Phase Changes of Water (Daniel O'Brien)
Principle(s) Investigated:
- Energy Transfer and Changes of Phase
- Evaporation
- Heat of Fusion
PS1.A: Structure and Properties of Matter - In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position, but do not change relative locations.
PS3.A: Definitions of Energy - Heat and temperature.
C7.D: Chemical Thermodynamics - Students know how to solve problems involving heat flow and temperature changes, using known values of specific heat and latent heat of phase change.
Materials:
- Water - 2 x 500 ml
- Crushed ice: 500 ml
- 2 - 500 ml flask
- Temperature Probe w/DataStudio software
- Gloves (for handling the dry ice and/or the flasks of boiling water)
- Hot Plates
- Tongs
- Dry Ice - 2- 3 lbs
- Salt - 3 tsp
- Slotted Spoon
- Drill with bit
Student prior knowledge: Matter all around us exists in three common phases: solid, liquid and gas. Matter can change from one phase (or state) to another. A familiar example is of water as ice, liquid water and water vapor (steam).
Thermal conductivity: Recall that thermal energy flows from warmer matter to cooler matter.
Effects of solutes: Solutes lower the freezing point and raise the boiling point of a solvent.
Temperature: The measure of the average kinetic energy at the molecular level of a substance.
PROCEDURE: Begin with a review of relevant prior knowledge concepts and purpose of demonstration.
Phase Change #1: Liquid to Gas - Fill a 500 ml flask with room temperature water. Insert a probe with DataStudio software loaded and running. Place on a hotplate and begin heating. Have students observe gradual increase of temperature. When a clear trend is visible, have them predict what will continue to happen. When the heating stops at 100° Celsius, ask them to hypothesize why this happened.
Phase Change #2: Solid to Liquid - Fill a 500 ml flask with crushed ice. Insert a probe with DataStudio software loaded and running. Have students predict what will happen. Place the flask on a hotplate and begin heating gradually, while the students observe. After all the ice has melted, ask them to hypothesize why this happened and how it is different than the previous demostration.
Additional Demonstrations: Simulated seawater
1 - Mix a solution of approximately 3 teaspoons of table salt with room temperature water to create a solution simulating the salinity of seawater (3.5%)
2 - Have students hypothesize what will happen.
3 - Add dry ice to demonstrate that the water will remain liquid below 0° Celsius
4 - If time permits, run a version of Phase Change #2 above with a block of ice and the seawater solution.
Explanation: The phase of matter depends upon its temperature and the pressure that is exerted upon it. Changes of phase generally involve a transfer of energy.
A substance typically requires more energy to change from one phase to the next than it does to absorb or release energy within a given phase.
ICE - Ice absorbs about 1/2 calorie per gram to raise its temperature 1° Celsius, but it will absorb 80 calories/gram before it will melt.
LIQUID - Liquid water will absorb about 1 calorie/gram thereafter until it begins to boil at 100° Celsius, yet it requires an incredible 540 calories/gram of heat energy to vaporize that same gram of water!
- Evaporation is truly a cooling process! (Heat is taken away as fast as it is added).
Questions & Answers:
1. How can a substance absorb or release energy with no resulting temperature change?
2. Why does water with substances dissolved in it freeze at a lower temperature that pure water?
3. Why can it be said that boiling is a "cooling process"?
Applications to Everyday Life:
1. Cooking at altitude: At high altitudes, water boils at a lower temperature. In Denver, Colorado, the mile-high city, for example, water boils at 95° Celsius, instead of the 100° Celsius characteristic at sea level. A "three-minute-egg" boiled in Denver would be a bit runny. Since adding solutes to a solvent raises its boiling point, this can be used to partially compensate for the effect. Most people add salt to the water when cooking pasta to raise its boiling point. Adding salt to the water decreases cooking time because the water is hotter.
2. Car manufacturers make use of the effects of solutes to protect engines from heat and cold. the coolant in a car radiator is a solution of water and another liquid called antifreeze, (generally ethylene glycol). The mixture of the two liquids has a higher boiling point and a lower freezing point than pure water.
3. Refrigeration: a refrigerator's cooling cycle is a good example of the energy interchange that occurs wht the changes of phase of the refrigeration fluid. The liquid is pumped into the cooling unit, where it is forced through a tiny opening to evaporate and draw heat from the things stored inside the food compartment. The gas is then directed outside the cooling unit to coils located in the back of the unit. As the gas condenses in the coils, heat is given off to the surrounding air. The liquid returns to the cooling unit, and the cycle continues.
An air conditioner uses the same principles. When the roles of vaporization and condensation are reversed, the air conditioner becomes a heater, called a heat pump.
Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.
Simulated Seawater @ 22° Fahrenheit / -6° Celsius
Videos:
Freezing Water with Dry Ice - no this is NOT boiling water. It is actually a pot of water at simulated seawater at
-6° Celsius