Separation & Purification (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Flashcards
Exam code: 0654 & 0973
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- FrontPaper Chromatography
What is the purpose of paper chromatography?
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What is the purpose of paper chromatography?
Paper chromatography is used to separate mixtures of soluble coloured substances and to identify the components by comparing them with known reference samples.
Rf = distance moved by ______ divided by distance moved by ______.
Rf = distance moved by substance divided by distance moved by solvent.
Why must the pencil line on chromatography paper be drawn above the level of the solvent?
The pencil line must be above the solvent level so that the sample spots are not washed directly into the solvent, which would prevent them from separating on the paper.
True or False?
A pure substance will produce more than one spot on a chromatogram.
False.
A pure substance produces only one spot on a chromatogram. More than one spot indicates a mixture or an impure substance.
How do you identify whether an unknown substance on a chromatogram matches a known reference substance?
If the spot from the unknown substance travels the same distance as a spot from the known reference substance (they are at the same horizontal level), the two substances are the same.
A substance that is more ______ in the solvent will travel further up the chromatography paper.
A substance that is more soluble in the solvent will travel further up the chromatography paper.
Why is pencil used to draw the baseline on chromatography paper rather than ink?
Pencil is used because ink would dissolve in the solvent and run up the paper along with the samples, interfering with the results. Pencil marks do not dissolve.
Which separation technique is used to separate an undissolved solid from a liquid?
Filtration is used to separate an undissolved solid from a liquid. The solid remains as a residue in the filter paper while the liquid passes through as the filtrate.
Simple distillation separates a dissolved solid from its ______, whereas fractional distillation separates two ______ liquids.
Simple distillation separates a dissolved solid from its solvent, whereas fractional distillation separates two miscible liquids.
How does crystallisation separate a dissolved solid from a solution?
Crystallisation separates a dissolved solid from a solution by heating the solution to concentrate it, then allowing it to cool slowly so that crystals form as the solubility of the solid decreases. The crystals are then collected by filtration.
True or False?
A pure substance melts over a range of temperatures, while an impure substance melts at a single sharp temperature.
False.
A pure substance has a single, sharp melting point. An impure substance (a mixture) melts over a range of temperatures because it contains different components.
How do impurities affect the melting point and boiling point of a substance?
Impurities lower the melting point and raise the boiling point of a substance compared to the pure substance. The more impure the sample, the greater the effect on these values.
In fractional distillation of ethanol and water, ______ distils out first because it has a lower ______ point of 78 °C.
In fractional distillation of ethanol and water, ethanol distils out first because it has a lower boiling point of 78 °C.
Why is fractional distillation used rather than simple distillation to separate ethanol from water?
Fractional distillation is used because ethanol and water have boiling points that are relatively close together (78 °C and 100 °C). Fractional distillation achieves a higher degree of purity when separating liquids with similar boiling points.
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