Read the descriptions below of two substances and an experiment on each. Decide whether the result of the experiment tells you the substance is a pure substance or a mixture, if you can.
• Sample A is 100 g of a coarse grey powder with a faint unpleasant smell. 15 g of the powder are dissolved in ethanol. 0.5 mg of the resulting black solution is carefully dropped onto a thick sheet of paper laid flat in a tray. After 30 minutes the initial round black stain has spread out and faded in color to a deep purple. Additionally, there is a dark green ring surrounding the inner stain.
• Sample B is 100 ml of a clear liquid. The density of the liquid is measured, and turns out to be 0.77 g/ml. The liquid is then cooled in the refrigerator. At 10.0 °C crystals begin to appear until the liquid is about half crystal, half liquid. After 30 minutes, no more crystals appear, even though the temperature is lowered to 6.3 °C.
Is sample A made from a pure substance or a mixture? If the description of the substance and the outcome of the experiment isn't enough to decide, choose "can't decide.
a. mixture
b. pure substance
c. (can't decide)
Is sample B made from a pure substance or a mixture? If the description of the substance and the outcome of the experiment isn't enough to decide, choose "can't decide."
a. pure substance
b. mixture
c. (can't decide)

We know a mixture as a sample that is made up of two or more substances. Based on the results from the experiment conducted on sample A, the sample is a mixture. Each colour that appeared on the paper represents one of the components of the mixture.

For Sample B, at a particular sharp temperature, the crystals begin to appear. That temperature at which the first crystal appears is actually the melting point of the solid. We were also told that only half of the clear liquid was crystallized meaning that other substances may still be contained in the remaining liquid. Crystallization is a separation technique that depends on differences in melting points of substances. We can't decide if the sample is pure because we have no further information about what happened to the remaining liquid. That would have told us if the liquid remaining was just the solvent used to dissolve B which could have also been evaporated to leave only the pure sample.