Demo: Colbie Bell

Using Ice to Boil Water

(Teacher Copy)

Grade: Science 9, Fluids and Pressure (Unit 2)

Science Concepts: Students will be expected to predict the response of fluids to external pressures.

Questions: Pre Demo

When heat is applied to a flask, how is it that water heats according to particle theory?

Heat is transferred through the glass by conduction, and then convection occurs in the water itself. Warm water rises and spread outward, while cold water moves downward, creating convection current.

2) At what temperature does water boil? How does boiling occur?

Water boils at 100°C at standard sea level atmospheric pressure of 760mmHg. At the boiling point the saturated vapor pressure equals the atmospheric pressure, this allows bubbles to form and rise since the vapor pressure can break free of the force exerted by atmospheric pressure.

Questions: Post Demo

1) What is the ice on the outside of the flask doing to system inside the flask?

When the flask is cooled off with the ice, the water vapor above the water condenses on the cooled surface and a partial vacuum is created above the water. This lowers the atmospheric pressure inside the flask, allowing the water to boil at a lower temperature. Since the atmospheric pressure is lower, the saturated vapor pressure does not have to be as high for boiling to occur.

2) If we were hiking in the Himalayans and wanted to have boil potatoes for supper would we have to bring more or less propane for our camp stove?

Boiling potatoes would require the use of more propane than usual. Although the water boils at a lower temperature, this actually causes the potatoes to take longer to cook.

3) Give a brief description on how a pressure cooker can speed up the cooking time of foods.

When using a pressure cooker the escape of the water vapor is slowed, causing an increase in atmospheric pressure. This causes the boiling point to rise past 100°C. Therefore food cooks faster at the higher temperature.

Materials:

Flask
Rubber stopper
Clamp
Burner
Stand
Boiling chips (optional)
Water
Ice
Safety glasses

Procedure:

1) Fill the flask 2/3 full with water and heat to boiling using the burner and stand.

2) As the flask will now be hot use a clamp to hold the flask. Allow the water to boil for a bit and then remove from heat. Immediately stopper the flask.

3) Allow water to cool so that students can see that boiling has stopped.

4) Invert the flask and place a bag of ice on the base of the flask so that cooling occurs.

Observe the results.

Explanation:

The boiling point of a substance is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure. This allows bubbles to form and rise up to the surface since the saturated vapor pressure can now escape the force exerted by the atmospheric pressure. As steam is formed inside the flask it replaces the air above the water. Placing the stopper in the flask causes the displaced air to be trapped outside. Cooling the flask causes internal water vapor (steam) to condense on the glass surface and a partial vacuum is created above the water. This lowering of the systems atmospheric pressure causes the water to boil at a lower temperature. Under normal atmospheric pressure at sea level (760mm Hg {mercury} the boiling point of water is 100°C. However, raising or lowering the atmospheric pressure by about 28mmHg will cause the boiling point to change by 1°C.

Resources:

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/vappre.html

Liem, T. (1987). Invitations to science inquiry. Lexington, MA: Ginn Press