|Using Ice To Boil Water
Ice pack / bag with crushed ice
Water (enough to fill half the flask)
Heat source (Bunsen burner, eg.)
Heat resistant gloves
This particular demonstration could be applicable in several areas of the science curriculum. For example it could be applied to S1-2-11 Investigate properties of substances and explain the importance of knowing these properties. Also it could be applied in the Weather cluster of the senior 2 Science curricula.
How To Present Activity
This discrepant event is meant to create a sense of wonder and inquiry in students about the effect of atmospheric pressure on the boiling point of water. The teacher may find it advantageous to begin by ensuring the student’s recollection of key pertinent facts such as the boiling point of water, phase change events, and atmospheric pressure information.
If time permits, the teacher may want to have students in small groups bring water to a boil and observe the change in temperature when the flask is placed in an ice bath (without a stopper in place...this time). Then explain that they will be shown a phenomenon that makes them think twice about what they already know!
The teacher can facilitate this discussion while bringing the water to a boil, asking the students to explain what is going on while the bubbles begin to form and roll.
The teacher will then remove the flask from the heat source, just as the boiling point has been reached, and stopper the flask. The teacher will ask students to describe things they’ve noticed (Why did the boiling stop? How hot is the water? What happened to the glass when the stopper was put in? Why? What do you think is happening to the atmospheric pressure inside the flask?).
Then the teacher will invert the flask on the ring stand (so the round part is facing up), and place the ice pack on top of the flask. Because it usually takes a minute or so for the effect to take place, the teacher may ask the students to predict what effect the ice will have.
Soon, the water will begin to boil inside the flask. The teacher will ask the students to try to explain what they saw. They can facilitate inquiry by asking such questions as:
What is the temperature of the water after the stopper was put in? Target response: Just under 100°C, as it had a few moments to cool down.
What effect does the ice pack have on the glass? Target response: It cools it down.
What is happening on the inside of the flask? Target response: The steam is condensing into water.
What happens to the atmospheric pressure inside the flask as the steam condenses? Target response: There is less vapour in the flask, so the pressure decreases.
What does this tell us about the boiling point of water? Target response: There are different boiling points of water depending on the surrounding atmospheric pressure?
Using this knowledge, if you lived on a mountaintop, do you think it would be easy or hard to hard-boil an egg? Target response: The pressure on a mountaintop is lower, thus the boiling point of water is lower. It should be hard to hard-boil an egg because the water won’t get hot enough.
If I stoppered the flask before putting it on the heat source, would the water boil before, on, or over 100°C? Target response: The pressure inside the flask would increase, so the boiling point would also increase. Thus, it wouldn’t begin to boil until the temperature was higher than 100°C.
Is it possible for water to boil at only a few degrees above 0°C? Target response: It is possible, as long as the pressure was low enough.
Explaining the Event
When the flask is stoppered, after it has reached its boiling point and is removed from the heat source, the evaporating steam is trapped in the flask. When the ice pack is applied, however, the cooling effect causes the gaseous water molecules to condense and return to the liquid state. Since 1 mole of water occupies much less space in its liquid form than its gaseous form, there will be a decrease in pressure as this condensation occurs, and a vacuum is temporarily created inside the flask. With lower vapor pressure come lower boiling points, and the water begins to boil.
Why is This Event Discrepant for Students?
This event will cause disequilibrium in students due to the fact that boiling events are usually related to the heating of the substance to be boiled. Many students may see the application of ice as a cooling of the water, when in fact, it is the air in the flask being cooled creating a decrease in the pressure of the gas in the flask. It will seem very discrepant that as the temperature of the overall system is decreased slightly, the water will be brought to a more vigorous boil than after it was removed from the heat source.
Equipment & Safety Considerations
Students should always exercise caution around flame usage in the lab. If the teacher is presenting this demonstration, have the students stand back a few feet from the flame, and ensure that long hair and loose clothing is tied back. The flask should not be thin-walled, and should be scratch free. Also, you must wait until the flask is removed from the heat source, and has ceased boiling, before you insert the stopper – otherwise the pressure can increase so rapidly that the stopper pops out and hot water may follow. Additionally, it is important that gloves are worn when the flask is being inverted, as it will be very hot.
Boiling with Ice. J., Tom. (n.d.) http://van.hep.uiuc.edu/van/qa/section/States_of_Matter_and_Energy/Boiling_Evaporating_and_Condensing/20020811181627.htm
Vapor Pressure. Making Water Boil Using Ice. Decker, T. (n.d.) http://tiger.coe.missouri.edu/~pgermann/DiscEvent/Gases_Air/Boiling/Vapor_Pressure/vapor_pressure.html
Ask a Scientist. Boiling Water on Ice. Calder, V., Bradburn, G. Oct 12, 2002. http://www.newton.dep.anl.gov/askasci/chem00/chem00894.htm