Prepare pure, dry hydrated copper sulfate crystals from copper oxide and dilute sulfuric acid. Copper oxide is an insoluble base; copper sulfate dissolves in the reaction mixture.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
Use a beaker/water bath, spatula, filter funnel/paper, evaporating basin and watch glass. Wear eye protection; acid is irritant/corrosive depending on concentration and copper compounds are harmful.
Warm dilute sulfuric acid in a water bath, as required by the core method. Gentle warming speeds reaction; boiling acid is unnecessary and increases risk.
CuO + H₂SO₄ → CuSO₄ + H₂O. Black copper oxide reacts to form a blue salt solution. The final hydrated crystal contains water of crystallisation.
Preparation sequence
Add small portions of copper oxide to warm acid while stirring, until some remains unreacted after mixing. Insoluble excess shows the acid has been used up.
Filter to remove excess copper oxide. The residue is unwanted black solid; the blue filtrate contains dissolved copper sulfate.Filter off unreacted solid, then concentrate the filtrate gently using a water bath. Do not evaporate to dryness; crystals form on cooling.
Gently heat the filtrate in an evaporating basin, using the instructed Bunsen setup, to remove some solvent. Do not heat to complete dryness.
Allow the concentrated solution to cool so crystals form. Concentrating reduces solvent amount; cooling reduces solubility and allows crystallisation.
Separate crystals from the remaining solution and dry them on a watch glass, with gentle blotting if instructed. Do not strongly heat hydrated crystals to drive out their structural water.
Keep equipment clean and labelled. Retain a sensible amount of solution during evaporation; splashing loses product and overheated residues can be altered.
Purity, yield and evaluation
Excess insoluble base ensures no acid remains, then filtration removes it. A soluble base cannot be removed this way; soluble reagents normally need measured neutralisation such as titration.
When crystals form, some dissolved impurities stay in the liquid left behind (the mother liquor). This can improve purity, although it does not guarantee that the crystals are completely pure.
Some copper sulfate stays dissolved after cooling, and transfer/filtration losses lower yield. Evaporating more solvent can recover more but risks overheating; explain the trade-off.
Dry crystals have no liquid coating their surfaces, but still contain the water molecules in their crystal structure (water of crystallisation). Surface liquid makes the measured mass too high.
For percentage yield, compare actual and theoretical masses of the same form of the salt. Include water of crystallisation if required. Do not compare a measured hydrated salt mass with a theoretical mass for the anhydrous salt (the salt without water of crystallisation).
Use measured reagent quantities where calculating yield, careful transfers and gentle concentration. Repetition assesses consistency but does not correct systematic product loss.
Do not mistake the filtered excess oxide for product or heat the filtrate until all dissolved salt becomes a powder. The required product is hydrated crystals.
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 external liquid consistently left on weighed crystals.
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