In this new video, we're going to take a look at heating and cooling curves. We're going to say here in heating and cooling curves, we have the representation of the amount of heat absorbed or released during phase changes. Now let's pretend that this heating curve represents that of water. We call it a heating curve because you can see that over time as our time increases going this way, our temperature is increasing. We're going to say here that we know that water either freezes or melts at 0 degrees Celsius. And then we should know that water either starts to condense or starts to boil at 100 degrees Celsius. These are key temperatures you need to know for water. If it were some other type of compound, like let's say methanol or hexane, you wouldn't know what their melting or boiling points were. So they would have to tell you those numbers. Okay. So just remember, for water, it's expected that you do know the values of 0 degrees Celsius and 100 degrees Celsius. But for other compounds, they would give it to you whether you're taking that as a quiz, homework, or an exam. Now we're going to notice that at these temperatures of 0 degrees Celsius and 100 degrees Celsius, our line is flat, meaning there is no change in our temperature. So here and here there's no change in our temperature. But then at temperatures that are not 0 degrees Celsius or 100 degrees Celsius, our temperature does change. Now we're going to need room guy. Let's talk about these different phases. We know that below 0 degrees Celsius, it's so cold that water will exist as a solid. We're going to say here that water exists as a solid up to 0 degrees Celsius. Once it hits 0 degrees Celsius, then we're undergoing a phase change. And what's happening here is that our solid water which is ice starts to melt. On this plateau on this line here that's not increasing, we're going to be a solid-liquid mix. The solid is slowly melting into a liquid. At this point here, all of it has melted and now it's completely a liquid. So this part here that's increasing is all liquid. Then when we get to 100 degrees Celsius, which is right here, our water starts to boil and again we're undergoing another phase change. On this line here, we have liquid as well as gas, sometimes called vapor. You can say a liquid-gas mix or liquid-vapor mix. And once we get to this end part here, all of the liquid has evaporated and now it's all gas. As we start to climb up again, it's all gas now. Now let's talk about what's happening at each one of these spots. So here we're going to say this is 135. This is 2 and 4. We're going to say here a few key things that we need to recognize in terms of this heating curve. We're going to say during phase changes. So during phase changes, that means we're talking about segments 24. We're going to say we can tell that temperature remains constant. That's pretty obvious. It's not increasing. It's flat. But here's some other things that are not as obvious. Because your temperature is staying constant, that means your average kinetic energy is remaining constant as well. Just remember, your average kinetic energy is connected to the temperature of your substance. If the temperature of the substance is not changing, your average kinetic energy for that substance won't change either. Now we're going to say here that during these phase changes because this is a heating curve, our particles are going to start to spread themselves out. Because if you think about it, in a solid, all the particles are tightly packed together. Then as we become a liquid, they're moving around more freely but they're still in pretty close vicinity to each other. They're just sliding on top of each other. Then as we become a gas, that's when they really spread themselves out. During our phase changes, again which are these blue parts where the temperature is remaining constant, we're going to say here the particles are spreading out and that's because the kinetic energy again is not changing. The average kinetic energy is staying the same and heat is being transferred into potential energy. During phase changes, heat is transferred into potential energy. Remember, potential energy is just the energy of your position, or in this case, the energy of your state. Solids have the lowest potential energy, liquids have the next highest, and then gases have the highest potential energy. Remember, during these phase changes we're going from one phase to another phase. Now, during temperature changes, what can we say? Here, during temperature changes, we're going to say that heat energy is converted into kinetic energy. Because this is a heating curve and the temperature is increasing, we're going to say increasing temperature would mean that we're going to have an increase in our average kinetic energy. Finally, the last thing we're going to talk about in terms of this heating curve is what type of equations do we use at each one of these positions. We're going to say here our temperature is changing for segment 1 here and so that's going to be q=mcΔT. M equals mass, c represents the specific heat of the substance, ΔT is the change in temperature. That's final minus initial temperature. Now here, water exists as a solid until it gets to 0 degree Celsius where it starts to melt. Here the specific heat will be for the solid. Now we're all accustomed to remembering the specific heat of water when it's a liquid but there's also specific heat of water when it's a solid, so it's ice, and when it is a gas or steam. We'll talk about the Delta H values in a moment. Remember, for line segment 1, because the temperature is changing, it's q=mcΔT. During phase changes, there is no change in temperature so that portion of the equation drops out. It then becomes q=m⋅Δh. M here could be either in grams or moles. How do we know which one it is? We look at the units for delta h. And here in these examples that I give to you, Delta H's have grams in them. So m in this case would represent grams. If it was joules per mole or kilojoules per mole, then m would represent moles. Now, here on this first phase change, we're going from a solid to a liquid. That means we're melting or fusion. Another name for melting is fusion. Then in line segment 3, temperature starts to change again so it's going back to q=mcΔT. On this part of the line, we're a liquid completely. Here this will be the specific heat for the liquid. Then on line segment 4 again, we're undergoing a phase change so there's no change in temperature. So q=m⋅ΔH. On this phase change, we're going from a liquid to a gas. That represents vaporization. We'll use delta H vape. Temperature is changing again. And the temperature is changing again. So q=mcΔT one more time. Here because it's a gas, we're going to use the specific heat for the gas. That's how we look in terms of this heating curve. Below we have the cooling curve. Check out the very next video where I go into looking at the cooling curve. But remember, if you know what the parts of the heating curve are, the cooling curve is just everything in the opposite direction.
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8. Gases, Liquids and Solids
Heating and Cooling Curves
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