Purity, Formulations and ChromatographyAQA GCSE Chemistry: Revision notes
Section 1
What is a pure substance and how does it differ from a mixture?
A pure substance is a single element or compound that contains no other substances mixed within it. The key characteristic of a pure substance is that it has a specific melting point and specific boiling point — these are fixed, unchanging values that can be used to identify the substance.
A mixture contains two or more elements or compounds that are physically combined but not chemically bonded. Mixtures do not have fixed melting or boiling points; instead, they melt and boil over a range of temperatures because the different components have different melting and boiling points.
| Property | Pure Substance | Mixture |
|---|---|---|
| Composition | Single element or compound only | Two or more substances |
| Melting point | Fixed, specific value | Range of temperatures |
| Boiling point | Fixed, specific value | Range of temperatures |
| Can be separated physically | No | Yes |
| Example | Pure water (H₂O), pure iron | Seawater, brass, air |
You can test whether a substance is pure by carefully measuring its melting or boiling point — if the value matches known data exactly, it is pure.
A pure substance is like a single ingredient (pure sugar), whilst a mixture is like a recipe (sugar and flour combined). You can separate the recipe back into its ingredients, but you cannot 'unmake' the sugar itself.
Examiners often ask you to explain why a melting or boiling point range indicates a mixture rather than a pure substance — always state that the range shows different components with different melting/boiling points.
Section 2
What is a formulation and why are formulations important?
A formulation is a mixture that has been carefully designed and blended to have specific, useful properties. Formulations are engineered products created by combining different substances in precise proportions to achieve desired characteristics.
Formulations are used across many industries because they allow products to be tailored to specific purposes. Common examples include:
- Medicines: combinations of active ingredients and excipients (fillers, binders) to ensure the right dose reaches the correct part of the body
- Fuels: blends of hydrocarbons with specific energy content and burning properties
- Alloys: mixtures of metals (e.g. copper and tin for bronze, iron and carbon for steel) to create stronger, harder materials than pure metals
- Cleaning products: detergents, water, and other chemicals combined to remove different types of dirt
- Cosmetics: oils, water, pigments, and thickening agents blended for appearance and skin safety
- Fertilisers: nitrogen, phosphorus, and potassium compounds in specific ratios for plant growth
The key advantage of formulations is that they can be designed to combine the best properties of individual substances — a property a single pure substance alone cannot achieve.
Paracetamol tablets are a formulation: the active ingredient (paracetamol) is mixed with starch (binder), cellulose (filler), and magnesium stearate (lubricant). Each component serves a purpose in making the tablet effective and easy to manufacture.
Students often confuse a formulation with a compound — remember that a formulation is a physical mixture that can be separated, while a compound is a chemical substance with fixed atom ratios that cannot be easily separated.
Section 3
How does paper chromatography separate mixtures?
Paper chromatography is a separation technique used to separate and identify the different components in a mixture. It works on the principle that different substances in a mixture have different solubilities in a solvent and different levels of adsorption to filter paper.
How paper chromatography works:
- A baseline is drawn in pencil near the bottom of filter paper (never use pen — the ink will separate)
- A small spot of the mixture to be separated is placed on the baseline
- The paper is stood upright in a beaker containing a small amount of solvent (usually water, ethanol, or a mixture)
- The solvent gradually travels up the paper by capillary action (the attraction between the solvent and paper fibres)
- As the solvent moves up, it carries the components of the mixture at different rates
- Components that are more soluble in the solvent or less adsorbed to the paper travel further and faster
- Components that are less soluble or more strongly adsorbed travel more slowly
- This difference in travel rate causes the mixture to separate into individual spots, each representing a different component
- The solvent front (the furthest point reached by the solvent) is marked in pencil before the paper dries
Each spot on the finished chromatogram represents a different substance from the original mixture.
Examiners expect you to explain the separation in terms of solubility and adsorption — state that different components have different solubilities in the solvent, so they travel at different rates up the paper.
Do not use a pen to draw the baseline or mark the solvent front — the ink will separate and contaminate your results. Always use a sharp pencil.
Section 4
How do you calculate and use Rf values to identify substances?
The Rf value (Retention Factor) is a number between 0 and 1 that describes how far a substance travels relative to the solvent front. Rf values are used to identify unknown substances by comparing them against known standards.
The Rf value formula:
Rf = distance travelled by substance ÷ distance travelled by solvent
Both distances are measured from the baseline to the relevant point (the centre of the substance spot or the solvent front).
Step-by-step calculation:
- Measure the distance from the baseline to the centre of the substance spot (in mm or cm)
- Measure the distance from the baseline to the solvent front (in mm or cm)
- Divide the substance distance by the solvent front distance
- Record the Rf value to 2 or 3 decimal places
Example calculation:
- Distance travelled by substance = 6 cm
- Distance travelled by solvent front = 10 cm
- Rf = 6 ÷ 10 = 0.6
Key features of Rf values:
- Rf values are characteristic for each substance in a particular solvent — the same substance will always have the same Rf value in the same solvent
- Rf values are independent of paper size or chromatogram length — they only depend on the solvent used
- Rf values are always between 0 and 1 (a substance cannot travel further than the solvent front)
- Changing the solvent changes all Rf values, so you must use the same solvent as the standard for comparison
Using Rf values to identify substances:
- Run chromatography on an unknown mixture and calculate Rf values for each spot
- Run chromatography on standard (known) solutions using the same solvent
- Calculate Rf values for the standards
- Compare: if an unknown substance has the same Rf value as a standard, they are the same substance
- If no standards match, the substance is unknown or the solvent was different
A chromatogram shows a spot at 4.5 cm and the solvent front at 9 cm. Rf = 4.5 ÷ 9 = 0.5. If a standard of substance X has Rf = 0.5 in the same solvent, the unknown spot is substance X.
Always state that you must use the same solvent as the standard when identifying unknowns — different solvents produce different Rf values for the same substance, which would lead to incorrect identification.
Section 5
How do you interpret chromatograms to draw conclusions?
Interpreting a chromatogram means looking at the spots produced and drawing conclusions about the purity and identity of substances.
Interpreting for purity:
- One spot only = the substance is pure (contains only one component)
- Two or more spots = the substance is a mixture or is impure (contains two or more components)
- The more spots present, the more components in the mixture
Interpreting for identity:
- Compare the Rf values of unknown spots with the Rf values of known standards
- If an unknown spot has the same Rf value as a standard in the same solvent, they are the same substance
- If multiple standards are run alongside the unknown, you can identify several components in a mixture
- Different solvent systems may separate substances differently — use the same solvent as the standard for accurate comparison
Example interpretation:
If a sample shows three spots with Rf values of 0.2, 0.5, and 0.8, and standards show substance A at Rf 0.2, substance B at Rf 0.5, and substance C at Rf 0.8, then the mixture contains A, B, and C.
Important points when interpreting:
- Always measure distances carefully from the baseline to the centre of each spot
- Record Rf values to the same number of decimal places for consistent comparison
- A small variation in Rf values (typically ±0.05) may be due to experimental error, so values very close to standards should be considered a match
- The shape and size of spots are not used for identification — only position (Rf value) matters
A sample of food colouring is tested alongside standard dyes. The sample shows two spots at Rf 0.3 and Rf 0.6. Standard Red dye shows Rf 0.3 and Standard Yellow shows Rf 0.6. Conclusion: the food colouring is a mixture of Red and Yellow dyes.
Students sometimes try to identify substances by the darkness or size of the spot — this is wrong. Rf value (position) is the only property used for identification. Spot darkness indicates concentration, not identity.
Must Know
- Pure substances have fixed melting and boiling points; mixtures melt and boil over a range of temperatures — this is the key way to identify if something is pure
- A formulation is a deliberately designed mixture (e.g. medicines, alloys, fuels, cosmetics, fertilisers) created to have specific useful properties that single pure substances cannot achieve alone
- Paper chromatography separates mixture components because they dissolve to different extents in the solvent and stick to the paper with different strengths — components that dissolve more readily travel further up the paper
- Rf = distance travelled by substance ÷ distance travelled by solvent; Rf values are characteristic for each substance in a particular solvent and are used to identify unknowns by comparing with standards — always measure from the baseline and use the same solvent as the standard
- One spot on a chromatogram = pure substance; multiple spots = mixture or impure substance — identify components by matching Rf values to known standards
- Always use pencil (never pen) for baselines and solvent fronts, measure distances carefully, and remember that Rf values change if you change the solvent
That's the notes covered.
Carry on to the next subtopic.