Add weighed portions of powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid and track pH. This is not the separate-science burette titration core practical.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
The independent variable is the total mass of solid base added so far (cumulative mass). The dependent variable is pH, measured using a pH meter or universal-indicator paper.
Control initial acid concentration/volume, solid identity, portion size, temperature, stirring and time before readings. Change one chosen base at a time.
Use a balance, small spatula, beaker, measuring apparatus and pH probe or paper with a clean glass rod. Wear eye protection; acid, bases and dust can harm skin/eyes, and calcium oxide hydration releases heat.
Method
Measure a fixed acid volume and initial pH. Calibrate a pH meter with suitable buffers as instructed, rinse its probe and blot gently between samples.
Weigh a small portion of solid, add it, stir and allow the reaction and reading to stabilise. Record pH and cumulative mass; repeat additions consistently.
If using universal-indicator paper, transfer a small drop with a clean glass rod and compare under consistent lighting. Do not dip used paper into the whole mixture.
Add smaller portions where pH is changing steeply. Large additions may jump past the neutral point. A pH meter usually shows smaller changes than a broad indicator colour scale.
Repeat the entire series with fresh equal acid samples, not just the same reading in one beaker. Control heat changes and avoid inhaling powder.
Chemistry, graph and evaluation
Calcium hydroxide + hydrochloric acid produces calcium chloride and water. Calcium oxide also neutralises acid to form calcium chloride and water; it can first react with water to form hydroxide.
Hydrogen ions are removed during neutralisation, so pH rises as base is added. Excess acid keeps it acidic; excess accessible base can make it alkaline.
Plot total base mass added on the horizontal axis and pH on the vertical axis. Describe the measured curve. An increase of one pH unit means the hydrogen-ion concentration has become ten times smaller.
Equal masses of different bases do not necessarily neutralise equal amounts of acid: formula masses and reactions matter. Keep solid identity fixed unless investigating that comparison.
Stirring distributes the reagent, and waiting for stable readings avoids measuring local concentrations. Undissolved excess base is not proof of a particular exact pH.
An uncalibrated probe may shift all results away from the true values. Use small, consistent additions, rinse the probe between samples, choose suitable resolution and control temperature. Explain which measurement problem each improvement tackles.
Record the actual near-neutral points rather than inventing pH 7. Neutrality depends on conditions; pH around 7 is the usual school-temperature reference.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as an uncalibrated pH probe.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom