Part 1: Planning, measurements and observations

Turn an experimental question into a fair, reproducible investigation, then record data another chemist can interpret.

Written skills and practical competence

Module 1.1 is assessed in written AS and A-level papers: plan, implement, analyse and evaluate. You may meet an unfamiliar experiment, so explain the purpose of each step instead of memorising one recipe.

The full A level also has a separately reported Practical Endorsement (H432/04), based on demonstrated competence under Module 1.2 and the CPAC criteria. AS has no separate endorsement. PAGs organise practical opportunities; reading notes does not replace observed practical work.

Plan a fair comparison

State an independent variable, a measured dependent variable and relevant controls. For acid–carbonate rates, vary acid concentration; measure gas volume at regular times; keep temperature, acid volume and the mass and surface area of carbonate constant. Use acid in a suitable excess if comparing effects without changing the limiting quantity.

Choose a useful range with repeats, a method for starting the clock consistently, and apparatus able to measure the expected quantity. A 100 cm³ gas syringe cannot record 180 cm³ without losing information. Pilot work helps choose the range.

Identify the chemical hazard, the actual exposure risk and a specific control: use eye protection for splash risk; use a fume cupboard for toxic vapour; keep flammable liquids away from ignition sources. Follow the centre’s risk assessment and disposal instructions.

Turn “keep it fair” into a reproducible plan

An independent variable needs actual values and a method of setting them. For example, prepare several acid concentrations by diluting a stock while keeping final volume constant. A dependent variable needs an operational definition: gas volume every ten seconds, or time to a defined visual end point, rather than simply “how fast it reacts”.

Controls must name quantities and explain why they matter. Control temperature because it changes the energetic collision fraction; control carbonate mass because it affects possible product amount; control chip size because it affects exposed surface. Using identical apparatus helps consistency but does not replace those chemical controls.

Pilot the range so the fastest reaction is still measurable and the slowest finishes in a practical time. Select enough intermediate values to identify a trend, then repeat each condition. If mixing and starting the stopwatch take appreciable time relative to the reaction, redesign the starting method instead of treating the delay as negligible.

Record measurements without inventing precision

Put quantity and unit in each table heading, not repeatedly in every cell. Record raw measurements before processing, including initial and final burette readings. Match decimal places to the instrument; retain full calculator values until the final answer.

Describe observations before interpretations: “a white precipitate formed” is an observation; “sulfate ions are present” is an inference. A colour change alone does not prove a unique ion unless the reagent and test conditions are stated.

Graphs, gradients and worked rates

Put the independent variable on x and dependent variable on y, label both with units and use a scale that occupies most of the grid. Draw an appropriate best-fit line or smooth curve, not point-to-point zigzags.

For a tangent to a gas-volume curve, use a large triangle on the tangent, not two distant data points on the curve. If the tangent passes through (10 s, 12 cm³) and (50 s, 44 cm³), gradient = (44 − 12)/(50 − 10) = 0.80 cm³ s⁻¹. A secant instead gives a mean rate across its interval.

Use the graph that answers the scientific question

A graph of gas volume against time shows progress through one reaction. A graph of initial rate against initial concentration compares a series of reactions. These graphs have different independent variables and cannot be substituted for each other. Labelling only “rate” when you plotted gas volume hides this distinction.

Use a best-fit trend rather than forcing a line through every point. An anomalous point should be identified and investigated; retain it in the raw table. The best-fit curve should reflect the chemistry, including any plateau, without using the anomalous point to create an artificial kink.

When interpreting the graph, describe the evidence before the cause: “the initial gradient is steeper” is a graph comparison; “more particles per unit volume cause more frequent effective collisions” is the explanation. Merely saying the graph goes higher can confuse rate with final yield.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources