Chemical Analysis and Laboratory Skills
Separating and identifying substances
Filtration separates an insoluble solid; evaporation recovers a dissolved solid; distillation separates liquids by boiling point; chromatography separates by differing affinities for stationary and mobile phases. In paper chromatography, Rf = distance travelled by spot / distance travelled by solvent front; Rf depends on the solvent and temperature.
Qualitative tests
Flame tests identify metal ions by characteristic colours. Precipitation reactions identify anions: chloride gives a white precipitate with silver nitrate, sulfate a white precipitate with barium chloride, and carbonate effervesces with acid. Confirm precipitates by solubility in excess reagent.
Quantitative skills
Use volumetric glassware correctly: read the meniscus at eye level, rinse burettes with the solution, and record readings to two decimal places. Repeat titrations and average concordant results. Report values with appropriate significant figures and uncertainty.
Traps
- Using a measuring cylinder where a volumetric flask is required.
- Reading the top of the meniscus.
- Reporting an average of non-concordant results.
- Forgetting that Rf is unitless and solvent-specific.
- Confusing the purpose of a control with that of a blank.
Rule of thumb
For any analysis, ask: what is being measured, what is the uncertainty, and what would a valid result look like?
Worked example
A spot travels 3.0 cm while the solvent front travels 8.0 cm. Rf = 3.0/8.0 = 0.38. Comparing this with a reference value under the same solvent identifies the substance.