Mood Rings
The cause of these color changing rings is investigated.

Concepts
Observations, physical changes, conduction

Materials
  • Mood ring
  • Oven mitt
  • Water dropper

Procedure
For each of the steps below observations should be made and recorded. Initially the mood rings should start on a table and be handled a minimal amount so observations can be made of it at room temperature.

1. Inspect the color of the mood rings when they are lying on a table top.
2. Put on an oven mitt and pick up the mood ring between your index finger and thumb. Look to see if the color of the ring changes.
3. Place the ring back on the table. Obtain a dropper and put four drops of room temperature water on it at a different place on the ring (spread the drops around the “four corners” like top, bottom, left, right). Look for color changes.
4. Pick the mood ring up with your bare hand between your index finger and thumb. Look to see if the color of the ring changes.

Drawing Conclusions
Write about the following:
  • The color of the mood ring when on the table, when picked up when wearing an oven mitt, when drops of water were placed on it, and when the ring was placed directly on a hand.
  • Make an inference which explains any color changes. If no color changes were observed make an inference explaining this occurrence.

Read about the two ways matter changes below:
Chemical Change – a process where new substance are formed
  • Chemical changes are not (easily) reversible
  • Chemical changes create new substances

Physical change – a process where nothing new is formed, but its appearance is altered
  • Physical changes are reversible!
  • Physical changes do not create anything new.

Based upon the information you read and the observations you made about mood rings, would you consider the color change of a mood ring a physical or chemical change? Explain.


figure7.jpg
Figure 7: Mood ring touching a hand directly versus touching with an oven mitt on.


Extension 1: Mood Ring Conductivity Experiment
Materials – 2 mood rings, metal block, plastic block

Method – put the two mood rings on a finger until they turn dark blue. Then place one mood ring on a metal block and the other mood ring on a plastic block. Observe their color changes (if any) for a couple of minutes.

Results – describe the colors and color changes of the two mood rings. Make an inference that explains why there is a difference or why there isn’t a difference if this is the case.

figure8.jpg
Figure 8: The mood ring on the metal block returns to room temperature (green) before the mood ring on the plastic block.


Extension 2: Simple Circuits with Mood Rings
Warning: do not use a battery with a Voltage greater than 1.5 Volts
Warning: do not hook the battery up to the mood ring for more than 2 minutes

Materials - Mood ring, battery holder, 1.5 V battery, miniature light socket, miniature 3 V light bulb, knife switch, 3 wires (or alligator clips)

Method – First hook up a 1.5 V batter (in holder) in series with a lamp (light bulb in a socket), knife switch and a mood ring. When hooking the mood ring in the circuit place the wires on opposite sides of the ring. Turn the switch on and the light bulb should turn on. Observe the color of the ring for 2 minutes.

Second, while the circuit is off remove the ring from the circuit. You will be left with a circuit consisting of the battery, switch and the mood ring. Remember, the mood ring needs to have wires on opposite sides of it. Turn the switch on and observe the color of the mood ring for 2 minutes.

Note: the second circuit you created is a short circuit and leaving the switch on for greater than two minutes may cause the battery to become very hot. If you allow students to perform this experiment ensure all batteries are disconnected after 2 minutes is up.

Results - describe the colors and color changes of the mood ring in a circuit with a light bulb and the mood ring in a circuit without a light bulb. Make an inference that explains any differences or why they remained the same.

figure9.jpg
Figure 9: Mood rings in two different series circuits; one with light bulb (left) and one without a light bulb (right). The one without a light bulb changes color.


Extension 3: Relating Mood Ring Color to a Specific Temperature

Materials - 250 mL beaker, water, pipe cleaner, pencil, thermometer, hot plate, goggles

Method – Fill a 250 mL beaker to the 200 mL mark with cold water. Wrap one end of a pipe cleaner around the center of a pencil and the other end around a mood ring. Bend the pipe cleaner so it makes almost an “L” shape. Submerge the mood ring in the water by placing the pencil across the top of the beaker. Put goggles on, set the beaker in the center of the hot plate and turn it on medium-low to medium setting. Measure the temperature of the water and the corresponding color of the mood ring. Monitor the mood ring and as it changes color record the temperature associated with the color. When the temperature of the water reaches about 40 oC turn the hot plate off. It is unnecessary to increase the temperature any further.

Results – make a table of data that includes the temperature of the water the corresponding color of the mood ring. Explain how this data could be useful in a variety of different situations.

figure10.jpg
Figure 10: Mood ring suspended by a pipe cleaner in a beaker of water. A temperature probe can be inserted to determine the temperature when the ring is different colors.
Extension 4: Other Thermochromic Materials
There are several different products sold that change color when exposed to different temperatures. Two of these include color changing pencils and heat sensitive paper. Both work on the same principles as a mood ring with heat causing molecules to twist and reflect light differently. Students can be asked to observe their properties and describe what is occurring as a means of checking their understanding.

thermo pencils paper.jpg
Figure 12: Beside mood rings, there are also pencils and paper that turn color with heat.
Explanation
Trapped inside the band of a mood ring are thermotropic liquid crystals. Liquid crystals (LC’s) are temperature sensitive and change to many colors. LC’s start black below their temperature range, go through the colors of a rainbow, and then turn black again above the temperature range. LC's are reversible and can be used over and over again. Hence, a mood ring is a good example of a physical change.
LC’s respond to changes in temperature by twisting. The twisting changes their molecular structure, which alters the wavelengths of light that are absorbed or reflected. 'Wavelengths of light' is another way of saying 'color', so when the of the liquid crystals changes, so does their color. Check out the example below.

Here is a picture of a liquid crystal made from carbon (grey), hydrogen (light grey) and nitrogen (black). Let’s say this molecule currently reflects red light.
liquidcrystalred.jpg
Now we put on the ring and the metallic part of the ring conducts heat from us to the LC’s causing the molecule to twist a little. Now instead of reflecting red light, it now reflects blue light.

liquidcrystalblue.jpg
In the main activity the mood ring will probably not change color until bare hands touch the ring as this is the only behavior which increases the temperature of the ring.

In Extension 1 the mood ring on the metal block will return to room temperature color faster than the plastic block. This is due to the better conductivity of the metal block and subsequently the faster heat transfer between the ring and the metal block.

In Extension 2 there will be little to no color change of the mood ring in a series circuit with the light bulb. When the light bulb is removed from the circuit the mood ring will change color due to the heat produced from the rapidly moving electricity (short circuit).

In Extension 3 a typical table of data generated by a student is shown below. This data may differ for a given type of mood ring. Regardless, slow heating of the water should enable a student to turn a mood ring from a novelty item into a simple thermometer.

Temperature (degrees Celsius)
Color
20
Black
22
Amber
25
Green
28
Blue-green
30
Blue
33
Dark blue
Table 1: Temperature versus color of a typical mood ring




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