If you take 50 mL of ethanol and add it to 50 mL of water (you can do this in a lab if you have a 100 mL graduated cylinder) you will find, perhaps surprisingly, that the volume of the resulting solution is less than 100 mL, it is in fact about 98 mL, assuming good technique (no spilling). How can we explain this observation? Well, first we can reassure ourselves that matter has not been destroyed – if we weigh the solution, it weighs the same as 50 mL of water plus 50 mL of ethanol. This means that the density of the water:ethanol solution must be greater than the density of either the water or ethanol solutions alone. If we think molecularly, we can immediately deduce that in the ethanol and water mixture the molecules are closer together than they are when pure (before mixing). On the other hand, if you take 50mL of oil and 50 mL of water, we find that they do not mix – no matter how hard we try, they always separate away from one another in two layers. What factors determine whether substances will form solutions?

6.1 Solutions
6.2 Solubility
6.3 H-bonds
6.4 Free Energy
6.5 Polarity
6.6 Temperature

First, we need to be aware that solubility is not an all or nothing property. In the case of oil and water, if we were to look carefully, we would find that a small number of oil molecules are present in the water (the aqueous phase), while a small number of water molecules will be found in the oil. There are a number of ways to describe solubility; probably the most common is to define the number of moles of solute per liter of solution. This is called the solution’s molarity (M, mol/L). Another common way is to define the number grams of solute per mass of solution (for example: 1 mg (10^-3 g) of solute dissolved in 1Kg (10³ g) of solution is 1 part per million (10⁶) solute - 1ppm. As you might expect, given the temperature term in the free energy equation, solubility data are always reported at a particular temperature. At a given temperature, if no more solute will dissolve, the solution is said to be saturated (and of course if more solute could dissolve it is unsaturated)

Question to answer:

• How would you design an experiment to determine the solubility of a solute?
• How would you determine whether a solution was saturated?
  

Questions to ponder:

• You have a saturated solution, with some solid solute present.
• Do you think the solute particles that are in solution are the same ones over time?
• How would you determine whether they were the same?

Question to answer:

• What kinds of things dissolve in water? Make a list
• Which of these kinds of materials dissolve? Metals, ionic compounds, molecular compounds (polar, non-polar), network solids (diamond graphite).
  

Questions for later:

• What would you predict for the sign of ΔS on formation of any solution? Why?
• What would you predict for the sign of ΔH on formation of any solution? Why?
• What would you predict for the sign of ΔG on formation of any solution? Why?