the salt or the compound is insoluble. This, adding the solubility rules, is enough to know how soluble a compound is.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Wikipedia. (2020). Solubility chart. Recovered from: en.wikipedia.org
- Merck KGaA. (2020). Solubility Rules: Solubility of Common Ionic Compounds. Recovered from: sigmaaldrich.com
- Helmenstine, Anne Marie, Ph.D. (January 29, 2020). Solubility Rules of Ionic Solids. Recovered from: thoughtco.com
- The Bodner Group. (sf). Solubility. Recovered from: chemed.chem.purdue.edu
- Prof. Juan Carlos Guillen C. (nd). Solubility. University of the Andes.. Recovered from: webdelprofesor.ula.ve