Every card in Chemical analysis, Year 11: formulations and chromatography
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- A mixture that has been designed as a useful product, with each ingredient included in a measured amount for a particular purpose
A formulation
HintThe word shares its opening with 'formula', as in a recipe to be followed.
WhyPaints, medicines, fuels, cleaning agents, alloys, fertilisers and foods are all formulations. What makes a mixture a formulation is the design: the ingredients and their amounts are chosen so that the product does its job.
- Why are the components of a formulation mixed in carefully measured quantities?
So that the product has the properties it needs
HintThink what would happen to a paint or a medicine if the recipe changed from batch to batch.
WhyChanging the proportions changes the properties: a paint with too little binder flakes off, and a tablet with too much of the active drug could be harmful. So every batch is made to the same recipe.
- A sun cream contains one ingredient that absorbs ultraviolet light, another that helps it spread and another that stops it going off. What does this show about the ingredients of a formulation?
Each one is there for a particular purpose
HintAsk what job every ingredient is doing.
WhyA formulation is not a random mix: every component is chosen because the product needs what it does. You are not expected to know the ingredients of real branded products, only to recognise this idea.
- Which one of these is a formulation: distilled water, oxygen gas, a household cleaning spray, or copper wire?
The household cleaning spray
HintThree of them are single substances; look for the one made by mixing several.
WhyDistilled water is a single compound, and oxygen and copper are single elements. The cleaning spray is a designed mixture of several chemicals, each with a job, which is what a formulation is.
- A pupil says: 'A formulation has a fixed recipe, so it must be a compound.' Why is the pupil wrong?
Its ingredients are only mixed, not chemically joined
HintCompare how salt and water are held in sea water with how sodium and chlorine are held in salt.
WhyIn a compound the elements are bonded together in proportions fixed by the chemistry. In a formulation people choose the proportions and each component keeps its own properties, so it is a mixture.
- In chromatography, the name of the phase that moves: the solvent travelling through the paper and carrying the dissolved substances with it
The mobile phase (in chromatography)
HintOne of the two parts of the system stays still; this is the other, named for the fact that it travels.
WhyChromatography always has two phases. In paper chromatography the paper is the stationary phase, which does not move, and the solvent is the mobile phase, which carries the substances along.
- On a chromatogram, the centre of a blue spot is 3.6 cm above the start line. The solvent front is 8.0 cm above the start line. Calculate the Rf value of the blue dye. (number only)
0.45
HintDivide the smaller distance by the larger one.
WhyRf = distance moved by the substance ÷ distance moved by the solvent = 3.6 ÷ 8.0 = 0.45. Both distances are measured from the start line. An Rf value has no unit, because it is one length divided by another. The expression is given in the specification content for this section.
- Two dyes are spotted on the same start line. Why does one travel further up the chromatography paper than the other?
It is more soluble in the solvent and less attracted to the paper
HintEach dye is pulled two ways: by the liquid that is rising and by the sheet it is rising through.
WhyEach substance is shared between the two phases. One that spends more of its time dissolved in the moving solvent is carried further; one that clings to the paper is held back. Different substances are shared differently, so they separate.
- A food colouring gives a single spot on a chromatogram run in water. Why should it be run again in a different solvent before deciding that it is one pure dye?
Two dyes can travel together in one solvent; a pure substance gives one spot in every solvent
HintA mixture can hide if its components happen to move the same distance.
WhyA single spot shows only that nothing separated in that solvent. If the colouring still gives one spot in several different solvents, the evidence that it is a single substance is much stronger.
- A dye has an Rf value of 0.45 when the solvent is water. Would you expect the same Rf value if the chromatogram were run in ethanol?
No — an Rf value depends on the solvent used
HintThink about whether the dye dissolves equally well in both liquids.
WhyAn Rf value belongs to a substance in a particular solvent. To identify an unknown by comparing its Rf value with known values, both must have been measured in the same solvent.
- The distance moved by a substance is measured from the start line to the ____ of its spot.
centre
HintSpots are blobs, not points, so one agreed place on the blob has to be used.
WhyA spot can be several millimetres tall, so measuring to its top or its bottom would give different Rf values. Measuring to the middle every time makes results comparable. The solvent distance is measured from the same start line to the solvent front.
1★ GCSE-CHEM-PAR-0012
2★ GCSE-CHEM-PAR-0013
3★ GCSE-CHEM-PAR-0014
4★ GCSE-CHEM-PAR-0015
5★ GCSE-CHEM-PAR-0016
6★ GCSE-CHEM-PAR-0017
7★ GCSE-CHEM-PAR-0018
8★ GCSE-CHEM-PAR-0019
9★ GCSE-CHEM-PAR-0020
10★ GCSE-CHEM-PAR-0021
11★ GCSE-CHEM-PAR-0022
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