The point of this lesson is to show the students that water is a compound made up of the two pure substances oxygen and hydrogen. This lesson also demonstrates the procedure which separates water. But why is someone interested in separating a substance into components? Ask the students for their suggestions. (Hint – think of baking). Perhaps one reason is to identify the individual components of a substance so that the substance can be made over and over. Another reason is that an understanding of a substance’s components gives clues as to the properties of the substance. These ideas are easy to understand in the context of a recipe. A cook cannot continue to bake a cake if he or she doesn’t know which ingredients go into the cake. Furthermore, a lot can be understood about the cake (its taste, color, its nutrition value) if one knows what ingredients are in the recipe.

If this lesson comes after a unit on mixture and chemical reactions, it may be appropriate to review some of the ways that mixtures and reactions can be separated. For example, distilling, freezing, and heating are a few of the physical methods to separate a substance into its individual components. But water is a little more tricky. None of these methods separate water. After freezing, distilling, and heating water, what remains is still water.

Fortunately a new method has been developed that was first used at the beginning of the nineteenth century. This method uses electricity; hence the term used to describe the separation of water is electrolysis.

Objectives:

1. To separate water into its component parts, hydrogen and oxygen.
2. To synthesize water from hydrogen and oxygen.
3. To develop some lab techniques useful in other lab experiments.

compound

pure substance

hydrogen

oxygen

electrolysis

electrode

synthesis

electrolyte

for a group of 2-3 students – however it may be easier for the teacher to do this experiment as a class demonstration.

The apparatus should be assembled as shown in the figure.

Procedure:

1. Set up the apparatus without connecting the wires to the battery using the following method: Fill the beaker half full of water, fill the test-tube with water and gently tap the stopper into the top of the test-tube. Invert the test-tube into the beaker of water, holding your finger on the stopper. When the test-tube is in the water, remove your finger. Put the test-tube into the clamp. Do the same with the other test-tube. Use the scoopula to dislodge the stoppers from the test-tubes. Leave the stoppers in the bottom of the beaker where they will be convenient to use later.
2. Connect the wires to the battery. Not much will happen, perhaps a few bubbles of gas will form.
3. Add 20-30 ml of washing soda. Washing soda is an electrolyte. An electrolyte speeds up a chemical reaction. (To make the washing soda solution, dump some washing soda into a beaker. Fill the beaker with water. Some washing soda will dissolve and the rest will remain on the bottom. Use the solution).
4. Gas should form in the test-tube. Leave the battery connected about 30-40 minutes or until one test-tube is about 1/3 to 1/2 full of gas. Note which test-tube was connected to the "-" and which to the "+" side of the battery. (The test-tube with the most gas is the one with the electrode connected to the "-" side of the battery.)
5. Disconnect the battery. Remove the electrodes being careful not to allow the mouth of the test-tube to get out of the water. Replace the stoppers in the test tubes by moving the test-tube around so the mouth is over the stopper and push down on the stopper until the stopper is firmly in the test-tube. Remove the test-tube and set it in the test-tube rack. Remove the other test-tube.
6. Invert each test-tube (stopper down) and place a rubber band around the test-tube to mark the level of gas in the test-tube.
7. Prepare a table, such as the one shown below, for data and information to be gathered.

8. Test the test-tube that was connected to the "-" side of the battery in the following way: Prepare a "burning" wood splint. Remove the stopper with the test-tube vertical and quickly insert the splint in the test-tube. Do not drop it in the water in the test-tube. The gas should "pop" indicating "hydrogen" gas. Hydrogen explodes when ignited.
9. Test the test-tube that was connected to the "+" side of the battery in the following way: Prepare a "glowing" wood splint. Remove the stopper with the test-tube vertical and quickly insert the splint in the test-tube. Do not drop it in the water in the test-tube. The wood splint should burst into flame indicating "oxygen" gas. Oxygen supports combustion.
10. Enter the information in the table, including the names of the gases under "Gas".
11. In all this, the position of the rubber band should not be disturbed. To measure the volume of each gas, empty the water or put in more water up to the level of the rubber band in each test tube. Measure the amount of water using the graduated cylinder. Put the numbers in the table. Be careful to put the larger volume under "Hydrogen".
12. Find the "Volume Ratio" by dividing the volume of hydrogen by the volume of oxygen. (The ratio should be 2:1)
13. Compare the ratio with other groups in the class. (Some ratios may not be 2:1, but if all the ratios in the class are averaged, the result should be close to 2:1.)
14. If the students are up to it, the mass of each gas may be found by multiplying the volume of each gas by its density because mass = (volume)(density)

            Mass of hydrogen = (Volume of hydrogen) x (Density of hydrogen)

            Mass of oxygen = (Volume of oxygen) x (Density of oxygen)

            Density hydrogen = .000084 gm/cm³ Density oxygen = .0016 gm/cm³

15. Enter the mass numbers in the table and find the mass ratio. (The mass part of the experiment may easily be omitted if the students are younger than grade 8.)

Have each student fill out the data table. Also, have them write a conclusion to make sure they understand the main ideas of the experiment. That is, by performing this experiment, the students should understand that water is a compound and, therefore, must be separated by electrolysis, not the physical methods that we commonly think of. More specifically, this lab shows that the two molecules that make up water are oxygen and hydrogen in a two to one ratio. Hence, the molecular formula for water is H₂0 (two hydrogens for every oxygen).

Errors would include losing gas, problems testing the gas, moving the rubber band, errors in reading the graduated cylinder, errors in calculations.

The synthesis of water may be preformed by collecting both gases in the same test-tube. Set up the apparatus as before. When the test tubes are about 2/3 and ¸ full, switch the connections to the battery and finish collecting gases in the test tubes. Try to collect a full test-tube of gas. Each test-tube will have both gases in it. Note that the gases in the test tubes are still gas, not water. Remove the test tubes as before and set them in the test-tube rack. Each test-tube should be full of gas. It is not necessary to use any rubber band to mark the level of gas. Prepare a burning splint. When ready, open one test-tube and quickly insert the burning splint. There will be a large "bang". The hydrogen explodes and the oxygen supports the combustion, therefore a big bang. Water has been produced - but the amount of water is too small to notice - probably a drop of water was made. A chemical method (electrolysis) was required to separate water and a chemical method (synthesis) is required to make water from hydrogen and oxygen gas.

In this experiment, we learn that water is made by the chemical reaction of oxygen and hydrogen. For other labs dealing with chemical reactions check out "Volcano!", "Chemical Reactions - Acid Rain", "A Green Penny?", "Must it Rust?", and "Lose the indicator Blues".

Science Content Standard #1:Nature of Science

This experiment satisfies Benchmark #3 for grades 5-8:"Collect and summarize data from an experiment and interpret the results in terms of the data."

Science Content Standard #2: Physical Setting

This experiment partly satisfies Benchmark # 6: "Investigate the properties of density and conservation of matter." Furthermore it partly satisfies Benchmark # 7: "Understand the cycling of water..."

This lesson would fit well into any unit on water, but it goes best in a chemistry that introduces chemical reactions.