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Better Equipped Practical Teaching Guides
A complete GCSE Chemistry practical guide to preparing hydrated copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid, covering safe neutralisation, filtration, controlled concentration, crystallisation, percentage yield, troubleshooting, retrieval practice, technician preparation and examination support.
Teacher note: This resource supports practical teaching and equipment planning. Teachers should adapt the method and risk assessment to match their examination-board specification, exact reagent concentrations, current supplier safety data, local laboratory procedures and departmental policies.
Students prepare hydrated copper(II) sulfate crystals by reacting dilute sulfuric acid with copper(II) oxide, an insoluble base. The acid is warmed and then removed from the heat before copper(II) oxide is added in small portions. Copper(II) oxide is added until a persistent excess remains, showing that all the acid has reacted. The excess solid is removed by filtration. The clear blue filtrate is then concentrated using a water bath or approved electric heater and left to cool so that hydrated copper(II) sulfate crystals form.
The practical develops controlled heating, neutralisation, recognition of an excess reactant, filtration, evaporation, crystallisation, recording observations, quantitative chemistry and evaluation of yield and product quality.
Core enquiry question: How can an acid and an insoluble base be used to prepare relatively pure, surface-dry hydrated copper(II) sulfate crystals?
A salt is an ionic compound formed when hydrogen ions in an acid are replaced by metal ions or ammonium ions. In this practical, copper(II) oxide is an insoluble base. Hydrogen ions from sulfuric acid react at the solid surface. Water forms, copper(II) ions enter the solution and sulfate ions remain in solution throughout the neutralisation.
Word equation
Copper(II) oxide + sulfuric acid → copper(II) sulfate + water
Balanced equation
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
Net ionic equation
CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l)
Copper(II) oxide is added until black solid remains after thorough stirring. This persistent excess shows that the sulfuric acid has reacted completely.
The unreacted solid can be removed because it is insoluble. It becomes the residue on the filter paper, while the blue copper(II) sulfate solution passes through as the filtrate.
Controlled evaporation removes some solvent and increases the concentration of the dissolved ions. As the hot concentrated solution cools, hydrated copper(II) sulfate crystals form from the mother liquor.
The blue crystals are commonly represented as CuSO4·5H2O. Surface drying removes adhering mother liquor and surface water; it must not be described as removing the water of crystallisation from inside the crystals.
Particle-level distinction: the blue solution contains dispersed Cu2+(aq) and SO42-(aq) ions, not separate copper sulfate molecules.
Before starting: check that glassware is clean; the receiving flask, evaporating basin and crystallising dish are dry where required; the gas tap is off; the gauze is centred; the filter paper fits correctly; the acid concentration and volume have been checked; the water-bath and hot-apparatus cooling areas are ready; and a labelled copper-containing waste container is available.
This model risk-control information does not replace the school's own risk assessment. The final assessment must reflect the exact concentrations and quantities used, current supplier safety data sheets, current CLEAPSS or equivalent guidance, student experience, equipment and local emergency procedures.
| Hazard or activity | Possible harm | Required control measures | Emergency or corrective action |
|---|---|---|---|
| 1.0 mol dm-3 sulfuric acid | Skin or eye irritation; damage from splashes | Wear splash-resistant eye protection; use small quantities; measure below eye level; keep the container stoppered when not in use. | Alert the teacher immediately; irrigate affected skin or eyes with plenty of water and follow local emergency procedures. |
| Copper(II) oxide powder | Dust may irritate eyes or skin and may be harmful if inhaled or swallowed | Avoid raising dust; transfer with a spatula; keep the container closed; do not touch or taste; wash hands after use. | Stop work, alert the teacher and follow the product safety data sheet and local spill procedure. |
| Adding CuO to warm acid | Frothing or splashing of hot acidic mixture | Turn off the Bunsen burner first; add small portions; stir carefully; do not lean over the beaker. | Move away from the splash, alert the teacher and follow local exposure procedures. |
| Copper(II) sulfate solution and crystals | Eye or skin exposure; harmful if swallowed; environmental contamination | Wear eye protection; avoid contact; no eating or drinking; wash hands; keep product in labelled vessels. | Clean exposures and spills according to the safety data sheet and school procedure. |
| Bunsen burner and gas supply | Burns, fire or ignition of hair and clothing | Check hose and connection; use a heatproof mat; tie hair back; secure ties; keep flammables away; close the gas tap after warming. | Turn off the gas if safe; alert the teacher; follow the laboratory fire procedure. |
| Hot acid, water bath and salt solution | Burns or scalds | Use controlled heating; do not overfill; keep apparatus stable; allow cooling before filtration; use suitable tongs for hot basins. | Cool minor thermal exposure under running water and alert the teacher; follow school first-aid procedures. |
| Evaporating solution | Spitting, splashing or boiling dry | Use a water bath or approved electric heater; do not leave unattended; stop at the concentration endpoint. | Turn off the heat, stand back and allow the apparatus to cool. |
| Glassware | Breakage and cuts | Inspect before use; keep away from bench edges; support the funnel securely; do not force glassware. | Do not pick up broken glass by hand; inform the teacher and use the approved brush and container. |
| Copper-containing waste | Environmental harm | Collect copper-containing solution, crystals, CuO residue and contaminated filter paper in labelled waste streams; follow local disposal procedures. | Contain spills and inform the technician or teacher. |
Measure 40 cm3 of 1.0 mol dm-3 sulfuric acid into a 100 cm3 beaker.
Why? A known starting quantity makes the procedure repeatable and supports quantitative work.
Heat gently until the acid is hot and close to boiling. Do not allow it to boil.
Why? Warming increases the rate of reaction while keeping the process controlled.
Close the gas tap and carefully place the beaker onto the heatproof mat before adding copper(II) oxide.
Why? This reduces the risk of frothing or splashing and prevents unnecessary heating while solid is added.
Add copper(II) oxide in small spatula portions. Stir after each addition and continue until some black solid remains after thorough stirring.
Why? The persistent excess shows that all the sulfuric acid has reacted. Because the excess base is insoluble, it can be removed by filtration.
Allow the mixture to cool sufficiently, then filter it into a conical flask. The black excess solid is the residue and the clear blue copper(II) sulfate solution is the filtrate.
Why? Cooling improves safe handling, and filtration removes the unreacted insoluble copper(II) oxide.
Transfer the blue filtrate to an evaporating basin and heat it using a water bath or approved electric heater. Do not boil it to dryness. Stop when a cooled test drop crystallises or when the first crystals begin to appear at the edge of the basin.
Why? Controlled evaporation removes some solvent so crystals can form when the concentrated solution cools.
Transfer the concentrated solution to a labelled crystallising dish and leave it to cool slowly and undisturbed.
Why? As the concentrated solution cools, hydrated copper(II) sulfate crystals form from the mother liquor.
Separate the crystals from the mother liquor. If a wash step is required, rinse rapidly with only a very small amount of cold distilled water.
Why? Washing removes adhering mother liquor and soluble surface impurities while limiting dissolution of the soluble product.
Gently blot the crystals between filter papers or air-dry them on a labelled watch glass. Record their appearance and mass if required.
Why? Surface-dry crystals can be weighed more accurately; colour and appearance alone do not prove complete purity.
Excess unreacted black copper(II) oxide retained by the filter paper.
Clear blue copper(II) sulfate solution collected in the conical flask.
The remaining solution surrounding the crystals after crystallisation.
| Stage | Expected observation | Interpretation |
|---|---|---|
| Initial sulfuric acid | Colourless solution | No copper(II) ions are present initially. |
| First additions of CuO | Black solid gradually disappears and the solution becomes blue | Copper(II) oxide is reacting and Cu2+ ions are entering solution. |
| Final CuO addition | Some black solid remains after thorough stirring | Copper(II) oxide is in excess and the acid has reacted completely. |
| Filtration | Black residue on the paper and clear blue filtrate collected | Excess CuO has been removed. |
| Concentration | Volume decreases and the blue colour becomes more intense | Some solvent has evaporated. |
| Cooling | Blue crystals begin to form on the outer edges | Hydrated copper(II) sulfate is crystallising. |
| Collection and drying | Blue surface-dry crystals are obtained | Volume of water has been reduced. |
| Measurement or observation | Student result |
|---|---|
| Sulfuric acid concentration / mol dm-3 | |
| Sulfuric acid volume / cm3 | |
| Initial appearance of sulfuric acid | |
| Appearance while copper(II) oxide is added | |
| Evidence that copper(II) oxide is in excess | |
| Appearance of residue | |
| Appearance of filtrate | |
| Evidence that the filtrate is sufficiently concentrated | |
| Appearance of crystals after cooling | |
| Mass of surface-dry crystals / g, if measured |
When crystal mass is measured, the actual yield can be compared with the maximum theoretical yield. For the blue hydrated crystals, calculations must use CuSO4·5H2O rather than anhydrous CuSO4.
Actual crystal mass
mass of watch glass + crystals
minus
mass of empty watch glass
Percentage yield
(actual mass ÷ theoretical mass) × 100
n = c × V
V = 40 cm3 = 0.040 dm3
n(H2SO4) = 1.0 × 0.040 = 0.040 mol
The acid and copper(II) sulfate ratio is 1:1. Copper(II) oxide is in excess, so sulfuric acid is the limiting reactant.
M(CuSO4·5H2O) = 249.6 g mol-1
mass = 0.040 × 249.6 = 9.984 g ≈ 10.0 g
For an actual mass of 7.5 g:
(7.5 ÷ 10.0) × 100 = 75%
Interpreting the result: a yield below 100% may result from crystals remaining in the mother liquor, transfer losses, spitting, excessive washing or incomplete crystallisation. An apparent yield above 100% may be caused by wet crystals, contamination, a calculation error or use of the wrong formula mass.
| Source of error | Likely effect | Improvement |
|---|---|---|
| Too little copper(II) oxide added | Acid may remain in the filtrate and contaminate the product | Continue adding small portions until black solid remains after thorough stirring. |
| CuO added too rapidly to hot acid | Frothing, splashing and loss of mixture | Turn off the burner and add small portions with stirring. |
| Mixture filtered while still very hot | Increased burn risk and potentially poor handling | Allow the beaker and mixture to cool before filtration. |
| Torn or poorly fitted filter paper | Black particles pass into the filtrate | Use intact, correctly fitted paper and re-filter a contaminated filtrate. |
| Solution lost during transfer | Actual and percentage yield decrease | Pour carefully and rinse the reaction beaker with only a small quantity of water. |
| Filtrate heated too strongly | Spitting and product loss | Use controlled heating in a water bath or approved electric heater. |
| Solution evaporated too far | Crust formation, spitting or dehydration of the product | Stop when a cooled test drop crystallises or first crystals appear. |
| Too much or warm wash water used | Product dissolves and yield decreases | Use only a very small amount of cold distilled water. |
| Crystals not surface-dry | Measured mass is too high | Blot gently and allow surface moisture to evaporate before weighing. |
| Crystals left in the mother liquor | Yield is lower than it could be | Collect crystals carefully and allow sufficient crystallisation time. |
| Problem | Likely cause | Corrective action |
|---|---|---|
| No crystals after cooling | Solution was not concentrated enough | Return the mother liquor to the evaporating basin and concentrate it gently in the water bath. Test a cooled drop before stopping. |
| Crystals appeared during heating | Solution has become highly concentrated | Stop heating immediately and allow the solution to cool. |
| Black powder in the filtrate | Filter paper was torn, overfilled or poorly fitted | Re-filter the blue liquid through fresh, correctly fitted filter paper. |
| Black specks in the final crystals | Excess CuO passed through the filter | Redissolve the product, re-filter and recrystallise, or repeat the preparation. |
| Only very small crystals form | Cooling was too rapid or the dish was disturbed | Allow slower, undisturbed cooling using the same starting concentration. |
| Solution remains blue after crystals form | Some product remains dissolved in the mother liquor | This is expected. Further careful concentration may recover more product but can reduce purity. |
| Green, brown or unusual-coloured product | Contamination, incorrect reagent or dirty apparatus | Do not treat the current product as correct. Check reagent labels, apparatus and procedure before repeating. |
| Low yield | Spillage, transfer loss, excessive washing, incomplete crystallisation or crystals left in mother liquor | Review each stage; transfer carefully, minimise wash volume and allow sufficient crystallisation time. |
| Apparent yield above 100% | Crystals are wet or contaminated, or the wrong formula mass was used | Surface-dry the crystals, check for black particles and recalculate using CuSO4·5H2O. |
| Solution spits during concentration | Heating is too vigorous | Reduce heating immediately and use the approved water-bath arrangement. |
The burner must be off before solid is added to reduce frothing and splashing.
The required hydrated product forms by cooling a concentrated solution.
Copper(II) sulfate is soluble, so excessive wash water reduces the yield.
Important: excessive heating can remove water of crystallisation from blue hydrated copper(II) sulfate, changing its colour and measured mass. More severe heating can cause further decomposition.
Independent variable: cooling condition, such as insulated cooling, room-temperature cooling or ice-bath cooling.
Dependent variable: mean longest dimension of a defined number of crystals, measured using a millimetre grid or suitable magnifier.
Control variables: solution volume and concentration, concentration endpoint, starting temperature, crystallising vessel, total cooling time, degree of disturbance, and the method used to collect and dry the crystals.
Reliability: repeat each cooling condition and calculate the mean crystal dimension.
Safety: move hot solutions using the approved equipment and place them only in designated cooling areas.
A strong scientific conclusion should answer the aim directly; use observations and measurements as evidence; explain the chemistry rather than only list the method; include actual and percentage yield where available; distinguish product identity from proven purity; identify likely losses or contamination; and link suggested improvements to those limitations.
This guide uses an AQA-aligned sequence in which a Bunsen burner warms the acid and a water bath or approved electric heater concentrates the filtrate. The source guide identifies this as AQA Required Practical Activity 1 and identifies the preparation of pure, dry hydrated copper sulfate crystals from copper oxide, including use of a water bath, as Pearson Edexcel Core Practical 3.17. Teachers should check the exact terminology and method required by their examination board.
To ensure that all the sulfuric acid has reacted. The remaining black solid shows that copper(II) oxide is in excess.
To reduce the risk of frothing or splashing while solid is being added and to prevent unnecessary heating.
Copper is below hydrogen in the reactivity series and does not react with dilute sulfuric acid, whereas copper(II) oxide is a base and neutralises the acid.
Excess unreacted copper(II) oxide.
The clear blue copper(II) sulfate solution that passes through the filter paper.
Concentration produces a solution from which hydrated crystals can form on cooling. Evaporating to dryness may cause spitting and alter the hydrated product.
It provides more controlled heating and reduces the risk of vigorous boiling, spitting and overheating.
Slow, undisturbed cooling generally produces larger, better-formed crystals.
Cold water removes adhering mother liquor while limiting dissolution of the product.
Adhering mother liquor and surface water would make the measured mass artificially high.
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
(7.5 ÷ 10.0) × 100 = 75%.
Examiner advice summary: avoid saying the acid “disappears”; state that excess solid ensures complete neutralisation; do not say evaporation makes the salt; distinguish concentration from boiling to dryness; do not say filtration removes dissolved salt; distinguish residue and filtrate; do not say surface drying removes water of crystallisation; do not claim blue colour proves complete purity; include state symbols; use CuSO4·5H2O for theoretical-yield calculations on the blue hydrated crystals; and always explain how an improvement affects yield, accuracy or product quality.
These quick questions can be used as a starter, end-of-practical plenary or retrieval task in the following lesson.
| Question | Expected answer |
|---|---|
| Why is the acid warmed? | To increase the rate of reaction. |
| Why is the Bunsen burner turned off before CuO is added? | To reduce splashing or frothing and prevent unnecessary heating. |
| Why is excess copper(II) oxide added? | To ensure all the sulfuric acid reacts. |
| What is the residue? | Excess unreacted copper(II) oxide. |
| What is the filtrate? | The clear blue copper(II) sulfate solution. |
| Why is the filtrate concentrated? | So crystals can form when the concentrated solution cools. |
| Why is the solution not evaporated to dryness? | Vigorous or excessive heating can cause spitting, product loss and alteration of the hydrated product. |
| Why are the crystals surface-dried? | To remove adhering mother liquor and surface water before weighing. |
| What is the formula of the hydrated crystals? | CuSO4·5H2O. |
| What type of reaction occurs? | Neutralisation. |
Teacher demonstration point: before students start, demonstrate how to recognise persistent excess solid, fit and use the filtration setup correctly, judge when enough water has evaporated, and identify proper crystal formation during cooling. Students often struggle most with recognising the correct stage to stop heating.
| Lesson stage | Approximate time |
|---|---|
| Starter and learning objectives | 5 minutes |
| Safety briefing and teacher demonstration | 10 minutes |
| Measure and warm acid | 5 minutes |
| Add copper(II) oxide to excess | 8-10 minutes |
| Cool sufficiently and filter | 10 minutes |
| Concentrate filtrate using water bath | 10-12 minutes |
| Transfer, label and place in cooling area | 5 minutes |
| Crystallisation between lessons | Preferably overnight |
| Observe and collect crystals in next lesson | 10 minutes |
| Surface-dry, weigh and calculate yield | 10-15 minutes |
| Evaluation and plenary | 10 minutes |
Many plant fertilisers are manufactured as salts, including ammonium sulfate.
Pharmaceutical production requires salts with carefully controlled composition and purity.
Salts such as aluminium sulfate help remove impurities from drinking water.
Table salt and many preservatives are produced and purified through controlled chemical processes.
Metal salts support the storage and transfer of electrical energy in many battery systems.
Crystallisation is widely used to isolate and purify chemicals on an industrial scale.
Explore practical equipment, laboratory-planning resources and related support from Better Equipped.
This guide was written and reviewed by Better Equipped's technical team and further reviewed by former A Level Science Teachers. Our technical team draw on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. They include ex-school laboratory technicians and are here to support schools, colleges and laboratories. If you have feedback on this guide, we'd love to here it so please contact us. Don't forget we will be regulalrly updating our guides and resources on our Better-Resources Hub. If you would like us to cover a particular subject matter in these guides or have some top tips you'd like to share then again we'd love to hear from you.
Last reviewed and updated: September 2026
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