Solutions and Solubility

Before we can get into solutions and solubility, there a some terms that you need to become comfortable with:SolventSoluteSolution

Solvent: A solvent is any liquid (a solid, or gas for that matter) that doesn’t change state and is present in the largest amount. I realize that whole gas thing may freak you out, but think of our atmosphere as a solution of gases. Nitrogen makes up about 74% of the air we breathe and is considered the solvent.

Solute: a substance that does change state and is present in lesser amounts. In the atmosphere oxygen would be considered a solute. In the case of solids (here you’d thought I’d forgotten about them), we consider alloys like bronze to be solutions as we combine copper (solvent) with tin (solute).

Solution: A solution is a homogenous mixture of a solvent and one or more solutes. A solution of water, sugar, and salt can be an example of a single solvent (water) and 2 solutes (sugar and salt). BTW – water is considered the universal solvent.

A general rule for solutions is like-dissolves-like. This is a simple way to remember that polar solutes and solutions go together and non-polar solutes and solutions go together.

A polar solute or solution has a positively charged end and a negatively charged end. An example of a polar solvent is water.

A non-polar material lacks charged sides. An example of a non-polar material is cooking oil.

Remember: Polar and non-polar materials will not mix

Oil and Vinegar salad dressings, or milk, tend to form emulsions is mixed vigorously (milk is mixed very well indeed – hence the term homogenized milk).

Solubility is maximum amount of solute that will dissolve in a given amount of solvent at a specific temperature.

Solubility is normally measured in terms of grams of solute per 100 mL of solvent

(g/100mL).For solids, like sugar, solubility generally goes up

as the temperature of the solvent increases.For gases, the opposite is true. The solubility of

gases tends to go down as the temperature increases. This is one reason we have fish die-offs in Southern Saskatchewan lakes – they usually are fairly shallow and as the temperature goes up in the summer, the concentration of O2 gas in the water goes down and the fish suffocate.

SaturationA solution is said to be saturated

when the maximum possible amount of solute is dissolved in a given volume of solvent at a specific temperature. This is called the saturation point.

You also, in rare cases, create supersaturated solutions. These solutions are very unstable and you will typically see a precipitate (solid) form. This precipitate may grow in size until the saturation point is re-established.

If the solution is unsaturated, then the amount of dissolved solute will vary. We use 2 terms to describe the relative amounts of solute present.

Dilute solutions – dilute solutions are described as containing relatively small amounts of solute dissolved in a given volume of solvent. An example would be the accidental mixing of a can of orange juice concentrate with 4 litres of water instead of 4 cans of water.

Concentrated solutions – concentrated solutions contain large amount of solute per given volume of solvent. An example would be a can of frozen orange juice – lots of juice compared to the solvent (water).

Solution concentration unitsIn order to accurately describe the

concentration of solutions – you can’t really say “Hey mix me up a dilute solution of saline for this patient” – you need to use different units, depending on what you’re doing.

We’ve already looked a one unit used to describe concentration, and that is Molarity (M) aka mole per litre.

Another group of units is the percent-composition groupings. We have three different percent composition sets and these are mass/mass percent (m/m%), mass/volume (m/v%), and volume/volume (v/v%).

Please note the following examples are aqueous solutions.

Mass/mass %: m/m% = mass of solute/mass of

solution x 100%Example: You dissolve 5 gram of NaCl in 50g of water.

m/m% = 5g/55g NOTE this is the mass of the solute and solvent together.

m/m% = 9.1%

Mass/Volume percentage: m/v% = mass of solute /

volume of solvent x 100%. Example: If you wanted to make up 1 liter of a 20 percent (m/v) NaCl solution. You know you are going to need 200 grams of NaCl (200/1000 = 20%). You would weigh out the NaCl and place it in a 1 litre volumetric flask. You would then add about 500 mL of water to dissolve the salt. Once the salt is dissolved, you would then add water up to the line on the flask and you would have a 20%(m/v) solution of NaCl.

Volume/volume percent: (v/v)%(v/v)% = volume of solute/volume

of solvent x 100%Example: If you wanted to make a 50%(v/v) solution of ethyl alcohol, you would take 50mL of ethyl alcohol and add water to make 100mL.You would not take 50mL of ethyl alcohol and 50 mL of water and mix ‘em together. Because both liquids are polar and the polar alcohol molecules would fit together with the polar water molecules and this would prevent the 2 volumes from being added together (you would end up with less than 100 mL).