Comparing the solubility's of copper sulphate, sodium chloride and potassium nitrate
Molecular solids (sugar) and ionic solids (salts) both dissolve in water. However, they both dissolve in different ways.
The intermolecular forces holding molecules of sugar together are relatively weak so when sugar is placed in water these bonds are broken and individual C12H22O11 molecules are released into solution. It takes energy to break bonds between C12H22O11 molecules and it also takes energy to break the hydrogen bonds in water. These hydrogen bonds have to be disrupted in order to insert a sugar molecule into the substance. The energy needed for this is produced by the forming of bonds between slightly polar sucrose molecules and polar water molecules. This process works so well between sugar and water that up to 800g of sugar can dissolve in 1 litre of water.
The positive and negative ions in ionic solids (or salts) are held together by the strong force of attraction between particles with opposite charges.
When a salt dissolves in water the ions are released and become associated with the polar solvent molecules. Salts dissociate with their ions when they dissolve in water.
e.g. NaCl (s) Na+ (aq) + Cl (aq)
There are several factors that will affect solubility between different compounds:
Temperature - If the solution process absorbs energy then the solubility will be
increased if there is a temperature increase. If the solution
releases energy (exothermic, i.e. between sugar and water)
then solubility will decrease.
Molecular Size - If the size or weight of the individual molecules is large then
solubility will be low because larger molecules are difficult to
surround with solvent molecules.
Polarity - Generally only polar solute molecules will dissolve in polar solvents
and only non-polar solute molecules will dissolve in non-polar
solvents. Polar solute molecules have positive and negative ends.
So if a polar solute molecule is placed in a polar solvent then the
positive ends of solvent molecules will attract the negative ends of
solute molecules. This type of intermolecular force is known as di-
pole-dipole interaction. The type of intermolecular force in present in
non-polar molecules is called London Dispersion forces. Here the
positive nuclei of the atoms of the solute molecules will attract the
negative electrons of the atoms of a solvent molecule.
Branching - This factor only applies to organic compounds. The amount of
carbon branching will increase solubility because more branching will reduce the size of the molecule, making it easier to solvate.
The chemicals I will be using will be:
1. Copper Sulphate - A salt soluble in water and methanol. Blue crystals.
(CuSO4 SH2O) White colour when hydrated. May irritate skin, eyes,
nose and throat. May affect liver if inhaled or
2. Sodium Chloride - Common rock salt.
3. Potassium Nitrate - Colourless crystals. May ignite combustible materials.
(KNO3) May evolve toxic fumes.
Below is information from a data booklet:
Copper Sulphate - 1.39 Ã 10?? moles dissolve in 100g of water.
Sodium Chloride - 6.15 Ã 10?? moles dissolve in 100g of water.
Potassium Nitrate - 3.75 Ã 10?? moles dissolve in 100g of water.
Working out the quantities that will dissolve in grams shows that according to this information 29.12g of copper sulphate will dissolve, 35.98g of sodium chloride will dissolve and 37.88g of potassium nitrate will dissolve in 100g of water.
I carried out some preliminary tests that showed that more of the salts dissolved as we increased the temperature. Obviously these tests could be accurate, as I did not have time to plan them carefully. However, they do show that as the temperature was increased, the solubility also increased.
Having looked at the information given by the data booklet and my own preliminary work, I think that as the temperature increases the solubility of all the salts will increase.
3 boiling tubes, a burette, a thermometer, ceramic mat, Bunsen burner
Weigh out about 10g of copper sulphate and record its mass. Place this in a boiling tube and add 15cm? of distilled water with the burette. The burette is better for measuring small amounts of water than a measuring jug because it is more accurate. Now heat the boiling tube gently until all the crystals have gone into solution (should be around 70?C).
Allow the solution to cool while stirring with a thermometer. When crystals start to appear record the temperature.
Now add another 5cm? of water into the boiling tube. Reheat until all crystals have again gone into solution. Allow solution to cool while stirring with thermometer. When crystals start to appear record the temperature.
Add another 5cm? of water and repeat the same method, recording the new temperature at which crystals appear.
You now have recordings with 15cm? of water, 20cm? of water and 25cm? of water. Continue the experiment so you have readings using 30cm? of water, 35cm?, 40cm?, 45cm? and 50cm?. This gives a good range to plot a solubility curve from.
Now weigh out about 10g of sodium chloride and record its mass. Place in a new boiling tube so the sodium chloride does not get contaminated by the copper sulphate. Repeat the method shown above, recording the temperature at which crystals of sodium chloride appear. Wash the thermometer before using it to stir the solution as it will have traces of copper sulphate solution on. This will contaminate the sodium chloride and it will not be a fair test. Also try and use the same people doing the same jobs, i.e. the same person taking the temperature that took it when you were using the copper sulphate. This will help to reduce human error. Take readings using 15cm? of water, 20cm?, 25cm?, 30cm?, 35cm?, 40cm?, 45cm? and 50cm?.
Weigh out about 10g of potassium nitrate and record its mass. Again take a new boiling tube and place the potassium nitrate in it. Repeat the method above taking readings from the same volumes of distilled water.
Now repeat the whole experiment another two times and average the readings you get. Ignore any readings which are completely different to the other two as these will probably be due to an error.
Now work out the solubility of each salt at different temperatures per 100g of water. For instance if 10g of copper sulphate dissolves in 20cm? of water then the solubility of copper sulphate at the temperature you recorded will be 50g per 100g of water.
Once you have worked out all the solubility's at different temperatures
you can plot a solubility curve graph. Put solubility per 100g water on the vertical axis and temperature on the horizontal axis. Plot the points for each salt and join them with a line of best fit. Now you should have three separate
solubility curves on the same graph.
Sources of Error
Human error -it is very hard to tell the exact temperature at which crystals
come out of the solution, so mistakes here are probable. To
reduce the chance of error use the same thermometer and the
same person taking the readings.
Holman's Chemistry textbook