In early Greece, water was considered one of the four basic "elements" (earth, wind, and fire as the others) in the universe. With the discovery of more basic constituents, the term "element" took on a different meaning that we use today. Compounds are predictable units of matter that can be reduced to their constituent elements. Elements cannot be broken down into simpler substances that still retain their identities. When you write the chemical formula of water as H2O, you are stating the elementary composition of the compound. Water can be "broken down" into simpler substances (hydrogen and oxygen gas).
However, we also can observe that, in typical environments, water does not just fall apart into its hydrogen and oxygen elements. In fact, you have previously studied the thermodynamic factors that determine when reactions can spontaneously take place. What is the ?G° for the conversion of water to its elements (~ + 470 kJ/mol)? The positive sign on that value indicates that some input of energy is required for water to break down into its elements.
Using information that you have gained in this course, you can also see that the oxidation numbers of hydrogen and oxygen in water must both change in order to become neutral atoms as elementary molecules of H2 and O2. Therefore, you can see that changing water into its elements must be an example of a redox reaction.
In this project, you will investigate the electrolysis (literally tearing apart using an electrical force) of water.
Required Materials
From your supply kit, you will need:
- One nine-volt battery.
- Two alligator clips (or two paper clips to attach to the battery).
- Two pieces of graphite pencil lead.
- One clear glass container (beaker).
- One medicine dropper.
You are expected to furnish the following materials on your own:
- Tape (to attach graphite "electrodes" to the clips).
- Water (de-ionized if available) ~50 mL needed).
- Cabbage juice saved from Project 1 in this course (if you retained the frozen cabbage juice indicator solution as recommended, allow the frozen juice to melt).