Physics 102 - phases of matter




Chapter 17




birth of an iceberg

 

 

Phases of matter

 

You have learned about the phases of matter: solid, liquid, gas and plasma. In this section, we will discuss transitions between these phases.

 

Evaporation and condensation

When a liquid changes to a gas, or a gas to a liquid, it is called evaporation or condensation when the change occurs only near the surface of the liquid.

Recall that temperature is related to the average velocity of the molecules. The fastest of the molecules have enough speed to escape the fluid. This is evaporation. These fast molecules carry away energy from the liquid - making it a cooling process.

 

 

Fast molecules leave the liquid and get away. If the container is closed, they bounce off the walls of the container and may go back into the liquid - in other words, they may condense. This will continue until the molecules leaving and entering the liquid balance out and come into equilibrium. The evaporated molecules produce a vapor pressure.

If the temperature increases, more molecules will be moving faster and leave the liquid, but more molecules will also condense. A new equilibrium will be established with a higher vapor pressure.

 

We can plot the vapor pressure vs. temperature and the vapor density vs. temperature. As you would expect, the plots look very similar.

Why does water condense on a cold drink on a hot day? Air always contains some water vapor. Humidity is defined as the amount (mass) of water contained in a volume of air. Relative humidity is defined as the amount of water vapor in the air compared to how much water vapor the air could possibly hold at that temperature. If the relative humidity is 100%, the air is said to be saturated. When saturation occurs, water begins condensing out of the air.

 

time lapse cloud video

atmospheric cloud wave video

 

Water evaporates from the ocean, condenses into clouds and fog, rains or snows down onto the land, and then returns to the sea via rivers, streams and underground seepage or passageways.

 

 

How does it work?

 

 

The bird's body has ether in it, which evaporates inside the glass at room temperature.

Water evaporates from the bird's head, which cools it. This stops the ether in the tube from warming up and exerting pressure down the tube.

The evaporating ether increases the vapor pressure inside, which pushes down on the liquid ether and forces it up into the bird's neck.

The weight in the bird's neck increases until it tips him forward and allows the ether to flow back down.

The bird's head dips into the water, which cools it off again, and he starts over...

 

 

rubeGoldberg.jpg

 

 

 

Boiling

When a liquid changes to a gas and the change happens throughout the liquid, it is called boiling. Bubbles form in the water when the vapor pressure equals that of the water pressure. Why do the bubbles rise? Would you expect the size of the bubbles to increase or decrease as they rise? Boiling is another cooling process. The molecules carry off energy when they leave the liquid.

Equilibrium vapor pressure of water = 1 atmosphere at the 'normal' boiling temperature. Is this a coincidence?

 

Bubbles can form when the vapor pressure inside the bubbles is large enough to withstand the pressure of the water.

Air pressure pushes down on the water, making the water pressure higher.

At high altitude, air pressure is lower, so bubbles can form at lower temperature.

This means that food cooked using boiling water will take longer at high altitude.

 

 

Conversely, if we lower the pressure enough, we can boil water even at room temperature. Remember, boiling is a cooling process. If we keep lowering the pressure further, we can reach a state where the boiling water is cold enough to freeze! This is what happens in the process of freeze drying.

 

freeze dried pants

 

A liquid boils when heat is added at constant temperature. This heat lets the molecules overcome the energy barrier so that they enter in the vapor phase. This heat is called a latent heat of vaporization. The latent heat of vaporization for water is 2260 kJ/kg (or 540 cal/gram). Latent heats are often fairly large (particularly for water). This means it takes a lot of energy to boil water.

This is a plot of temperature vs. time for a pot of water that has been brought to a boil. When the heat is applied to the water, the temperature goes up. Notice that when it is boiling, the temperature no longer rises. This is because all of the heat energy imparted to the water goes into changing the phase of the water to steam.

Suppose I wanted to calculate how much energy it took to raise the temperature of 100 grams of water from 50°C to boiling, and then convert half of the water to steam. I would need to take account of both of the steps involved.

 

What makes a geyser erupt?

 

 

Sublimation

Sometimes a solid can transition directly into a gaseous phase. This process is called sublimation. A common substance that readily sublimes is dry ice or carbon dioxide. This will happen when the atmospheric pressure is too low to stop molecules from escaping the solid. Water ice can also sublime.

sublimationGraphs.jpg

 

 

A solid has a vapor pressure just like a liquid does, so we can make similar pressure vs. temperature plots for a solid.

Sublimation is especially evident in the low pressure environment of a comet.

comet Temple 1 animation

comet movie

comet movie 2

The process opposite of sublimation is called deposition - a vapor changing to a solid without going through an intermediate liquid phase. Frost is an example of deposition.

 

Melting and freezing

When a substance changes from solid to liquid it is called melting, or liquid to solid - freezing. Substances melt and freeze at the same temperature, and that temperature is a property of the material at a given pressure. Similar to the latent heat of vaporization we discussed for changing between a liquid and a vapor, a substance has what is called a latent heat of fusion for changing between a solid and liquid. The latent heat of fusion for water is 333 kJ/kg (or 80 cal/gram).

Suppose you raise the temperature of 100 grams of ice from -50°C to water at 80°C. To calculate how much energy is needed for this, you need to take account of warming up the ice to its melting point, then melting the ice, then raising the temperature of the water. Notice that the specific heat of ice is different than the specific heat of liquid water!

 

 

Phase change energetics

1. How much energy is transferred when a gram of steam at 100°C condenses to water at 100°C?

2. How much energy is transferred when a gram of boiling water at 100°C cools to ice water at 0°C?

3. How much energy is transferred when a gram of ice water at 0°C freezes to ice at 0°C?

4. How much energy is transferred when a gram of steam at 100°C turns to ice at 0°C?

 

Water ice has the property that adding pressure can cause it to melt, by crushing the ice crystal structure. If the pressure is reduced, it will refreeze. This process is called regelation.

 

Gliese_581.jpg

It's also possible to create solid hot water by exerting extreme pressure. We think that is the case with an extrasolar planet we have discovered and named Gliese 436b. It is about 20 light years away and has a mass about 12 times that of Earth. It has a surface temperature of some 400°C, much higher than the boiling point of water at 1 Eather atmosphere (sea level) but with its immense gravity, the high pressure would serve to keep water liquid. It is suggested that it may harbor extremely deep oceans, capable of forming solid hot water called ice-seven.

 

strQuarkStar.jpg

In astrophysics, we study effects that happen at extremes. One such effect is the phase change from nuclear matter to a quark gluon plasma. It is theorized that such a phase change may happen to neutron stars, creating a state between neutron stars and black holes. This intermediate compact object is called a strange quark star.