3 - Experimental design and practical measurements

Hypotheses, variables and representative samples

A useful investigation starts with a question that observations or measurements can answer.

A hypothesis is a proposed explanation or relationship that can be tested. “Increasing the concentration of an acid increases its reaction rate with a metal because reacting particles collide more frequently” predicts a measurable change and gives a scientific reason. Choose a dependent measurement that tests that prediction, such as gas volume produced in a fixed time. An observation that a reaction “looks vigorous” is harder to compare consistently.

The independent variable is changed or selected by the investigator. The dependent variable is measured for each independent-variable value. Control variables are kept the same because they could also influence the dependent measurement. In the acid investigation, change concentration, measure gas volume after 30 seconds, and control temperature, metal mass and exposed surface area. Explain the effect each control prevents: warmer acid could increase rate even if concentration stayed unchanged.

Choose a range wide enough to reveal a pattern, with several values and repeated trials at each. Plan how to start timing consistently and when to take readings. If the aim is to compare rates, equal total time matters. If the aim is to compare final yield, allow each reaction to finish. The method must match the intended conclusion.

Representative sampling matters when the material varies across a place or batch. A water sample collected from one convenient bank may not represent an entire pond. Define what is being represented; select several appropriate locations and, when time variation matters, several times. Random selection within that defined area can reduce deliberate selection bias. Use clean labelled containers and the same collection procedure so contamination or different depths do not create a misleading comparison. Combining equal-volume samples can give an overall sample, while keeping samples separate preserves differences between sites. Choose according to the question.

Choosing apparatus and controlling practical risks

Apparatus is suitable when its function and measurement quality match the investigation.

A balance measures mass; a stopwatch measures time; a thermometer or temperature probe measures temperature; a measuring cylinder measures an approximate liquid volume. A volumetric pipette delivers one specified volume precisely, while a burette allows a variable delivered volume to be calculated from two readings. A gas syringe measures collected gas volume directly. Select a range and resolution that can distinguish the expected change: a balance displaying 0.1 g steps is unsuitable for a change of about 0.02 g.

Read a clear liquid's meniscus at eye level with upright apparatus on a level surface. For a burette, check that the jet is filled and read the scale in its correct increasing direction. A gas syringe should move freely and be connected by an airtight delivery system. A liquid level or plunger that sticks can change the reading, so correct setup is part of the method.

Other apparatus choices follow the chemical purpose:

Risk control is part of apparatus choice and handling. Identify the hazard, exposure and a suitable control: eye protection for splashes, tongs and cooling time for hot equipment, stable clamps for apparatus, and suitable ventilation for harmful gases. Use a pipette filler rather than mouth suction. Keep flammable liquids away from ignition sources. Gas-generating apparatus needs a safe collection or release route; heating a sealed vessel can build pressure. Follow the approved practical method and risk assessment for the substances and quantities used.

Written planning prepares you to explain these choices. Practical competence also involves carrying out the work, manipulating apparatus correctly and making a contemporaneous record of observations.

Recording and processing results

A clear results record preserves the original observations so that another reader can understand how the conclusion was obtained.

Prepare a table with the independent variable in the first column and headings that include units. Put repeat measurements in separate columns and add a calculated mean column when appropriate. Record a balance reading to the resolution shown, rather than inventing extra decimal places. Include qualitative observations such as bubbling, a colour change or a precipitate with the conditions under which they occurred. A contemporaneous record also includes incidents that might explain an anomalous result.

Delivered volume, mean and range

Illustrative repeat trials: a burette starts at $1.20\,\mathrm{cm^3}$ and finishes at $25.20\,\mathrm{cm^3}$. Two further valid delivered volumes are $23.8$ and $24.1\,\mathrm{cm^3}$.

Delivered volume is the final reading minus the initial reading. Preserve both readings, and keep unrounded intermediate results until the last step.