What is Solubility?
The amount of solute needed to form a saturated solution at any particular temperature is the solubility of that solute at that temperature🌡️.
The solubility of one substance in another depends on:
Polar and ionic solutes tend to dissolve in polar solvents, and non-polar solutes tend to dissolve in non-polar solvents. (Remember “like dissolves like”!)
There are a few solubility rules that will be helpful on the AP Exam (though all but few of these are necessary to commit to memory):
Image Courtesy of Quizlet
💡You can easily notice that many of the same ions (Notably Ag, Hg2 2+, and Pb2+) are often exceptions to these solubility rules. However, do not try to memorize these completely!
These, like things like polyatomic ions, are memorized implicitly through usage! That's why by the end of AP Chemistry, most students don't think about charts like these, they simply know them through doing a bajillion problems.
Saturation of Solutions
Every solution, no matter the solute and solvent, has something called a saturation point. Essentially, this is the point at which no more solute can be dissolved in the solvent.
It depends on three things: the temperature, the solvent, and the solute. At a higher temperature, more solute can be dissolved (for the majority of solutes). This can be seen in the following solubility curve:
Image Courtesy of Dynamic Science
For example, looking at a saturated solution (note, saturated is important here, we'll get to that in a moment) of 100g H2O and KCl at approximately 70C, there will be around 50g KCl dissolved in the solution.
A saturated solution is a solution in which all possible solute has been dissolved in a solution. Once you have reached the saturation point, any additional solute (at that temperature) will fall out of solution and not dissolve. Essentially, with saturated solutions, you are on the solubility curve.
Undersaturated solutions are what their name implies - a solution that has not yet reached saturation. With an undersaturated solution, you can add more solute at the current temperature that still dissolves. Like saturated solutions, an undersaturated solution can be modeled using a solubility curve, representing a point below the solubility curve.
Supersaturated solutions are a type of solution where you have more solute than you can dissolve in the solvent. In order to accomplish this, a chemist heats up the solution and then adds enough solute to saturate it at that temperature, and then slowly cools it down to the temperature they need. This accomplishes a supersaturated solution that when agitated will produce crystals as the extra salt falls out of solution.
A supersaturated solution of CH3COONa, GIF Courtesy of Gyfcat
Factors Affecting Solubility
As mentioned before, polarity has a huge effect on solubility. Like dissolves like! Nonpolar substances are more likely to dissolve in nonpolar solvents and polar substances are more likely to dissolve in polar solvents.
Pressure only affects the solubility of gases. As pressure increases, solubility increases as well. This phenomenon explains why soda🥤 becomes flat overtime.
This can also be seen using Henry's Law: C = kP, or (solubility of gas in M) = (k Constant) (Partial pressure of [g] above solution). This equation is just an extra piece of information - not necessary for the AP.
For solids and liquids, pressure does not affect solubility.
For gases only: as temperature increases, the solubility of the gas decreases. As mentioned before, as temperature increases, the solubility of liquids and solids increase as well.
👉More about this: Unit 7 later in the course!