Experimental Techniques and Chemical Analysis

The One Thing

Experimental chemistry identifies and separates substances by exploiting differences in their physical properties or characteristic chemical reactions. Reliable conclusions require appropriate apparatus, controlled procedures, accurate measurements, and careful observation.

Reactions, Processes and Experiments

Key Terms

• Mixture: Two or more substances together that are not chemically bonded and can be separated by physical methods.
• Filtration: Separation of an insoluble solid from a liquid using filter paper; the solid is the residue and the liquid is the filtrate.
• Crystallisation: Obtaining a soluble solid by evaporating some solvent, cooling the concentrated solution, and forming crystals.
• Simple distillation: Obtaining a solvent by boiling it and condensing its vapour; suitable when the dissolved substance has a much higher boiling point.
• Fractional distillation: Separating miscible liquids with different boiling points using a fractionating column.
• Separating funnel: Apparatus for separating immiscible liquids that form layers because of different densities.
• Chromatography: Separation based on the different distribution of substances between a stationary phase and a moving solvent.
• Stationary phase: The material that remains in place during chromatography, such as paper.
• Mobile phase: The solvent that moves through the stationary phase and carries dissolved substances.
• Chromatogram: The pattern of separated spots produced by chromatography.
• Rf value: The ratio of the distance travelled by a substance to the distance travelled by the solvent front: Rf = distance travelled by substance ÷ distance travelled by solvent front.
• Pure substance: A substance containing only one element or one compound; it has a sharp melting point and boiling point.
• Impure substance: A substance containing more than one substance; it usually melts or boils over a range of temperatures.
• Qualitative analysis: Identification of substances or ions using characteristic chemical reactions rather than measuring their amounts.
• Precipitate: An insoluble solid formed when two solutions react.
• Flame test: Identification of some metal ions by the characteristic colour produced in a flame.
• Acidified silver nitrate test: Test for halide ions; chloride gives a white precipitate, bromide a cream precipitate, and iodide a yellow precipitate.
• Acidified barium nitrate test: Test for sulfate ions; a white precipitate of barium sulfate indicates sulfate.
• Test for ammonium ions: Warming with aqueous sodium hydroxide releases ammonia, which turns damp red litmus paper blue.
• Gas test: Identification of a gas from a characteristic observation, such as a glowing splint relighting in oxygen.
• Titration: Quantitative determination of concentration by reacting a solution with one of known concentration.
• Endpoint: The point in a titration at which an indicator changes colour, showing that the reaction is complete.
• Accuracy: How close a measured value is to the true or accepted value.
• Precision: How close repeated measurements are to one another.
• Reliability: The consistency of results, improved by repeating measurements and identifying anomalous values.
• Anomalous result: A result that does not fit the pattern of the other results and may result from experimental error.
• Control variable: A variable that is kept constant.
• Independent variable: A variable deliberately changed.
• Dependent variable: A variable that is measured.
• Burette: Apparatus that delivers measured volumes accurately.
• Pipette: Apparatus that transfers a fixed accurate volume.
• Conical flask: Flask commonly used to contain the reaction during titration.
• Meniscus: The curved surface of a liquid; for colourless solutions, the bottom is read at eye level.

Easily Confused

• Filtration and crystallisation: Filtration separates an insoluble solid from a liquid, whereas crystallisation obtains a soluble solid from a solution.
• Simple and fractional distillation: Simple distillation obtains a solvent from a solution, whereas fractional distillation separates miscible liquids with different boiling points.
• Stationary phase and mobile phase: The stationary phase remains in place, whereas the mobile phase moves through it.
• Pure and impure substances: A pure substance has fixed melting and boiling points, whereas an impure substance usually has lowered or broadened melting and boiling ranges.
• Qualitative and quantitative analysis: Qualitative analysis identifies what is present, whereas quantitative analysis determines how much is present.
• Accuracy and precision: Accuracy concerns closeness to the true value, whereas precision concerns closeness between repeated values.
• Reliability and an anomalous result: Reliability concerns consistency across results, whereas an anomalous result does not fit the overall pattern.
• Endpoint and concordant titres: The endpoint is the indicator colour-change point in one titration, whereas concordant titres are closely agreeing repeated titre measurements.
• Hydrogen and oxygen gas tests: Hydrogen gives a squeaky pop with a lighted splint, whereas oxygen relights a glowing splint.
• Carbon dioxide and chlorine tests: Carbon dioxide extinguishes a lighted splint and turns limewater milky, whereas chlorine bleaches damp litmus paper and may first turn it red.
• Acidified silver nitrate and acidified barium nitrate tests: Acidified silver nitrate identifies halide ions, whereas acidified barium nitrate identifies sulfate ions.
• Burette and pipette: A burette delivers measured variable volumes accurately, whereas a pipette transfers a fixed accurate volume.
• Residue and filtrate: The residue remains on the filter paper, whereas the filtrate passes through it.

What Gets Asked

• Selecting a separation technique: Questions require matching filtration, distillation, crystallisation, chromatography, or a separating funnel to the relevant property difference. Marks are lost when the method is chosen without referring to insolubility, boiling point, solubility, movement through phases, or density.
• Describing chromatography: Questions may require a valid procedure or interpretation of an Rf value. Marks are lost by using ink, placing spots below the solvent level, failing to use a pencil baseline, or not marking the solvent front immediately.
• Identifying ions and gases: Questions provide observations such as precipitate colours, flame colours, litmus changes, limewater changes, or splint results. Marks are lost by confusing chloride, bromide, and iodide colours or by assigning hydrogen, oxygen, carbon dioxide, chlorine, or ammonia the wrong test.
• Explaining purity: Questions compare melting or boiling behaviour. Marks are lost by stating that an impure substance has a sharp fixed melting point rather than a lowered or broadened melting range.
• Setting up or explaining titration: Questions may test apparatus, meniscus reading, endpoints, concordant titres, or calculation. Marks are lost by confusing the burette and pipette, reading the meniscus incorrectly, failing to convert cm³ to dm³, or ignoring the balanced-equation mole ratio.
• Evaluating practical reliability and safety: Questions may ask about variables, repeated measurements, anomalous results, uncertainty, or precautions. Marks are lost by failing to identify control, independent, and dependent variables, or by omitting eye protection, careful handling of acids and alkalis, fume-cupboard use for harmful gases, and the rule never to directly smell chemicals.