Overview
This lecture explains the concept of purity in chemistry, how to determine if a substance is pure, and methods such as melting/boiling point analysis and chromatography.
Understanding Purity
- A pure substance contains only one type of substance and nothing else mixed in.
- All elements are pure because they consist of only one type of atom (e.g., hydrogen, nitrogen).
- Individual compounds are pure if only their specific molecules are present (e.g., pure water is only H₂O).
- Everyday language may misuse "pure"; for example, bottled mineral water is not chemically pure.
Mixtures vs. Pure Substances
- Mineral water is a mixture, containing water, potassium, calcium, and other dissolved minerals.
- A mixture contains more than one substance not chemically joined and not in a fixed ratio.
Methods of Obtaining and Testing Purity
- Distillation separates pure liquids from mixtures by evaporation and condensation.
- Medical drugs and food additives must be pure; impurities can be harmful.
- Impurities are unwanted substances that prevent a sample from being pure.
Melting and Boiling Points
- Pure substances have fixed melting and boiling points (e.g., pure water boils at 100°C, melts at 0°C).
- Impure substances melt and boil over a range of temperatures.
- Impurities lower melting points and raise boiling points (e.g., salt on roads lowers water’s freezing point).
Chromatography
- Chromatography separates different substances in a mixture, such as dyes in inks.
- A solvent (like water or alcohol) dissolves the sample, which moves up filter paper.
- Separate spots on chromatogram indicate multiple substances—an impure sample.
- The RF (Retention Factor) value = distance moved by substance ÷ distance moved by solvent front.
- RF values must be less than 1 and have no units.
Chromatography with Amino Acids
- Amino acids are colorless and require a locating agent (e.g., ninhydrin) to become visible on the chromatogram.
- RF values help identify which amino acids are present.
Key Terms & Definitions
- Pure Substance — Contains only one type of particle with nothing else mixed in.
- Impurity — An unwanted substance mixed with the main substance.
- Mixture — Contains more than one substance not chemically bonded or in fixed ratios.
- Distillation — A process to separate pure substances from mixtures by evaporation and condensation.
- Chromatography — Technique for separating mixtures into individual components.
- Solvent — A liquid that dissolves other substances.
- RF Value — Retention factor; calculated as distance moved by substance ÷ distance moved by solvent front.
- Locating Agent — Chemical that makes colorless substances visible in chromatography.
Action Items / Next Steps
- Practice identifying pure and impure substances using melting and boiling point data.
- Complete chromatography experiments and calculate RF values.
- Review key terms and be able to define and apply them.