University of Virginia
Physics Department

## The Chemical Switch

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2
• PS.4
• PS.5
• PS.11

Objectives

Students will

• Understand how electrical sockets provide charges and circuit current to light bulbs.
• Investigate conduction of electricity through different solutions.
• Classify how concentrations of solutions affect passage of current.

Motivation for Learning

Prelimiary Class Questions For Teacher Demonstration

• Can you make a "chemical switch" with a solution/mixture?
• Can you light a bulb without a socket?
• Can you cause water to "come apart" or decompose?

Materials

• A standard bulb holder
• Light bulb (6.3 volts)
• Lantern battery (6 volts)
• Two beakers (125 mL)
• Tap water or distilled water (50 mL in each beaker)
• Solutes - NaCl and Sucrose
• Marker or rubber bands
• Three sections of wire with insulation free ends. This should be standard wire suitable for your bulb holder.

**Note - The demonstration is based on accepted materials and connections. The creative connections is the basis for the student activity.

Procedure

1. Pour 50 mL of tap water or distilled water into each of the beakers.
2. Place bulb into the bulb holder. Connect two wires at least 25 cm long to each side of bulb holder. Connect one wire to battery. Strip insulation off about 3 cm of the other wire going to bulb holder and place the stripped end into a beaker of water.
3. Connect another wire to battery; strip 3 cm off open end and place this end into beaker of water.
4. See diagram labeled Figure 1 below. Two wires in the circuit are placed into the beaker of water.
5. Making sure the wires do not touch, ask students to observe closely for the answers to the following:
(a.) Is there enough current being conducted to light the bulb?
(b.) What other effects of the current do you see?
6. Sprinkle a few grams of NaCl into the water. Ask students to observe closely for the answers to the following:
(a.) Is there enough current being conducted to light the bulb?
(b.) All other factors being constant, what changed the results?
(c.) How do you know when there is exactly enough NaCl to carry the current through the liquid?
(d.) Can you control the current flow to make a "chemical switch?" If so, which is the best experiment segment to control the current?
7. In the second beaker, sprinkle a few grams of sucrose into the water. Then move wire ends from first beaker over to the second. Ask the students to observe closely for answers to the following:
(a.) Is there enough current being conducted to light the bulb?
(b.) All other factors being constant, what caused the results?
(c.) Would concentrating the solute make a difference?
(d.) Does every solute you add help in making the chemical switch? Why or why not?

**Note - The bulb will only emit a glow; it will not completely light up to its normal illumination.

Figure 1

Background Information

There are several components for the activity. Electrical, solution formation and concentration, as well as chemical activity sections, are coordinated to become variables of each other. The implications for each section are explained under the following headings.

Electrical - By making wire sockets, students see how it is possible that electricity reaches its destination as well as the effects of electricity passing through a "path" that they provide. The flow of electrons being pushed by the battery is demonstrated through the various mediums of solution and their concentrations provided for testing. The students recognize that the electrons are flowing out of the negative terminal towards the positive terminal. How they control the electrons route can effect electrical energy. The principle of conductivity dependence on the basis of the presence of ions is reinforced. It is observed that the differences in ion counts influences the passage of electrons going to the other terminal.

Solutions and Ions - As an ionic substance dissolves in water, each ion is surrounded by water molecules. The ionic material is attracted and broken apart by the polar nature of the water. This process of dissolving leaves ions available to attract the electrons placed into the solution by the negative terminal of a battery. This experimental situation simulates the conductivity of charges in everday situations, as in our bodies. How the concentration of ions can control this passage of electrons is a significant principle of this experiment.

Another observation is the decomposing effect of electricity on water. The additional solute influences the outcome rate in electrolysis. The influence of the charges being introduced from an outside source attracting the oppositely charged elements in water conducts at a faster pace as the concentration of ions increases. The students may then compare the distilled or tap water rate of decomposition by volume with that of molecular enhancement.

1. The pattern of charge flow is from negative to positive.
2. The breaks in the circuit would represent an open circuit.
3. The completed connections would represent a closed circuit.
4. The level of conductivity for "pure" water is not detected by the light bulb.
5. Ions are being placed into the water by the dissolving process.
6. 165 grams of NaCl in 450 mL provides the concentration necessary for this activity.
7. The solution is being decomposed.

### Student Activity

Materials

• Electrical cord (sliced), 3 sections of wire 1/2 to 1 foot in length -this is regular insulated electrical wire that is sliced into separate wires
• Electrical tape
• Light bulb (6.3 volts)
• Lantern battery (6 volts)
• Insulated ring terminal (5/16 inch - 3/8 inch stud or 12 - 1/4 inch stud)
• One 500 mL beaker
• 450 to 500 mL of water
• Choice of solute (NaCl or MgSO4) and (C12H22O11 - Sucrose)
• Balance scale or electronic balance
• Marker or two rubber bands

**Note - Students are to be aware that connecting wires to the battery is the last step for part 1.

Procedure

Part 1:

1. Strip the insulation from each end of the 3 wire sections.
2. Place one wire section end through the opening of the ring terminal and across the ring.
3. Wrap the wire end around the ring edge to secure.
4. Indent the wire across the ring opening to support the light bulb underneath.
5. Take another section of electrical wire and wrap the end around the bulb. * This would be at the top of the metal casing, but under the bent edge of the bulb. The tightly wrapped wire is held in place with a small piece of electrical tape.
6. Put the bulb with the attached wire onto the indented wire across the ring opening.
7. Secure all parts together with electrical tape.
8. Connect the free ends of the wire to the battery and test for closure of the circuit.
****Time is given for readjustments in the circuit permitting student decision making.

Part 2:

1. Mass the selected solute in several gram parts or allotments. This may be by 10 or 20 grams portions until or if you reach the 100 gram addition.
2. Place 450 to 500 mL of water in a beaker.
3. Mark very clearly the beginning water level with a marker or a rubber band.
4. Place the ends of the wires from Part 1 into "pure" water.
5. Look for changes at the ends of the wires.
6. Observe for conductivity.
7. Add 10 or 20 grams of solute to the water. Test with battery and bulb.
8. Continue to add solute in 10 or 20 gram increments or portions and test for conductivity.
9. Record results in the table
10. Calculate the solute concentration for each trial, using the formula, Mass/Volume.
11. Identify the concentration that initiates conductivity

Part 3:

1. Make observations of the very small bubbles coming from the wire ends into the solution. This should be done at the initial hook-up of the battery. This observation will be the "point of comparision" for bubble production as the solute is added in increments.
2. Compare the rate that each wire is producing bubbles. Is the negative wire more active than the positive?
3. Decide the "run time" and observe for any changes in the solution height.
4. At the end of the "run time," try to measure by number of mL or by mass of solution what has occured.

Data Table

Part 2 Data Table

 Grams of solute Observations (conductivity) Concentration (g/mL)

Part 3 Data Table

Initial water level __________________mL = _______________grams

Final water level __________________mL = _______________grams

Detectable change ________________mL = _______________grams

Extentions

The accumulated data for the concentration vs. conductivity observations may be used with the Microsoft Word Excel Program. The scatter plot works well with concentration as x values and grams of solute as the y values. An example is shown below. Another scatter plot may be done with concentration as x values and the constant volume as the y value. This arrangement may be useful for student comparisions as a visual aid. An example is also shown.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

1. What is the pattern in the circuit in which the charges flowed?

2. If any breaks occur in the wiring, which type of circuit would be represented?

3. If all the connections are complete, which type of circuit would be represented?

4. In the pure water, what was the level of conductivity? (light on or light off)

5. As the solute was added, what is changed to help charges "travel" in the liquid?

6. What was the concentration for which the light turned on?

7. Is the solution changing its state to gas or is it being broken down into component gases?

8. Compare different solute concentrations for conductivity.