The Glowing Penny
WHAT YOU NEED
- 250 mL beaker and white card.
- 5 mL acetone.
- glass stir rod.
- copper wire (10 cm).
- penny.
- Bunsen burner, small propane stove or torch.
- matches.
- safety goggles.
- oven mitts or gloves.
WHAT TO DO
To prepare:
- Wrap the copper wire around the penny so that when suspended the flat face is facing outwards (towards audience).
- Wrap the other end of the copper wire around the middle of the glass stir rod.
- Balance the stir rod over the beaker and measure the length of the wire so that the penny is suspended about 3-4 cm above the bottom of the beaker.
During the demo:
- Wearing the gloves and goggles, hold onto the glass rod heat the penny over the Bunsen burner until it glows red. Have students tell you when it is hot.
- Remove the penny from the burner, turn out the lights, and have the students count out loud until the penny is no longer glowing.
- Reheat the penny until it glows red.
- Add acetone to the beaker so that it is 1-2 cm deep.
- Quickly suspend the penny in the beaker over the acetone without touching the penny to the acetone (white card behind so the acetone is easier to see).
- Watch the penny glow red and have the students count out loud until they count significantly longer than the first time.
WHAT'S GOING ON?
This demonstration introduces the concept of a vapor, and demonstrate an exothermic reaction.
Think about the difference between a gas and a vapor. A gas is a substance that is normally in the gaseous state at standard temperature and pressure; a vapor is the gaseous form of a substance that is normally in the liquid or solid state at standard temperature and pressure. For example, at room temperature we speak of water vapor and oxygen gas. Evaporation is the process by which a liquid changes into a vapor. The molecules from the liquid enter the surrounding air. The acetone used in this demonstration is very volatile (it changes from a liquid to a vapor very easily at room temperature) and therefore a layer of acetone vapors surround the suspended penny.
Heat can be generated in many different ways. Did you know that some chemical reactions can produce heat? This is called an exothermic reaction. An exothermic reaction gives off heat to its surroundings as a chemical or physical change occurs. In this demonstration an exothermic reaction occurs because the copper is a catalyst (a substance that increases the rate of a chemical reaction without itself being consumed) for acetone changing to acetaldehyde, a very exothermic reaction. When bonds are broken energy is released (like a Christmas cracker). The heat from this reaction keeps the penny glowing red hot.
QUESTIONS
During the Demonstration:
- Do you think that the penny is still hot? Why? (it is still glowing).
- Is anything touching the penny? (wire, vapor from the acetone).
- Was the acetone hot before we put it in the beaker? (no).
- Was anything occurring between the acetone and the air? (evaporation like water in a glass left out or rain).
- What keeps the penny hot? (the penny helps the acetone change to acetaldehyde, which produces heat - an exothermic reaction).
- Would the penny glow if it was hanging higher above the acetone level? (no, it wouldn't be exposed to the vapors).
- How long will the penny stay glowing? (until all the acetone has evaporated).
After the Demonstration:
- Can anyone tell me the three states of matter? (solid, liquid, and gas).
- Optional: introduce the concept of a catalyst.
- Would this work with a dime, a quarter, or any other metals? (no, catalysts are specific to their reactions).
SAFETY
- Acetone is extremely volatile and will ignite very easily. Do not immerse the hot penny in it (although the hot penny will not ignite the acetone). Keep a tin pie plate on hand to smother a flame if one starts in the beaker.
- Always wear heat resistant gloves or mitts, the glass rod can get very hot.
- Always wear safety goggles.
- Warn that the students should not try this demonstration at home.
