AQA Chemistry 8462 - Chemical Analysis Vocabulary Flashcards

71 vocabulary flashcards covering section 4.8 of AQA Chemistry 8462, including purity, formulations, chromatography, gas tests, ion tests, and instrumental methods.

Pure substance (in chemistry)

A single element or compound, not mixed with any other substance.

Melting or boiling point of a pure substance

Occurs at a specific temperature.

Assessment of purity using melting point data

A pure substance has a sharp melting point; impurities usually change and broaden the melting range.

'Pure' in everyday language

A substance with nothing added, such as pure milk or pure orange juice, which is not the strict chemical meaning.

Formulation

A useful mixture designed as a product by mixing substances in carefully measured quantities.

Control of component quantities in a formulation

Ensures each component contributes a particular property so the proportions determine how the product performs.

Examples of formulations

Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Chromatography

A technique used for separating mixtures and helping identify substances.

Phases in chromatography

A stationary phase and a mobile phase.

Stationary phase

The phase in chromatography that does not move.

Mobile phase

The phase in chromatography that moves through or over the stationary phase.

Cause of substance separation in chromatography

Substances have different attractions for the stationary and mobile phases, so they move at different rates.

Chromatogram

The pattern produced after a mixture has been separated by chromatography.

Single spot on a chromatogram

Suggests that the sample may contain one substance under those conditions.

Multiple spots on a chromatogram

Suggests that the sample is a mixture containing several substances.

Identifying an unknown with a known substance in chromatography

Comparing how far they travel under the same conditions, often using $R_f$ values.

$R_f$ equation

$$R_f = \text{distance travelled by substance} \times \text{distance travelled by solvent front}^{-1}$$

Units of $R_f$

None; $R_f$ is a ratio.

Normal range of an $R_f$ value

Between $0$ and $1$.

Prompt marking of the solvent front

Prevents the solvent from evaporating, which would make its final position difficult to measure accurately.

Pencil start line in chromatography

Chosen because pencil graphite does not dissolve in the solvent and interfere with the chromatogram.

Positioning sample spots above solvent level

Prevents samples from dissolving directly into the solvent instead of travelling up the paper.

Use of small concentrated spots in chromatography

Prevents large spots from spreading and overlapping, which would make separation and measurement less clear.

Factors affecting $R_f$ values

The solvent, stationary phase, temperature and other experimental conditions.

Condition control for $R_f$ comparison

Required because changing conditions can change how far a substance travels.

Required Practical 6

Investigation using paper chromatography to separate and identify mixtures, including calculating $R_f$ values.

Measurements needed to calculate $R_f$

Distance from the start line to the centre of the substance spot and distance from the start line to the solvent front.

Matching substances in chromatography

Indicated when two samples produce matching spots or $R_f$ values under the same conditions.

Gas test

A characteristic chemical test used to identify a gas.

Test for hydrogen

Bring a lit splint to the gas; hydrogen burns with a squeaky pop.

Test for oxygen

Insert a glowing splint into the gas; oxygen relights it.

Test for carbon dioxide

Bubble the gas through limewater; the limewater turns cloudy or milky.

Test for chlorine

Expose damp litmus paper to the gas; chlorine bleaches it white.

Use of damp litmus paper for chlorine

Necessary because chlorine needs moisture to produce the bleaching effect.

Flame test

A test used for identifying certain metal ions by their characteristic flame colours.

Flame colour for lithium ions

Crimson.

Flame colour for sodium ions

Yellow.

Flame colour for potassium ions

Lilac.

Flame colour for calcium ions

Orange-red.

Flame colour for copper(II) ions

Green.

Limitations of flame tests with mixtures

Colours can overlap, and a strong colour such as sodium yellow can mask others.

Metal hydroxide precipitate test method

Add sodium hydroxide solution to the sample and observe the colour of any precipitate.

Sodium hydroxide test result for aluminium ions

A white precipitate forms which dissolves in excess sodium hydroxide.

Sodium hydroxide test result for calcium ions

A white precipitate forms.

Sodium hydroxide test result for magnesium ions

A white precipitate forms.

Sodium hydroxide test result for copper(II) ions

A blue precipitate forms.

Sodium hydroxide test result for iron(II) ions

A green precipitate forms.

Sodium hydroxide test result for iron(III) ions

A brown precipitate forms.

Distinguishing aluminium, calcium, and magnesium using sodium hydroxide

Difficult because they all initially form white precipitates, although aluminium dissolves in excess sodium hydroxide.

Test for carbonate ions

Add dilute acid; carbon dioxide is released, which turns limewater cloudy.

Initial observation for carbonate ion test

Effervescence or fizzing after acid is added.

Test for sulfate ions

Acidify with dilute hydrochloric acid, then add barium chloride solution; a white precipitate indicates sulfate ions.

Purpose of acidifying before the sulfate test

Removes interfering carbonate ions that could also form a precipitate.

Test for halide ions

Acidify with dilute nitric acid, then add silver nitrate solution and observe the precipitate colour.

Silver nitrate test result for chloride ions

White precipitate.

Silver nitrate test result for bromide ions

Cream precipitate.

Silver nitrate test result for iodide ions

Yellow precipitate.

Purpose of nitric acid in halide tests

Acidifies the sample without adding halide ions that would interfere with the result.

Required Practical 7

Use chemical tests to identify positive and negative ions in unknown ionic compounds.

Strategy for identifying an unknown ionic compound

Use appropriate tests systematically, record observations carefully, and use the results to identify both ions.

Importance of clean equipment in chemical testing

Prevents contamination that can cause false colours, precipitates, or other misleading results.

Difference between observations and conclusions

An observation records what is seen, whereas a conclusion identifies the substance or ion based on that evidence.

Instrumental analysis

Using analytical instruments to identify substances or determine their composition.

Advantages of instrumental methods

They are sensitive, accurate, and rapid.

Sensitivity in chemical analysis

The ability to detect very small amounts of a substance.

Flame emission spectroscopy

An instrumental method used for identifying metal ions and measuring their concentrations.

Process of flame emission spectroscopy

A sample is placed in a flame and emits light at characteristic wavelengths.

Emission spectrum element identification

Based on the fact that each element produces a characteristic pattern of wavelengths.

Concentration determination via flame emission spectroscopy

Determined because the intensity of emitted light is related to the concentration of the metal ion.

Advantage of flame emission spectroscopy over flame tests for mixtures

The instrument can distinguish characteristic wavelengths even when visual flame colours overlap.

Key rule for answering identification questions

Always state the exact observation from the test before naming the ion or gas.