This is the sixth in a series of 8 chemistry labs on the fundamentals of biodiesel. In
this lab we will learn about “Physical and Chemical Solubility”. Written procedures
and background information for all 8 labs is provided on the biodiesel education website.
Solubility is a chemical property referring to the ability for a given substance, the
solute, to mix or dissolve in a solvent to form a homogeneous solution. It is measured
in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The solubility
of sugar in water, for example, is about 2 grams per mL at room temperature. This is
a lot considering one mL of water only weighs 1 gram.
Solubility is temperature and pressure dependent. Generally, as the temperature and pressure
increase, the solubility of one substance in another will increase. For example, the
solubility of sugar in hot water can go up to as much as 5 grams per gram of water.
A useful rule of thumb for predicting solubility is "like dissolves like," meaning a solute
will dissolve best in a solvent that has similar chemical characteristics. One of these characteristics
is “polarity”. Polarity is where the electrons forming the bond are not equally distributed
causing a molecule to have a slight electrical charge
Some molecules are both polar and non-polar. Soap, for example, is a long molecule that
has a polar head and a non-polar tail, allowing it to dissolve in both water and oil. As you
may know, this makes it useful for cleaning non-polar oil and grease off your hands with
polar water.
When a solute in a solvent forms a solution, it is called dissolution. A solution can be
formed through dissolution, but it can also be formed because of a chemical reaction.
This is what distinguishes physical solubility, as in the sugar in water example, and chemical
solubility. The process of making biodiesel gives us several examples of chemical solubility.
The biodiesel reaction between methanol and triglycerides goes through different stages
of solubility between the reactants and the products of reaction. Initially methanol is
not soluble in oil. However, as the reaction begins, and methyl esters are formed, the
methyl esters act as co-solvent to allow the methanol and oil to become soluble in each
other. As the reaction progresses, free glycerol is produced, which is not soluble in the methyl
esters.
In this laboratory exercise we will be analyzing
the solubility of potassium nitrate in water at different temperatures. To conduct the
exercise, you will need a Bunsen burner, ring stand, ring and wire gauze, a 250 ml beaker,
four 20 mL test tubes, a tube holder and rack, thermometers, graduated cylinder, a marker
for labeling test tubes, glass stirring rod, and a weighing scale. For reagent you will
need 20 grams of potassium nitrate.
The first step is to number four test tubes and place in a rack. Weigh potassium nitrate
and water into the four tubes as indicated in this table. Prepare a water bath by filling
a 250 ml beaker ¾ full of water. Place the beaker on the ring stand and heat to 90º C
Place a thermometer and the test tubes in the water bath and stir each test tube with a glass stir rod until the potassium
nitrate is dissolved. Take one test tube out of the bath with a thermometer
and observe the first sign of crystallization as it cools. Record
the temperature in table. Repeat this with the three other test tubes.
Now you are going to use the information you discovered to draw a solubility vs temperature
curve. The Y axis (solubility) is often expressed in grams of solute per 100mL of water and
you will need to convert your data into grams of potassium nitrate per 100 mL of water.
Since there are 5 ml of water in each tube you can multiply the grams of potassium nitrate
by 20. Next, plot your data and draw the solubility curve for potassium nitrate vs. temperature.
With this plot, we know the solubility of potassium nitrate in water at different temperatures.
Thank you for watching the “Physical and Chemical Solubility” lab on the fundamentals
of biodiesel. Please go to the biodiesel education website for a complete written transcript
of this lab exercise.