The Physics of Heat
Lesson 5: Phase Changes
When you have completed this lesson and the homework, you will be able to:
In this lesson we will address the following standard from the Indiana Academic Standards for Physics I:
When you put a pot of water on the stove and turn on the burner, heat is added to the water. As a result, the temperature increases. After a while, however, the water starts to boil and a strange thing happens: even though the burner is on high, the temperature is no longer increasing! It just sits there, boiling away. So, where is all that heat energy going? To understand what is happening, we must look at the physics of phase transitions.
Heat vs. Temperature Graphs
Earlier we studied what happens to an object as heat is added. We learned that as heat is added, the kinetic energy of the individual molecules increases and the temperature rises. Not all materials are the same, however. They have different heat capacities. 450 J of heat may warm up a kilogram of iron a whole degree, but the same heat will change the temperature of a kilogram of aluminum only 1/2 of a degree. But regardless of the amount of heat, it is always true that adding some heat to a body increases it temperature. We can represent this by graphing the Temperature vs. Heat Added for a kilogram of the substance.
Since Iron has a smaller capacity for heat, the same amount of heat makes it change temperature more. If something has a huge capacity for heat, then you could all lots of heat and the temperature would barely change. The dotted red line shows how water, which has a very high heat capacity, compares to iron and aluminum.
It is important to understand this graph before we can talk about phase transitions.
Let's take a look at what happens in the case of water.
Let's say we start with some water in a big pan that has a lid, and for the purposes of this "thought experiment" let's pretend that none of the steam is able to escape. A thermometer is sticking out so we can see what the temperature is doing. The burner is turned on high so heat is constantly being added to our system: Here is what happens:
If we were to graph what was happening, it would look something like this:
You can see that the temperature stalled while the water was being converted to steam. This conversion from water to steam is known as a phase transition.
A phase transition is the process of changing states. This occurs anytime a substance undergoes a change in its state, such as from a solid to a liquid (melting) or from a liquid to a gas (boiling).
As you probably can tell from the example above, phase transitions require energy. Liquids do not naturally turn into gasses: it requires energy, in the form of heat, to get a pot of water to boil and turn into steam. While it is in the process of boiling, the energy going into the system does not change the temperature, but is diverted to breaking the attractive bonds that keep the water molecules together. Once these attractive bonds are broken the water molecule is free and becomes a gas.
After all the molecular bonds have been broken by the energy we are adding, then we have a pot full of gas (steam) and if we continue to add heat, the temperature will once again rise.
The previous graph is not really the whole story, however, because water also undergoes a phase transition ( a change of state) at 0 °C. At this temperature it turns from a solid into a liquid. The full graph of the behavior of water would look like this:
Let's summarize some important points so far:
Melting and Boiling Points
All substance are different, and respond to heat differently. 4186 J of energy will only heat a kilogram of water 1.0 °C, while the same amount of heat will warm up a kilogram of gold 23 °C. It is not too hard to imagine that the temperature at which substances boil is different too. Thanks to the work of many careful scientists, we can now look up the melting and boiling points for almost all known substances. A few of the more common ones have been collected in a table for you:
Almost all substances will eventually freeze, if we can get them cold enough. (Helium is the exception). All substances will eventually melt, too, if we can get them hot enough. Since our normal earthly temperature is around 20 °C, substances with a melting point that is higher than 20 °C are generally known to us as solids. If the boiling point is below 20 °C, we generally know them as a gas.
It will also not surprise you to learn that the amount of heat needed to convert a kilogram of a substance from a liquid to a gas is different for different substances. For example, it will take 333,000 J of heat to convert 1 kilogram of H2O from ice to water. This occurs at 0 °C. On the other hand, if you had some ethyl alcohol that you were trying to boil, you would need 104,000 J of heat to convert it from a solid to a liquid. To find how much heat is needed to convert a particular substance from one state to another, we can use the simple relationship
The constants Lv and Lf are given in the table along with the melting points for most common substances.
To help put this all together, let's solve an example problem:
Problem: How much heat is needed to completely boil 1.00 Liter (1 kg.) of water that is initially at room temperature?
Here is our plan to solve this problem
Okay, let's give it a try:
Everything you need to know is right here .....