AQA GCSE Combined Science Chemistry Required Practicals

how do you conduct the rates of reaction practical

• Mix sodium thiosulfate with hydrochloric acid in a flask over a paper with a cross.

• Time how long until cross is no longer visible.

what is the independent variable of the rates of reaction practical

Concentration of acid or thiosulfate

what is the dependant variable of the rates of reaction practical

Time for reaction to finish

what is the control variable of the rates of reaction required practical

Volume, temperature, paper used

what are the limitations of the rates of reaction practical

Judgement of end point is subjective

what are the common errors of the rates of reaction practical (disappearing cross)

• Starting/stopping timer inconsistently

• Not cleaning flask between trials

how do you conduct the chromatography practical

• Draw pencil line on filter paper.

• Add ink spots, dip paper in water (solvent).

• Watch the solvent carry dyes up the paper.

what is the independent variable of the chromatography practical

Type of ink or solvent

what is the dependant variable of the chromatography practical

Distance dyes travel / Rf value

what are the control variables of the chromatography practical

Type of paper, solvent volume

what are the limitations of the chromatography practical

Solvent front might not be clear

what are the common errors of the chromatography practical

Pencil line too low or ink dissolves in solvent pool

how do you conduct the identifying ions practical

• Flame tests (for metal ions)

• Sodium hydroxide tests (for metal hydroxides)

• Gas tests (e.g. limewater for CO₂, glowing splint for O₂)

what is the independent variable of the identifying ions practical

Substance tested

what is the dependant variable of the identifying ions practical

Observation (colour change, precipitate, gas produced)

what are the control variables of the identifying ions practical

Amount of substance used

what are the limitations of the identifying ions practical

Some colours may be hard to see

what are common errors of the identifying ions practical

• Cross-contamination of wires in flame tests

• Not recognising correct gas test result