1.3 Factors Affecting Solubility
Imagine pouring a bunch of salt into a glass of water. Eventually you'll reach the point where no more salt will dissolve, and any more you add will simply sit on the bottom. Whenever a solute is dissolved in a liquid, a point in reached in which no more solute will dissolve. This is the point of saturation.
Saturation is an equilibrium system, which you'll remember is a dynamic state in which both the forward and reverse reactions continue but at equal rates. We can write a general equation that illustrates this:
Because a saturated solution is an equilibrium system, the same factors that affect a system at equilibrium will affect our saturated system. The two key factors to consider are:
- the effect of temperature and
- the effect of pressure on the solubility of gases.
The Effect of Temperature on Solubility
If you want to get more salt to dissolve in our glass of water, what would you do? If you heated the water you would be able to dissolve more salt.
Generally, increasing the temperature will increase solubility of solids and liquids. But increasing temperature will lower the solubility of gases (the gas will escape from solution, going back to the gas phase).
We can explain this using Le Châtelier's Principle. (See Unit III: Equilibrium Section 3-4) The dissolving process of molecular substances in the solid and liquid form is an endothermic process:
Adding heat will favor the endothermic direction; in this case favoring the product, or aqueous, side of the reaction.
The dissolving process of gases, however, is exothermic:
Adding heat to this system will again favor the endothermic reaction, which in this case is the reverse reaction. The substance will come out of the dissolved, aqueous state and form back into a gas.
The Effect of Pressure on the Solubility of Gases
The solubility of gases increases when the pressure above the gas is increased. In other words, more gas will dissolve when pressure is increased.
This can again be explained by Le Châtelier's Principle (See Unit III: Equilibrium Section 3-3). As our example we will examine a can of coke, or other carbonated beverage.
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Carbonated beverages contain carbon dioxide which is dissolved under pressure. Our equilibrium system is:
On the reactant side of the equation is one mole of gas; on the product side are zero moles of gas. Increasing pressure will favor the side with the fewest moles of gas - in this case the dissolved carbon dioxide.
When you open a can of pop, you increase the volume (the entire room versus the inside of the can), thereby decreasing the pressure. According to Le Châtelier's Principle, this will favor the side with the most moles of gas. And that's what happens - when you open a can of pop it begins to fizz - this is the CO2 coming out of solution. The "fizz" are the gas bubbles forming and popping.
Complete Assignment 2 before continuing on to the next section.
