Solubility

Solubility is a measure of a solute's ability to dissolve in a solvent. Different solutes have different solubilities. For example, sodium chloride has a solubility of 39 grams in 100 milliliters of water, while silver chloride is only 0.002 grams.

So why is this? Solubility depends on the physical and chemical properties of both the solute and solvent. You've seen this yourself. For example, you know that butter doesn't dissolve in water, but it does dissolve in olive oil.

The term 'like dissolves like' reminds us that a solvent dissolves a solute with a similar polarity. So polar solvents dissolve polar solutes, while non-polar solvents dissolve non-polar solutes. In addition, we can influence solubility through other factors like pH, temperature, and pressure.

For example, solubility typically increases with increasing temperature. Think about dissolving sugar in iced coffee versus hot coffee. Now, if a solute does dissolve in a solvent, only so much can be dissolved. When this limit is reached, the dissociated ions are in equilibrium with the solid and the solution is saturated. This means that if you add more of the solute, it won't dissolve.

We can quantify this equilibrium using an equilibrium constant K_c. This equation uses the concentrations of the dissolved products in the numerator and reactants in the denominator, each raised to the power of their stoichiometric coefficient. We can use this equilibrium constant to understand the thermodynamics of the dissolution process as it goes from its initial undissolved state to its final dissolved state.

The enthalpy, H, of a solution is a quantitative measure of the total heat content of the system, and ΔH describes the change in this heat content. If ΔH is positive, it indicates that the reaction absorbed heat, or is endothermic. When ΔH is negative, it indicates that the reaction releases heat, or is exothermic. Next, entropy, S, describes the degree of disorder in a system. ΔS is 0 for a reversible reaction, but is otherwise positive, as the disorder of a system prefers to increase.

Finally, the Gibbs free energy, G, is a measure of the energy that can be used to do work. It is calculated from ΔH and ΔS and is dependent on temperature. When ΔG is positive, the reaction is not spontaneous and energy must be put in for the reaction to proceed. When ΔG is less than 0, it indicates that the reaction is spontaneous.

These properties tell us a lot about how a solute dissolves in a solvent. For example, we can use enthalpy and entropy to learn whether or not the solute prefers to remain undissolved in its ordered crystalline form, or disordered in solution. And we can use Gibbs energy to learn whether or not we need to put energy in, via heat, to dissolve a substance.

In this lab, you'll explore the solubility of a compound at varying temperatures, and use titration to determine the exact concentration of the saturated solution. Then, you'll use your data to calculate the thermodynamic properties of the dissolution.