Elements, compounds, mixtures and separation techniques

a pure substance made out of atoms that all contain the same number of protons and neutrons, and this cannot be split into anything simpler

a pure substance made up of 2 or more elements chemically bonded together in a fixed ratio

a combination of two or more substances (either elements and/or compounds) that aren’t chemically bonded

a substance that consists of only one type of element or compound

fixed melting and boiling points - e.g water boils at exactly 100 degrees celsius

a substance consisting of various elements or compounds that aren’t chemically bonded together

impure substances can melt and boil over a range of temperatures

• simple distillation • fractional distillation • filtration • crystallisation • paper chromatography

• separating a liquid and a soluble solid from a solution (e.g NaCl from water) • separating a pure liquid from a solution of liquids (e.g water from ethanol)

• solution is heated in a side-arm flask to the boiling point of the pure liquid in the solution until the liquid evaporates • the vapour rises through the neck of the side-arm flask and moves into the condenser, where it cools and condenses into the pure liquid • the liquid drips out of the condenser and is collected in a beaker • only the solute remains in the side-arm flask

separating two miscible liquids (e.g water and ethanol)

• the solution is first heated to the temperature of the substance with the lowest boiling point • this substance will evaporate first and its vapour will travel up though the fractionating column, which contains glass beads in it to increase the surface area for vapour condensation • the vapour then passes into the condenser, where it cools and condenses into a liquid • the liquid drips out of the condenser and is collected in a beaker • only the substance with the higher boiling point remains in the round-bottomed flask

separating an undissolved solid from a mixture of solid and liquid (e.g water and sand)

• a piece of filter paper is placed in the funnel above a beaker, and the mixture is poured into this • the filter paper only allows the smaller particles of the liquid to pass through, leaving the undissolved solid behind in the filter paper • the solid particles are too large to pass through, so remain as the residue and the extracted liquid is the filtrate

separating a dissolved solid from a solution, when the solid is more soluble in the hot solvent (e.g hydrated copper (II) sulfate)

• solution is heated until most of it has evaporated • this leaves behind a saturated solution, which can be tested by dipping a cold glass rod into it and observing crystal formation • then the solution is left to cool and crystallise, and crystals are filtered out of the solution and washed with cold distilled water to remove soluble impurities • crystals are then left to dry in a warm place

separating substances that have different solubilities in a given solvent (e.g the different dyes in black ink placed in water)

• draw a pencil line w/ruler 2-3cm away from the base of the paper (NOT pen as this may dissolve into the solvent and move up the paper) • place equally sized spots of substances evenly spaced apart on the pencil line • fill beaker with solvent + ensure that pencil line sits above the solvent so that the substances on it don’t directly dissolve into solvent • place paper in beaker of solvent and hold in place for 10-15 minutes until the sample has split into components and solvent has moved up the paper and some dyes may do too • chromatogram is complete when the solvent front is 2-3cm away from the top of the paper

• substances that rise further up the chromatogram are more soluble in the given solvent than those that don’t • some samples may remain on the pencil line, indicating that they are insoluble in the given solvent

samples that rise to the same height on the chromatogram are the same substance

• impure substances will have separated into many dots on the chromatogram; the mixture will separate into different spots to show the different components it consists of • pure substances will have only one dot on the chromatogram

• Rf = distance moved by spot or solute / distance moved by solvent • measure from pencil line to center of spot

• Rf values are ALWAYS less than one • this is because, the spot can never travel further than the solvent on the chromatogram