Elements Compounds And Mixtures

a compound contains elements chemically bonded in a fixed ratio; a mixture contains substances not chemically bonded that can be separated by physical methods

sodium and chlorine are chemically bonded together to form a compound; when elements chemically combine, the product has entirely different properties from its constituent elements

pure iron — element; sea water — mixture; carbon dioxide — compound

a pure substance melts at a fixed/sharp temperature; a mixture melts over a range of temperatures

measure the boiling point of the sample; if it boils at exactly 100 °C it is pure, but if it boils over a range of temperatures or above 100 °C it contains impurities/is a mixture

the salt water is heated until the water boils and turns to vapour; the vapour passes into a condenser where it is cooled and condenses back into liquid water; the pure water (distillate) is collected in a separate container, leaving salt behind in the flask

pure water has a fixed boiling point of 100 °C; because only one substance is evaporating, the temperature stays constant until all the water has boiled away

it allows vapours to condense and re-evaporate repeatedly, which improves separation of liquids with different boiling points

ethanol has a lower boiling point (78 °C) than water (100 °C); ethanol evaporates first and its vapour reaches the top of the column, passes into the condenser, and is collected first

fractional distillation uses a fractionating column to separate a mixture of miscible liquids with different boiling points; simple distillation does not use a fractionating column and separates a solvent from a dissolved solid or a single liquid from a solution

filtrate

the mixture is poured through filter paper in a funnel; the water passes through the filter paper as the filtrate, while the sand is trapped on the filter paper as the residue

heat the copper sulfate solution gently in an evaporating basin to evaporate some of the water; stop heating when the solution is saturated (test by dipping a glass rod — crystals form on the rod); leave the solution to cool slowly so that large, regular crystals of copper sulfate form

slow cooling allows solute particles to arrange in an orderly pattern, producing large regular crystals; heating to dryness produces small, irregular crystals and the substance may decompose

pencil is insoluble in the solvent so it does not dissolve or move; ink from a pen would dissolve in the solvent and interfere with the results

a pure substance produces only one spot on the chromatogram; a mixture produces two or more spots

if the solvent level is above the baseline, the spots dissolve directly into the solvent instead of being carried up the paper

$${R}_{f}=\frac{3.6}{7.2}$$ $${R}_{f}=0.50$$

calculate the Rꜰ value of the unknown substance from the chromatogram; compare it with Rꜰ values of known substances measured under the same conditions and in the same solvent — a matching Rꜰ value identifies the substance

the substance cannot travel further than the solvent front, so the distance moved by the substance is always less than or equal to the distance moved by the solvent