Revision notes · Chemical analysis
Purity, formulations and chromatography
Pure substances4.8.1.1
Definition: In chemistry, a pure substance is a single element or a single compound not mixed with any other substance.
Pure substances have distinct physical properties that distinguish them from mixtures.
- •Pure elements and compounds melt and boil at exact, specific temperatures.
- •Impurities in a substance lower the melting point and broaden the melting temperature range.
- •Impurities increase the boiling point and cause the substance to boil over a range of temperatures.
- •In everyday language, 'pure' can mean nothing has been added, whereas in chemistry it means a single element or compound.
⚠️ Common mistake: Thinking 'pure' orange juice is chemically pure when it is actually a mixture of water, sugar, and acids.
🧠 Remember: Pure = one exact melting point; Impure = melts over a range.
Formulations4.8.1.2
Definition: A formulation is a complex mixture designed as a useful product.
Formulations are carefully created mixtures where each component serves a specific function.
- •Formulations are made by mixing components in exact, measured quantities.
- •Each chemical component in a formulation is chosen to give the product required properties.
- •Examples of formulations include paints, fuels, medicines, cleaning agents, alloys, and fertilisers.
- •The composition of a formulation is strictly controlled so that the product remains consistent and effective.
⚠️ Common mistake: Confusing formulations with random mixtures; formulations always have precisely measured proportions.
🧠 Remember: Formulations are measured mixtures made for a specific purpose.
Chromatography4.8.1.3
Definition: Chromatography is a technique used to separate mixtures and identify dissolved substances.
Paper chromatography separates substances based on their differential movement across paper.
- •Chromatography uses a stationary phase (such as paper) and a mobile phase (a solvent like water or ethanol).
- •A pure substance produces a single spot in all solvents, whereas a mixture separates into multiple spots.
- •Substances move higher up the paper if they are more soluble in the solvent and less attracted to the stationary phase.
- •The Rf value is calculated using: distance moved by substance ÷ distance moved by solvent.
- •Rf values are always between 0 and 1 and depend on the specific solvent used.
- 1Draw a pencil baseline about 2 cm from the bottom of the chromatography paper (pencil doesn't dissolve, so it won't run with the solvent).
- 2Add small, concentrated spots of each sample onto the baseline, letting each spot dry before adding more if needed.
- 3Place the paper in a container with a shallow layer of solvent, keeping the solvent level below the baseline so the spots don't dissolve straight into it.
- 4Put a lid on the container and leave the solvent to rise up the paper, carrying the substances with it at different rates.
- 5Remove the paper just before the solvent reaches the top, and immediately mark the solvent front in pencil.
- 6Once dry, measure the distances moved by the solvent and by each spot to calculate their Rf values.
⚠️ Common mistake: Drawing the starting baseline in ink instead of pencil, which causes the ink to dissolve and run up the paper.
🧠 Remember: Rf = Spot distance divided by Solvent distance.
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