1.2 Elements, Compounds & MixturesEdexcel IGCSE Chemistry: Flashcards

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Define element.

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Define element.: A substance made of only one type of atom that cannot be broken down into simpler substances.
Define compound.: Two or more different elements chemically combined in fixed proportions.
Define mixture.: Two or more substances physically combined, not chemically bonded, in no fixed proportion.
How does the melting point of a mixture differ from that of a pure substance?: A pure substance melts at one fixed temperature; a mixture melts over a range of temperatures.
Which separation technique separates an insoluble solid from a liquid?: Filtration.
Which separation technique obtains dissolved solid crystals from a solution?: Crystallisation — evaporating solvent until saturated, then cooling.
What does simple distillation separate?: A solvent (pure liquid) from a solution, by evaporation then condensation.
What does fractional distillation separate, and how?: Miscible liquids with different boiling points; the mixture is heated in a fractionating column and each liquid evaporates and is collected at its own boiling point.
What is the formula for Rf value?: Rf = distance travelled by substance ÷ distance travelled by solvent.
In paper chromatography, why is the baseline drawn in pencil?: Pencil is insoluble in the solvent, so it will not dissolve and interfere with the results.
What is the 'stationary phase' in paper chromatography?: The chromatography paper, which stays fixed while the solvent moves through it.
What is the 'mobile phase' in paper chromatography?: The solvent, which moves up through the stationary phase carrying dissolved substances.
How can chromatography show a substance is a mixture?: A single spot separates into two or more spots, showing it contains more than one component.
If a spot travels 4.5 cm and the solvent front travels 9 cm, what is the Rf value?: Rf = 4.5/9 = 0.5.
How can chromatography identify unknown components?: By comparing the Rf values or spot pattern of the unknown to known reference substances run in the same solvent.