At which part of the graph will liquid and solid phases of water coexist?
This graph is called a heating graph or heating curve. And it shows us the change in temperature and energy as a substance, in this case water, is heated from the solid phase all the way to the gas phase. Each segment corresponds to a different process along the way.
In this question, we’re being asked to identify the segment of this heating graph where we’ll have both the liquid and the solid phase of water. First, let’s think about when liquid and solid water will coexist. In other words, when would we have both liquid and solid water in equilibrium with each other. This occurs at the melting point when we’re transitioning or undergoing a phase change from solid water to liquid water.
So in this question, our job is really to figure out which segment corresponds to melting. We know that the melting point of water occurs at zero degrees Celsius. Which is a temperature that’s been conveniently labelled on our graph for us. Since segment B is the process that is occurring at the melting point, it would make sense that B corresponds to melting. And so it’s the segment that we’re looking for.
You may have noticed that this segment is flat. Meaning that while energy is being gained by the water, the temperature of the water isn’t increasing. Why is this the case?
It’s worth thinking about this for a little bit so that we can fully understand this graph. If we compare a substance that’s a solid to a substance that’s a liquid, we’ll see that a solid is much more ordered. The particles are stuck closer together. In the liquid, the particles are still close together. But they’re free to move around each other.
Separating the particles like this requires energy. So when a substance is undergoing a phase change, that is, going from a solid to a liquid or a liquid to a gas. All of the energy that’s being put into the substance is being used to separate the particles. When the phase change is complete, energy will still be going into the substance. But that energy can be used to increase the temperature instead of separate the particles to change the phase of the substance.
We’ve already identified the segment that we’re looking for. But let’s go ahead and identify what the other segments on this graph are. Just so we have a good idea of what’s going on in this heating curve. We know that water exists as a solid below zero degrees. And segment A occurs exclusively below zero degrees. So segment A must correspond to solid water heating up.
We’ve already identified B as the melting point, where all the energy is being used to change the phase of the solid water into liquid water. After segment B, we have liquid water. So now in segment C, the temperature begins to increase again, meaning that the liquid is being heated up. So C corresponds to a liquid heating.
D corresponds to another flat segment, which means this is a phase change. Since we have a liquid, D must be boiling, where the liquid is beginning to change into a gas. After segment D, the water has boiled. So we now have gaseous water. And the temperature is beginning to increase again. So E must correspond to a gas heating.
But as we discussed, we wanted to identify where the liquid and solid phases of water would coexist, which occurs at the melting point of water. And melting corresponds to segment B.