C2.1 Purity and Separating Mixtures

Mixtures are made of 2 or more substances. (Elements or compounds) that haven’t been chemically combined. Mixtures can be separated. Their chemical properties don’t change because they have been mixed with another substance

Air: oxygen, nitrogen and carbon dioxide

filtration separates mixtures of insoluble solids and liquids. Done by pouring the mixture through filter paper. The insoluble solid is trapped. The liquid runs through the paper and is collected below

crystallisation separates solutions into their different parts: dissolved solids (solutes) and liquids (solvents). Heat the mixture so that the solvent evaporates. eventually crystals of the solute will form. we can collect the solvent by condensing it as it evaporates

Chromatography separates solutions with a number of different solutes in the solvent. Place a drop of the solution to be separated near the bottom of a piece of chromatography paper. Dip the very bottom of the paper into a suitable solvent. The solvent moves up the paper and carries the solutes in the solution with it. Different solutes move at different speeds, so they separate on the paper.

Simple distillation separates 2 liquids with different boiling points. The mixture is heated until the liquid with the lower boiling point starts to boil. The vapour released passes through a condenser, where the gas cools back into a liquid.

Fractional distillation separates lots of liquids with different boiling points. The mixture is slowly heated until the liquid with the lowest boiling point boils and then condenses. Then we increase temperature slowly to collect (boil then condense) the other fractions

A chemically pure substance is a single element or compound. It contains only one substance. Purity is worked out by looking at the melting or boiling points of samples. Pure substances have exact and specific melting/boiling points. Water, glucose and oxygen gas are all pure substances.

Impurities lower the melting point of a sample and widen the range of temperature at which the sample will melt.

Impurities increase the boiling point and widen the range of temperatures at which the sample will boil.

Formulations are mixtures of chemicals that rae designed to create useful products

in a formulation each component helps to decide what the mixture’s overall properties are.

to ensure that a formulation does what it is supposed to, each chemical component must be present in a precisely measured quantity. E.g. fuels, cleaning agents, medicines.

paper chromatography only has 2 stages. substances are picked up and carries by a mobile phase (liquid or gas). The mobile phase then moves through a stationary phase (solid or viscous liquid)

the separation of substances depends on the distribution of a substance across the 2 phases. A substance moves far if it’s more attracted to the mobile phase. a substance doesn’t move far if it’s more attracted to the stationary phase. different components can sometimes be equally attracted to a solvent. so, the number of spots a mixture produces can vary depending on the solvent used.

the components that travel furthest are highly soluble in the solvent. They are also minimally attracted to the chromatography paper

chromatography produces chromatograms to identify compounds in mixtures by calculating Rf values. Rf = distance travelled by substance / distance travelled by solvent. We can work this out for both reference substances and tested substances

Rf values depend on the solvent. We can learn more about the identity of the components by testing any references and the unknown mixture in a range of solvents. A reference substance is a pure sample that’s run next to the tested substance to see if its a component in the mixture. these substances provide valuable evidence but not proof.