Measure CO₂ produced by marble chips with hydrochloric acid, and time a disappearing cross when sodium thiosulfate reacts with hydrochloric acid to form sulfur.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
The gas method can vary acid concentration, temperature or marble surface area one at a time. Control marble mass, other conditions and total reagent amounts as appropriate.
For the cross method, Pearson specifies different temperatures by warming thiosulfate before adding acid. Concentration investigations are useful additional work, not a replacement for this temperature comparison.
Use eye protection and dilute school reagents; acid irritates, and thiosulfate/acid releases sulfur dioxide. Use small quantities with teacher-approved ventilation, avoid inhaling fumes and handle warm baths safely.
Gas collection and analysis
Connect a stoppered marble/acid flask to a freely moving gas syringe with secure airtight tubing. Mix reactants, fit the stopper promptly and start the clock consistently.
Record gas volume at fixed intervals until the reaction finishes. Choose a syringe capacity large enough for the expected gas and check that the plunger cannot jam.
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Initial volume/time gradient measures initial rate; a tangent to a curve estimates rate at a particular time.
The curve levels off when a reactant is used up. Higher concentration can increase initial rate, but final gas volume depends on limiting amount, not simply on being faster.
Changing marble particle size while keeping mass constant changes surface area. Small chips/powder have more exposed area and more frequent successful collisions.
Leaks or gas loss before stoppering underestimate collected gas. A method that starts mixing after sealing can improve timing, when safely implemented in the supplied apparatus.
Cross method and quality
Place a flask of measured thiosulfate solution over a cross. Warm it to each selected temperature and measure the actual mixture temperature; add a fixed acid volume, mix and start timing.Disappearing-cross method: compare temperatures using identical volumes, concentrations, viewing geometry and endpoint. Sulfur obscures the cross.
Look from the same position under the same lighting. Stop timing when you can no longer see the cross. The sulfur precipitate makes the liquid cloudy and hides the cross; the cross itself has not disappeared.
Keep concentrations, volumes, total liquid depth, cross size, lighting and endpoint observer/criterion matched. A light sensor can reduce subjective judgement when validated for the same threshold.
Record the temperature and time for each run. If you use the same visual endpoint and mixture conditions, 1/time lets you compare relative rates: a larger value means a faster reaction. Warmer mixtures usually give shorter times.
Let the reagents reach the intended temperature before mixing. Adding cooler acid can change the mixture’s temperature. Measure the actual mixture temperature rather than relying only on the water-bath setting.
Repeat independent runs and compare spread. Start/stop reaction-time errors dominate very fast reactions; choose a measurable range rather than inventing precision.
Higher temperature increases collision frequency and the fraction exceeding activation energy. A catalyst lowers activation energy but is not the changed variable in this temperature test.
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 gas consistently escaping before the stopper is fitted.
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