Separation & Purification (Cambridge (CIE) IGCSE Combined Science: Chemistry): Flashcards
Exam code: 0653
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- FrontPaper Chromatography
Define paper chromatography.
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Define paper chromatography.
Paper chromatography is a technique used to separate mixtures of soluble substances based on their different solubilities in a solvent. More soluble components travel further up the paper.
Why is a pencil used to draw the baseline on chromatography paper rather than a pen?
A pencil is used because the graphite does not dissolve in the solvent. Ink from a pen would dissolve and run up the paper with the solvent, contaminating the chromatogram.
True or False?
A pure substance will produce only one spot on a chromatogram.
True.
A pure substance contains only one component, so it travels a single fixed distance up the paper, producing just one spot. An impure substance or mixture produces multiple spots.
In chromatography, the solvent travels up the paper by .......... action, carrying the substances at different rates depending on their .......... in the solvent.
In chromatography, the solvent travels up the paper by capillary action, carrying the substances at different rates depending on their solubility in the solvent.
Define Rf value in paper chromatography. (Extended Tier Only)
The Rf value (retention factor) is the ratio of the distance moved by a substance to the distance moved by the solvent front. It has no units and is always less than 1.
How is the Rf value calculated in a chromatography experiment? (Extended Tier Only)
Rf = distance moved by substance ÷ distance moved by solvent. For example, if a substance moves 3 cm and the solvent moves 6 cm, the Rf value is 0.5.
True or False?
The Rf value of a substance stays the same regardless of which solvent is used. (Extended Tier Only)
False.
The Rf value depends on the solvent used. Changing the solvent changes the relative solubilities of the components and therefore changes their Rf values.
How can chromatography be used to tell if two substances are the same compound?
If two substances are the same compound, their spots will travel to the same height on the chromatogram (i.e. they will be in line horizontally). They will also have identical Rf values under the same conditions.
The Rf value is a .......... with no units and will always be .......... than 1. (Extended Tier Only)
The Rf value is a ratio with no units and will always be less than 1.
Why must the pencil baseline be placed above the solvent level in a chromatography experiment?
The baseline must be above the solvent so that the sample spots are not submerged. If the spots touched the solvent directly, the substances would dissolve into it rather than being carried up the paper by capillary action.
Define filtration as a separation technique.
Filtration is a technique used to separate an undissolved solid from a liquid or solution. The mixture is poured through filter paper in a funnel; the solid remains as a residue and the liquid passes through as the filtrate.
Why is it important to heat a solution for no more than half of the liquid off during crystallisation?
If more than half the liquid is removed by direct heating, the crystals may decompose or form too quickly and be impure. Removing only half ensures a concentrated, saturated solution remains, from which good-quality crystals form slowly on cooling.
In simple distillation, the liquid is heated and .......... rises through the flask, then .......... in the condenser to be collected.
In simple distillation, the liquid is heated and vapour rises through the flask, then condenses in the condenser to be collected.
True or False?
Fractional distillation is used to separate liquids with very different boiling points.
False.
Fractional distillation is used when the boiling points of liquids are close together. It gives a higher degree of purity than simple distillation and is used for mixtures such as ethanol and water.
How is a suitable solvent used to separate a mixture of two solids?
A solvent is chosen in which only the desired solid dissolves. The mixture is stirred in the solvent, the undissolved solid is removed by filtration, and the dissolved solid is recovered by evaporation or crystallisation.
In fractional distillation of ethanol and water, ethanol evaporates first because it has a .......... boiling point than water.
In fractional distillation of ethanol and water, ethanol evaporates first because it has a lower boiling point than water.
How do you test whether a solution is saturated during crystallisation?
Dip a clean, dry, cold glass rod into the hot solution. If crystals form on the rod when it is removed and cools, the solution is saturated and ready to be set aside to crystallise.
True or False?
Simple distillation can be used to separate two miscible liquids with similar boiling points.
False.
Simple distillation is not effective when boiling points are similar. Fractional distillation should be used in this case as it gives better separation and higher purity.
Define crystallisation as a separation technique.
Crystallisation is a method of separating a dissolved solid from a solution by evaporating the solvent to form a saturated solution, which is then cooled slowly so that pure crystals of the solute form.
What property difference does distillation exploit to separate the components of a mixture?
Distillation exploits differences in boiling point. The liquid with the lower boiling point vaporises first, passes through a condenser where it cools back to a liquid, and is collected separately.
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