The only safety consideration is when using the matches or lighters if the teacher sets up multiple stations for multiple groups of students. REMIND STUDENTS TO THAT THEY ARE TO BE USED FOR LIGHTING THE CANDLES ONLY.
In most cases, the teacher will perform the experiment and the students will observe.
This experiment lines up with the Manitoba Middle Years Specific Learning Outcome of:
5-4-03 (GLO: D3)
It states that students will…
Describe the properties of air:
Air has mass/weight and volume.
Air expands to fill a space.
Air expands and rises when heated.
Air contracts and sinks when cooled.
Air exerts pressure.
Air moves from areas of high pressure to areas of low pressure.
Raising their hands, some students should automatically assume that you will put it over the candle.
“What will happen when I put it over the candle?”
Students should know by now that when you put the beaker over the flame, it will eventually use all the oxygen and the candle will burn out. Let there be discussion if needed.
Raise the beaker in the air and ask if anything will happen.
As the room quiets down, slowly bring the beaker down. (This will keep them anxious and hold their attention)
As you get closer to the flame, move it over the candle as quick as you can without touching it. The beaker will now be sitting upside down on the plate of water.
The flame will go out in a matter of seconds, but as it is going out, water will begin to fill upwards in the beaker and the candle will float on top of the water rise to the top as it fills.
There is one main reason why this is a good example of a discrepant. Most students even after observing air pressure differential experiments still might not expect the water to rise upwards into the beaker. It defies their logic on the principles of gravity and how water should always flow downwards.
Choose students with their hands up for an explanation.
After some suggestions clarify what occurred with your explanation and theory.
EXPLANATION AND THEORY:
When the burning candle is covered with the beaker, the flame eventually goes out because it uses up all the oxygen. Fire needs oxygen to burn. The other gas left behind has less pressure compared to air. Remember that when the candle burns the oxygen turns to carbon dioxide. Carbon dioxide has less pressure than Oxygen.
As a result of the different pressure inside the beaker, the air pressure outside the beaker tries to equalize the lower pressure difference inside the beaker but does so by pushing the water into the beaker. This is why the water rises. It is sitting between the air outside of the beaker and the left over gases inside the beaker.
This isn't the main factor for the rise in the water level though. You will observe that when you place the beaker over the burning candle and on the water, bubbles form in water just outside the rim of the beaker. This is because as the candle becomes closed off in the beaker it increases the temperature of the air in the beaker. Because air and most other gases expand (spread out) when they are heated they take up more volume (space). Therefore the bubbles in the water are air trying to escape the beaker to find more "space."
When the flame goes out the leftover air and gases in the jar cool down and they want to contract (move closer together). As this happens, the air and other gases pull the water in and upwards while they "move closer together” or you can look at it as the water is being pushed into the beaker by the outside air pressure.
It is important that after the discussion and explanation, the students achieve a state of understanding or equilibrium about what occurred in the experiment.
Be prepared to change some variables of the experiment to accommodate their curiosity of what if we changed this or what if we changed that. Here are some examples of variable changes:
What if we had more multiple candles burning in the beaker?
It would burn up the oxygen more quickly and also increase the temperature of the gases even higher. As a result, more water would be drawn in when the beaker cooled afterwards. This is because more gas escaped the beaker initially because of the higher temperature and a higher need to increase volume (spread out even further.)
What if the beaker was larger?
It would take longer for the candle to burn up the greater amount of oxygen available. This would create a larger amount carbon dioxide and therefore lower pressure. However the main reason why the water rises in the beaker after cooling is because is determined by how much heat is given off by the candle(s). If you use the same candle in the smaller and larger beaker, you should see little difference in water drawn in.
Concluding Questions for Students:
Can you remember what Mr./Ms/.Mrs. _________ used to perform this experiment? What happened during the experiment?
What happens to air when it is heated up? What happens when it is cooled down? Explain why the water rose up in beaker.
What would happen to a balloon if you filled it with warm air, tied it up and then brought it outside during winter time? Would it get bigger, shrink or stay the same size? Why?
Can you think of any other objects you may have at home that may shrink or get bigger from changes in temperature?
Explain why the water rose up in beaker after the candle went out.
Steve Spangle Science: Why Does the Water Rise? 3, October 2007.