A pure substance contains only one kind of atom or molecule while a mixture has two or more pure substances mixed together. An example of a pure substance would be NaCl, by itself it is pure because it contains only one kind of molecule. An example of a mixture would be if you poured the NaCl within water(H2O) this would be a mixture because it contains two different pure substances.
In the lab we did today we separated water from copper (2) sulfate pentahydrate by heating the chemical intill the water evaporated. There is two reasons why CuSO4 – 5H2O is a pure substance: 1) The observation of a chemical reaction seen in the change of color of the hydrate as the water evaporated shows that it was pure substance because pure substances can only be separated through chemical reactions. At the end of the lab we added water back to the anhydrous salt and another chemical reaction occurred as seen through the release of heat and the change in the substances color as the water molecules bonded to the salt. 2) The second reason we know this is a pure substance is due to the fact that the water molecules are proportionate to the CuSO4 molecules. This can be proved with percent composition as shown below:
The combined molar mass of CuSO4 – 5 H2O = 249.71 g and the molar mass of the water is 5(18.02) = 90.1 g
To find the proportion of water to the hydrate we divide the molar mass of the water by the hydrate to obtain a percent.
90.1g H2O/ 249.71g CuSO4-5 H2O = .3608 x 100 = 36.08% that is the constant proportion of water to the hydrate.
In our lab we evaporated .78g of water out of the 1.97g of hydrate. which came out to 39.59% which does not support the hydrate being a pure substance, but a way to validate the results would be to take the hydrate from the lab and multiply it to the molar masses of the water and the hydrate as so 1.97g/1 x 90.1g H20/249.71g CuSO4-5H20 = .7108g this number represents the theoretical mass of water that should have evaporated from hydrate.
.7108g H20/1.97g CuSO4-5H2O = .3608 x 100= 36.08% which happens to be our constant proportion. Our lab results were within a five percent error which was most likely due to mass lost during measurements and stirring. All in all, That is why hydrates are considered pure substances.