072 – Water electrolysis

This experiment demonstrates the electrolysis of water, a process in which electrical energy is used to decompose water into hydrogen and oxygen gases. By applying a direct current through an acidified water solution, students observe gas formation at each electrode and test the properties of the resulting products. The activity illustrates the principles of oxidation and reduction reactions as well as the conversion of electrical energy into chemical energy. Through qualitative tests, learners confirm the identity of the gases produced — hydrogen, which burns with a “pop,” and oxygen, which supports combustion. This lab provides a foundational understanding of electrochemical reactions and their applications in energy and environmental science.

Educational Goals

    • Understand the principle of electrolysis — recognize that water can be decomposed into its elemental gases, hydrogen and oxygen, through the use of electrical energy.
      • Observe chemical evidence of decomposition, including gas formation at both electrodes and the characteristic tests for identifying each gas (reignition of a glowing splint for O₂ and the “pop” test for H₂).
    • Develop practical laboratory skills by:
      • Assembling an electrolytic cell with proper electrode connections.
      • Measuring and handling liquids accurately.
      • Operating a current generator safely and efficiently.
    • Apply theoretical concepts of oxidation and reduction, understanding that:
      • Oxidation occurs at the positive electrode (oxygen gas production).
      • Reduction occurs at the negative electrode (hydrogen gas production).
    • Reinforce critical thinking through prediction, observation, and explanation of experimental outcomes, connecting macroscopic results to microscopic chemical reactions.
    • Cultivate safe laboratory conduct, particularly when handling electricity, acids, and flammable gases.
    • Appreciate the broader significance of electrolysis in modern science and technology — including hydrogen fuel production, water purification, and renewable energy storage.
    • Encourage scientific curiosity and reflection, helping students see how chemical transformations link directly to real-world applications and sustainable innovation.

    Protocol

    PART A: The electrolysis of water

    1. Attach the universal clamps to the 2 universal supports on the right, one clamp per support, at approximately 20 cm from the base.
    2. Fill the beaker with 1 L of water using tap water.
    3. Place the 1 L beaker in the center of the 2 supports, under the 2 clamps.
    4. Fill to the brim the two test tubes with tap water.
    5. Put the stoppers on the openings of the test tubes.
    6. Fix the two test tubes to the universal clamps by positioning them upside down, so that they are immersed in the 1 L beaker filled with water (the opening of the test tubes must remain immersed at all times).
    7. Once the test tubes are attached to the clamps, remove the stoppers from the openings. Ensure that air bubbles have not lodged at their upper end.
    8. Attach an electrode to each test tube: the positive electrode (red electrode) to test tube 1, and the negative electrode (black electrode) to test tube 2.
    9. You will find two conducting wires (1 red, 1 black) in front of the current generator.
    • Connect the flat part of the red conductor wire to the positive terminal of the current generator (red terminal on the right), and the other end (crocodile clip) to the electrode of test tube 1.
    • Connect the flat part of the black conductor wire to the negative terminal of the current generator (black terminal on the left), and the other end (alligator clip) to the electrode of test tube 2.
    1. Measure 15 mL of hydrochloric acid using the graduated cylinder.
    2. Pour the contents of the graduated cylinder into the 1 L beaker.
    3. Mix everything for a few seconds using the glass rod.
    4. Turn on the current generator. Start the stopwatch.
    5. Let the reaction occur for about 1 minute. Stop the stopwatch.
    6. Turn off the generator.

    PART B: Product Analysis

    1. Attach a clamp to the left universal support, approximately 40 cm from the base.
    2. Remove the electrodes and the alligator clips from the immersed test tubes.
    3. Remove test tube 1 from the water, always upside down, and ensure that any residual liquid flows into the beaker below. Then fix the test tube upside down to the left-hand stand using its clamp.
    4. Light a wooden splinter then make it a red ember by shaking it.
    5. Bring the splint close to test tube 1 and, while keeping the test tube opening facing downward, quickly insert the wooden splint without touching the sides.
    6. Light a second wooden splint but keep the flame alive, then bring it close to the opening of test tube 1 (positive) and quickly insert the splint into the test tube without touching the sides.
    7. Put test tube 1 back on the metal test tube rack, on the top right shelf.
    8. Redo steps 18 to 22 with test tube 2 (negative).
    9. The results of the observations are found in the results table.
    10. Empty the liquids into the recovery beaker on the right counter, and deposit the used splints into the waste container.

    Anticipated Outcomes

    During electrolysis, electrical energy decomposes water into its elemental gases—oxygen (O₂) at the positive electrode (anode) and hydrogen (H₂) at the negative electrode (cathode). Because water itself is a poor conductor, the small amount of hydrochloric acid added acts as an electrolyte, increasing ionic conductivity and allowing current to flow efficiently between electrodes. Gas bubbles will gradually form on both electrodes, with approximately twice the volume collected in the hydrogen tube compared to the oxygen tube, consistent with the stoichiometric ratio in the reaction 2 H₂O → 2 H₂ + O₂.

    In Test Tube 1 (O₂), introducing a glowing red ember is expected to cause re-ignition or a visible brightening of the glow, confirming the presence of oxygen—an essential supporter of combustion. When a burning splint with a direct flame is applied, no noticeable reaction should occur beyond normal burning, since oxygen itself is not flammable but supports the combustion of other materials.

    In Test Tube 2 (H₂), bringing a glowing ember near the mouth should cause no reaction, as hydrogen alone does not reignite a weak ember. However, when a lit flame is applied, the hydrogen will combust rapidly with the residual oxygen in the air, producing a brief “pop” sound. This distinct popping noise serves as the classical qualitative test for hydrogen gas formation.

    Collectively, these outcomes demonstrate that electrolysis separates water into two distinct gases with dramatically different combustion behaviors—oxygen, which sustains fire, and hydrogen, which burns explosively in air. The experiment thus provides direct sensory evidence of water’s molecular composition and illustrates the conversion of electrical energy into chemical energy and back into thermal and light energy through combustion.

    Significance and lessons learned

    • Understanding electrolysis: This experiment provides a clear visualization of electrolysis, an important chemical process with applications ranging from industrial chemical production to the development of clean energy technologies.
    • Chemical principles and safety: Participants learn to handle chemicals and conduct experiments safely while observing firsthand the reactive properties of hydrogen and oxygen, two fundamental elements in chemistry.
    • Practical skills: The experiment enhances skills in setting up experimental apparatus, conducting controlled reactions, and interpreting observable results, which are critical competencies in scientific research and laboratory work.
    • Conceptual connections: By linking theoretical knowledge with practical experience, the experiment reinforces understanding of chemical reactions, stoichiometry (the 2:1 volume ratio of hydrogen to oxygen produced in water electrolysis), and the basic principles of electrochemistry. This lab exercise not only deepens the understanding of chemical and physical properties of water and its constituent gases but also exemplifies the interconnected nature of scientific concepts, demonstrating how they can be applied to understand and manipulate the natural world.

    Summary of Assignment by Grade Range

    Grades 3-5 (Ages 8-10)

    • Focus: Basic introduction to electrolysis and simple observations of gas formation.
    • Activities: Observing water electrolysis and noting the formation of bubbles (hydrogen and oxygen gases), simple discussions on the process of electrolysis, basic safety instructions.

    Grades 6-8 (Ages 11-13)

    • Focus: Intermediate understanding of electrolysis, chemical reactions, and gas production.
    • Activities: Conducting water electrolysis, measuring gas production at electrodes, understanding the basic principles of electrolysis, following detailed safety protocols.

    Grades 9-12 (Ages 14-18)

    • Focus: Advanced understanding of electrolysis, chemical reactions, and stoichiometry.
    • Activities: Accurately conducting water electrolysis, measuring and recording gas volumes, analyzing the reaction mechanisms, detailed recording and interpretation of results, adhering to advanced safety protocols, reinforcing concepts of chemical reactions and gas production.

    Laboratory essentials

    Instruments

    • Beaker (750 ml & 1000 ml)
    • Electrical wires
    • Glass rod
    • Graduated cylinder (25 ml)
    • Lab power supply
    • Lab stand & Clamps
    • Test tubes
    • Test tubes electrodes
    • Timer
    • Wood pieces

    Products

    • HCl 1.0M (solution)
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