In middle school chemistry labs, electrolysis of copper sulfate solution is one of the
most visually striking classic experiments. In this seemingly simple device, the gradua
covering of the surface of the metal electrode with a red substance, the subtle
change in the color of the solution, and the bubbles generated around the electrode
together interpret the wonderful world of electrochemical reactions. In this article, we
will take you to observe the whole process of electrolysis of copper sulfate and
reveal the scientific essence behind each phenomenon.
Experimental setup and basic principles
The experiment uses a DC power supply, carbon rod electrodes and a moderate concentration
of copper sulfate solution (recommended 0.5 mol/L). When 6-12V voltage is connected, Cu²⁺
and SO₄²- in the solution migrate to the two poles respectively under the action of electric field.
The reduction reaction (Cu²⁺+2e-→Cu) at the cathode causes copper metal to precipitate
continuously, while the oxidation reaction (4OH-→O₂↑+2H₂O+4e-) at the anode produces
oxygen. The whole reaction system follows the laws of conservation of mass and
conservation of charge, and the total reaction equation is: 2CuSO₄+2H₂O→2Cu+O₂↑+2H₂SO₄.
Record of observation of phased phenomena
1. Initial stage (0-3 minutes)
At the instant of energization, red spots immediately appeared on the cathode surface and
these copper nuclei expanded in all directions at a rate visible to the naked eye. Tiny bubbles
begin to precipitate from the anode area, and the bubbles drive the solution to convection
as they rise, forming a unique fluid pattern. The solution as a whole remains transparent
blue, but there is a color gradient near the electrode.
2. Middle stage of reaction (10-15 minutes)
A dense purplish-red copper layer forms on the cathode surface and the deposit takes on a
typical metallic luster. The rate of bubble generation at the anode is accelerated, and the
nature of the gas can be verified by collecting it by the drainage method. The blue color
of the solution becomes obviously lighter, and at the same time there is a local turbidity
phenomenon in the anode area, which is due to the formation of sulfuric acid from the
generated H⁺ and SO₄²-, resulting in a decrease in the pH value of the solution.
3. Long-term reaction (more than 30 minutes)
The cathode copper layer thickens to 0.5-1mm, and the deposition structure shows dendritic
fractal characteristics. The surface of anode carbon rod gradually becomes thin due to
oxidative corrosion, the solution changes to light blue-green, and white copper sulfate crystals
may precipitate at the bottom. At this time, the measurement of the solution pH value can be
found to be significantly reduced, verifying the generation of sulfuric acid.
Scientific explanation of the key phenomena
1. Dual mechanism of color change
The lightening of the solution color mainly originates from the reduction of Cu²⁺ concentration,
but the turbidity phenomenon in the anode area is the crystallization of copper sulfate due to
local oversaturation. This color change is not a linear process, and when the Cu²⁺ concentration
decreases to a critical value, the solution will suddenly change from blue to light green.
2. Formation law of deposition structure
The deposition pattern of copper cathode is significantly affected by the current density: a dense
coating is formed at low current, and loose dendrites are produced at high current. This
phenomenon can be explained by the theory of electrode polarization and crystallization
kinetics, and the shape of the deposit can be controlled by adjusting the voltage in practice.
3. Verification method of gas generation
The gas generated by the anode can rekindle the wooden bar with sparks, which is confirmed to
be oxygen. However, it should be noted that as the reaction proceeds, the anode may produce a
small amount of chlorine gas as a side reaction (if the solution contains impurities Cl-), so the
experiment needs to be carried out in a ventilated environment.
Innovative applications in teaching practice
1. Exploratory learning design
Guide students to design controlled experiments: compare the differences in reactions at different
concentrations of solution, different electrode materials (copper/graphite), and different voltages.
Through the controlled variable method, help students understand the concepts of concentration
gradient and overpotential.
2. Quantitative Analysis Extension
Instruct advanced learners to perform electrolysis efficiency calculations: weigh the cathode weight
gain, measure the gas volume, and analyze the current efficiency in comparison with the theoretical
value. This data-handling exercise fosters a scientific mindset.
3. Interdisciplinary links
Linking industrial applications of metal refining: explaining the practical application of electrolysis in
copper purification and extending the discussion on the principles of electroplating technology.
Through the comparison of experimental phenomena and industrial production, the ability of
knowledge transfer is enhanced.
Safety regulations and operational points
Attention is required during the experiment:
- Wear protective glasses and rubber gloves
- Control voltage within 12V
- Timely processing of waste electrolyte (acidic solution)
- Avoid electrode short-circuit high temperature
- Children's operation requires professional teacher guidance
The value of this classic experiment lies not only in verifying the chemical equations in the
textbook, but also in cultivating the scientific thinking of the observer. By recording each
phenomenon in detail and analyzing its formation mechanism, learners can truly understand
the essence of the electrolysis process. When the red copper layer appears at the cathode,
what we see is not only the deposition of metal, but also the visualization of the microscopic
ionic world. This teaching method of transforming abstract theories into concrete phenomena
is precisely the charm of chemistry experiments.
Interested readers are advised to conduct extended research under the guidance of professionals:
try to compare the electrolysis of different metal sulfate solutions, explore the behavioral
characteristics of composite electrolyte systems, or use digital sensors to monitor changes
in parameters such as pH and conductivity in real time. These extended experiments will
open a wider window of electrochemical research.