|This demonstration will explore chemical changes and introduce some of the properties of oxygen.||Materials:
|A classic classroom activity adapted by the Windows Team. Recommended by Dave Mastie.|
|5 minutes to set up, 20 - 40 minutes wait, 5 minutes for demonstration|
Student Learning Outcomes:
- This activity takes about 5 minutes to set up and then needs to be set aside for at least 20 minutes but not more than 40 minutes. It works well to introduce this at the beginning of a class or lesson period and then complete the activity as a concluding effort.
- Begin by asking students what they think would happen in a world where the oxygen level increased. Explain that you are going to demonstrate one impact of increased oxygen by increasing the concentration of oxygen in a contained environment.
- Into each of the glass jars, coarsely peel 2 potatoes, dropping the peels into jars. Cut the potatoes into chunks and drop into glass jar.
- Identify and label one jar will as the control and the other as the treatment.
- In the control jar, pour the water over the potatoes. Cover with lid.
- In the treatment jar, pour the hydrogen peroxide over the potatoes. Cover with lid.
- Set aside with the control jar.
- When you are ready to conclude the demonstration, ask students how we might test for increased levels of oxygen (pure oxygen is highly flammable).
- Put on safety goggles and light the candle.
- Place the wooden splint in to the flame. After it catches on fires, blow to extinguish the flame at the end of the splint.
- Quickly, uncover the control jar and place the splint in the jar (nothing should happen). Remove splint and re-cover the jar with lid.
- Repeat this procedure (steps 10 and 11) with the treatment jar (it should re-ignite almost immediately). Remove splint and re-cover jar with lid.
- Why did the splint re-ignite in the treatment jar? (see background information for complete discussion).
- Have students design an experiment to determine what would happen if O2 were significantly reduced in the jar. Would the flame re-ignite? Have them explain the role of O2 in fire.
- Have students describe a world with increased O2 concentration in the atmosphere. Ask them to describe a world with decreased 02 concentration in the atmosphere.
- Ask students if the steel wool got lighter or heavier during this process and explain their answer. (Most will say it got lighter as it burned. The opposite is true-- it got heavier because the combustion process added oxygen to the iron.)
- You can extend this demonstration to include a discussion of metal burning in a high oxygen environment.
- Tease the steel wool into a small golf-ball sized clump.
- Dim lights.
- Using the tongs, place the steel wool ball into the candle flame for a few seconds. Quickly uncover the control jar, and using the tongs. Lower the steel wool into the jar (nothing should happen).
- Repeat this procedure with the treatment jar.
Properties of Oxygen:
Oxygen forms about 21% of the atmosphere making it the second most abundant gas in our atmosphere. As a gas, it is colorless, odorless, and tasteless. Oxygen is very reactive and is required for most types of combustion. Without the presence of oxygen, we would not have fires. Of course, without oxygen, life as we know it would not exist. If we had a much higher concentration of oxygen in our atmosphere, we might experience a world on fire. But a world on fire would hardly be a fit place to live.
In this demonstration, we increased the concentration of oxygen in the gallon jar. At the beginning of the demonstration, the air in the jar was at room temperature and pressure, with a "normal" mix of atmospheric gases. As the enzymes in the potato interacted with the hydrogen peroxide solution, oxygen molecules were released and introduced into the jar's "atmosphere."
As the oxygen was released, a number of changes took place inside the jar:
- the atmospheric pressure increased slightly, due to the introduction of more O2 molecules
- the atmospheric temperature increased slightly, due to the increase in pressure
- the percentage of oxygen increased
- the increase in the percentage of oxygen accounts for the effects seen during this demonstration.
In the second part of this activity, we took a steel wool pad and lit it so that it began to burn. Steel wool (which is mostly iron) burns with a flame because it has a much greater surface area than does a nail (which doesn't burn) of the same mass. The combustion rate is also related to the amount of O2 available. When we placed the steel wool into the jar that contained extra oxygen, it began to burn much faster, which in turn released more heat. During the combustion process, a compound known as "ferrosoferric oxide (Fe3O4)" is formed in the fused blobs which fell into the bottom of the jar. It's important to note that the material in the melted blobs is not the same as the original steel wool-- it's gone through a chemical transformation:
3 Fe + 2 O2 ==>
Most people don't realize that iron burns in the presence of oxygen, even though they see it happening almost every day. The most familiar form of iron combustion is rust. As iron reacts with the oxygen in the atmosphere, it combines with the oxygen and releases heat.
There are two tie-ins to fire science in this demonstration:
1) Oxygen concentrations affect combustion rates. This is why people who put out oil well fires uses explosives. The explosives use up all the available oxygen, leaving none for the oil fire.
2) Surface area also affects combustion rates. This is why brush and shrubs are consumed by a forest fire more quickly than are large trees. This is also why we start campfires with kindling rather than a log.
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