ENGAGE
Phun Physics of Phase Change - Blab....
There are common experiences in our daily lives that reveal methods employed by heat's restless nature to move from where it's hot to where it's not. Shivering outside the shower while dripping wet is one example. Complaining about the August heat in New Orleans invariably leads to matter-of-fact statements to the effect that, "
it's not the heat, it's the humidity."
Let's boil this down, so to speak. You certainly know by now that water exists most commonly in nature in three
phases or states:
solid, liquid and
vapor. You also certainly understand the relative thermal energy level, or heat differences of each of these phases; ice is cold (low thermal energy) and vapor is hot (high thermal energy) and liquid is somewhere in between.
But did you know that movement between each of these phases requires a substantial exchange of energy that does not change the temperature of water one bit?
That's why boiling water on the stove can't get any hotter than 100 degrees Celsius. All the thermal energy moving into the water at that temperature can only cause evaporation because evaporation at the surface of water removes heat from water at the same rate as the heat is added.
Or we could condense it all just by saying,
evaporation is a cooling process. Huh?
You'll recall from our last module that water has a high heat capacity--that water requires 1 calorie per gram to change the temperature 1 degree Celsius.
Water also has a high
latent heat of vaporization. This is the amount of thermal energy required to break all the hydrogen bonds that hold water together in its liquid state. This value registers at a whopping 540 calories per gram!
Which is to say that 1 gram of water at 100 degrees Celsius requires the input of an additional 540 calories of thermal energy to change phase from liquid to vapor. No wonder you're shivering outside your shower with all that heat leaving your skin as water turns to vapor and drifts away--from
where it's hot to where it's not.
The same numbers and processes hold true in reverse; water condensing from vapor to liquid releases 540 calories per gram. Stated another way,
condensation is a warming process.
That's why steam is so dangerous. It's a double burn. First, it burns because of it's high temperature; Secondly, it burns again as it releases its latent thermal energy while changing phase from vapor to liquid.
And all this holds true for the other famous phase change,
freezing--except for the numbers. The
latent heat of fusion for water (freezing) comes in around 80 calories per gram. That is, 1 gram of solid water (ice) at 0 degrees Celsius requires an additional 80 calories of thermal energy to become 1 gram of liquid water at the same temperature.
Which is to say, like evaporation,
thawing is a cooling process. That's why we put ice in our beverages. And just like the condensation process described above,
freezing is a warming process.
This may sound strange, but water freezing in the arctic helps to warm the surrounding air as the water releases 80 calories of thermal energy per gram into its immediate environment. More cool information on ice to come in following modules.
Still Confused?
Here's the bottom line - Whenever water changes phase, whether melting, freezing, evaporating or condensing, thermal energy is always released or absorbed, depending on the direction of the change. In nature this energy either comes from, or is released into the environment depending, again, on the direction of the change.
EXPLORE
Phun Physics of Phase Change - LAB!
Let's try our hand at
backwards design. Examine the
graph and information. How would you design a lab to create similar results?
There are many simple labs that require little more than water and a thermometer to demonstrate these basic principles. Take a look around the net and find one that best suits your students.
Helpful Hint: Safety in the lab is always a real issue. Many seemingly simple labs hold real perils. From broken glass to bad burns, protect your students and yourself by understanding and employing authentic safety practices in your labs.
- What is the relationship between thermal energy and phase change in water?
- How is this different from the thermal energy required to change temperature?
- How do the phase changes of water help regulate Earth's temperature?
EXTEND
- What other labs or demonstrations reveal the flow of thermal energy through systems?
EVALUATE
- How valuable is an understanding the physics of phase change to understanding weather and climate?
ENGAGE
Life Under Pressure
We aren't surprised to hear about the tremendous pressure at the bottom of the ocean. It makes sense that the weight of all that water would have such an enormous effect. But it's oft said, we live at the bottom of an ocean of air with similar results.
The weight of the air on the surface of the Earth averages about 14.7 lbs per square inch. Or about 2100 lbs per square foot. Or about 42,0000 lbs of pressure on the surface of your body. But we don't notice it much unless we change altitude quickly in a plane or elevator.
Anybody who's watched a TV weather report knows that atmospheric pressure plays an important role in determining the forecast; That is, pressure affects the behavior of wind and water vapor. Therefore atmospheric pressure also plays an important role in determining when and where heat is transferred in the atmosphere.
Before TV weather reports, our more agricultural forebears and mariners monitored this law of physics by watching their handy barometer. They also utilized the power of this phenomenon by using pressure cookers to better preserve food. That is, by increasing the vapor pressure in a cooker, they could cook at higher temperatures.
Conversely, we have to cook food longer at higher altitudes because water boils at a lower temperature due to the decreased atmospheric pressure.
Low Pressure = Cloudy/Rain
Now, let's condense these concepts, so to speak. Reduced atmospheric pressure allows water vapor in the air to cool and condense into liquid more easily because reducing the pressure of a gas decreases its temperature.
As the water vapor condenses, it releases 540 calories per gram of thermal energy in to the atmosphere, thus warming it. (Interestingly, reduced atmospheric pressure also allows water to evaporate more easily because of its corresponding reduction in vapor pressure.)
High Pressure = Clear/Sunny
Conversely, increased atmospheric pressure limits the condensation of water because increasing the pressure of a gas also increases its temperature. You might correctly predict here that an increase in atmospheric pressure limits water's evaporation by increasing its vapor pressure, just like in grandma's pressure cooker.
Helpful Safety Hint: Because of the affects of pressure on boiling water, the explosive results of removing the cap from an overheated car radiator should be universally understood and avoided.
EXPLORE
YouTube
Check out some of the more surprising effects of pressure on phase change.
This is a YouTube video produced by the Edwards Vacuum Corporation, called Making Ice by Boiling Water. While you're there, see what related videos are posted.
Making Ice by Boiling Water
EXPLAIN
- What is the relationship between evaporation, condensation, vapor pressure and atmospheric pressure?
- Where does the energy to boil water in a vacuum come from?
- Why does the water freeze? Where does the heat go?
EXTEND
- What stories can you tell that relate to any of these scientific principles?
EVALUATE
- How valuable is an understanding the physics of pressure to understanding weather and climate?
