AS 1 Experimental Techniques as an internally assessed portfolio: planning a fair investigation, identifying variables and risks, gathering and processing data with appropriate units and uncertainty, presenting results, and evaluating reliability and validity.

What this unit is asking

AS 1 Experimental Techniques is an internally assessed unit, so there is no terminal written paper for it. Instead you build a portfolio of practical work that is marked by your centre against CCEA criteria and moderated by CCEA. The unit asks you to show that you can plan a valid scientific investigation, carry it out safely, collect and process quantitative data correctly, present it clearly, and evaluate how reliable and valid your results are. These are the same skills the externally assessed units (Human Body Systems, Physical Chemistry, Organic Chemistry, Genetics) test through written data questions, so mastering them here lifts your marks across the whole qualification.

Planning a valid investigation

Good planning starts with a clear, testable hypothesis that predicts how the dependent variable responds to the independent variable, with a reason from the underlying science. You then choose a sensible range and number of values for the independent variable (typically at least five, spread widely), and decide how the dependent variable is measured and in what units. Identifying every control variable and saying how each is held constant is what makes the investigation valid: if two factors change together, the result cannot be attributed to either one.

A risk assessment is a marked part of the portfolio. For each hazard (a hot water bath, a corrosive acid, a biohazard such as microorganisms or body fluids, sharp glassware) you state the risk it poses and the control measure that reduces it (goggles, gloves, aseptic technique). Selecting apparatus of an appropriate resolution for the precision you need is also a planning decision.

Collecting and processing data

Raw data go in a results table with the independent variable first, repeat readings of the dependent variable next, and a calculated mean. You decide whether a reading is anomalous (clearly off the trend) and, if so, exclude it with a reason. Percentage uncertainty lets you compare precision: a fixed absolute uncertainty is a smaller fraction of a larger reading, so reading larger volumes or longer times reduces it. Where rate is needed, it is change in the measured quantity divided by time, in units such as cubic centimetres per second.

Presenting and evaluating

Processed data are presented as a graph with the independent variable on the x axis, the dependent variable on the y axis, suitable scales that use more than half the grid, labelled axes with units, accurately plotted points and a line or curve of best fit. Trends are then described and explained using the science of the topic.

The evaluation is where the highest marks sit. You distinguish random errors (scatter around the trend, reduced by repeating and averaging) from systematic errors (a consistent offset, for example an uncalibrated balance or a parallax error read the same way each time, which repeating does not remove). You comment on whether anomalies were present, judge how reliable the data are from the closeness of repeats, judge how valid the conclusion is given the controls, and propose specific, realistic improvements (finer apparatus, more values, better temperature control) rather than vague ones such as "be more careful".

Examples in context

Example 1. Enzyme rate practical. Measuring how temperature affects amylase activity is a classic AS 1 task. The independent variable is temperature, the dependent variable is the time for starch to be digested, and the controls are amylase concentration, starch concentration and volume, and pH. A water bath holds each temperature, repeats give a mean, and the evaluation treats the difficulty of holding the mixture exactly at temperature as a source of error.

Example 2. Rate of an acid reaction. Measuring the gas produced when an acid reacts with a carbonate links AS 1 skills to the Physical Chemistry unit. A gas syringe of fine resolution lowers the percentage uncertainty, repeats establish reliability, and a tangent drawn at the origin of a volume-against-time graph gives the initial rate. The evaluation compares random scatter with any systematic offset such as gas escaping before the bung is fitted.

Try this

Q1. Distinguish between the reliability and the validity of an investigation. [2 marks]

• Cue. Reliability is how consistently the measurement repeats; validity is whether the investigation measures what it intends to, which depends on controlling variables.

Q2. State one source of random error and one source of systematic error in a titration. [2 marks]

• Cue. Random: judging the exact end-point colour. Systematic: a burette with a consistent calibration or zero error, or always reading the meniscus from the wrong angle.

Q3. A balance has a resolution of 0.01 grams and a reading is 2.50 grams. Calculate the percentage uncertainty in this reading. [2 marks]

• Cue. Half of 0.01 is 0.005; $0.005 / 2.50 \times 100 = 0.20\%$.