3.2 Solubility Tables & the Solubility of Ionic Compounds
Solubility tables allow us to predict whether a certain ionic compound will dissolve in water at 25°C. While this is not as much information as we could gain from a solubility curve, we will use a solubility table for much of what we want to cover in the remainder of this unit. There are different versions of solubility tables, but we will be working with a very basic one.
You'll need to print a copy of a solubility table and have it handy while you read the following notes.
Reading the table is not difficult. Have a look at the first row of data in the table. It tells us that all negative ions (anions) when combined with any alkali ion (the positive cations from the first column of the periodic table - Na+, K+, Rb+, Cs+, Fr+) form a compound that is soluble in water. Some examples:
- Sodium chloride, NaCl, will be a soluble compound because Na+ is an alkali ion.
- K3PO4 will also be soluble because the cation (K+) is an alkali ion.
Similarly, the second and third rows tell us that any ionic compound containing the hydrogen ion, H+, or ammonium, NH4+, will also be soluble in water.
- Ammonium sulfate, (NH4)2SO4, will be soluble in water.
Two negative anions - nitrate, NO3-, and acetate, CH3COO- - will also always form soluble solutions.
- Cu(NO3)2 is soluble
- Pb(CH3COO)2is soluble
What about silver chloride, AgCl? We see in the next row that Cl- ions combine with Ag+ ions to form a compound with a low solubility. Thus, if we add some solid AgCl to water it will dissolve very poorly; most of it will remain as a solid and not dissolve.
You will see a list of cations that form compounds with a low solubility with chloride ions. A compound with a cation not in that list, however, will form a soluble compound. Magnesium chloride, MgCl2, would be soluble.
Be careful with cations that form more than one ion such as copper. Recall that copper can form the copper(I) ion, Cu+, or the copper(II) ion, Cu2+. Which of these is an insoluble compound, and which is soluble: CuBr, CuBr2?
| CuBr | low solubility (insoluble) - will not dissolve well in water |
| CuBr2 | soluble - will dissolve in water |
In general, a substance is considered soluble if it will form a solution with a concentration of 0.1 M or greater, and it is insoluble if it forms a solution at concentrations of 0.000 1 M or less.
Saturated Solutions & Equilibrium
It is important to remember that saturated systems are equilibrium systems. This means that the ions are in equilibrium with their non-dissolved state.
For example, the equation illustrating a saturated solution of silver sulfate would be:
Ag2SO4 (s) 2Ag+(aq) + SO42-(aq) It is also important that you remember that equilibrium means "equal rates" not "equal amounts". In other words, in a saturated solution at equilibrium you will not be able to detect any changes in the concentration of the ions because the solid form dissolves into ions at the same rate that the ions form back into a solid (crystallization). Equilibrium does NOT mean that the concentrations of reactants and products are equal.
In the next section we will take a more detailed look at the relationship between solubility and ion concentration in saturated solutions.
Try the practice questions before continuing to the next section.
