Phase Diagram of Water vs Other Substances: Differences & Meaning

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  • 0:02 What is a Phase Diagram?
  • 2:55 Phase Diagram of Water
  • 5:00 Comparing Phase Diagrams
  • 6:33 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Phase diagrams give scientists specific information about how phase changes occur at different pressures and temperatures. This lesson examines phase diagrams, focusing on water and how it's a little different from most other substances.

What Is a Phase Diagram?

Imagine you have been transported to the future and it's your job to determine if Planet Z could possibly have liquid water on its surface. You have a scientific tool known as a phase diagram to help you figure this out. A phase diagram shows the state a substance is in at different pressures and temperatures.

Phase diagrams are related to phase changes. You might not be familiar with the scientific term phase change right now, but you've probably witnessed quite a few phase changes in your life. Have you ever noticed frozen puddles? Or dew on the leaves of plants? Or steam leaving a pot as you boil water? Well, those are all phase changes, or the change from one state to another state. In this context, the word 'state' refers to a substance's form: solid, liquid or gas.

Let's take a moment to talk about phase changes and give some examples. When a solid changes into a liquid, melting has occurred. Just think about an ice cube melting in a glass. Conversely, when a liquid changes into a solid, freezing has occurred. That's what happened to those frozen puddles.

When a liquid turns into a gas, vaporization has occurred. That is why you see water vapor leaving your pot as you boil noodles. A lot of that water leaves the pot as water vapor, a gas. The reverse of this is condensation. This is when a gas turns into a liquid. Think of the dew on the leaves. The gas in the air cooled, and some of it turned into liquid, or dew.

This last phase change is probably less familiar: when a solid becomes a gas, or sublimation. If you've ever seen dry ice, or solid carbon dioxide, you've probably watched it turn directly into a gas. When a gas turns directly into a solid, deposition has occurred. You might notice this when you drive through fog in freezing temperatures and ice gets on your windshield.

So what does this have to do with finding liquid water on Planet Z? Don't worry, I'm getting there. Let's get back to those phase changes.

All of these phase changes occur at certain temperatures and pressures. For example, water boils at 212 degrees F (or 100 degrees C) at a pressure of 1 atmosphere. But let's say you wanted to boil water at the bottom of a deep pit that has a pressure of 3 atmospheres. Because of this difference in pressure, water's boiling point would be higher than it is at sea level.

But what if you wanted to boil water on top of a tall mountain where there is less pressure? Yep, you guessed it! It is easier to boil water when there is less pressure, meaning the boiling point is lower.

So you can see boiling points change depending on factors such as pressure. This holds true for the other phase changes as well.

So you might be asking yourself, 'How in the world do people keep all of these pressures, temperatures and phase changes straight?' Well, that's exactly what phase diagrams are for. This phase diagram is going to help you figure out if there's liquid water on Planet Z:

Phase Diagram for Water
phase diagram for water

Phase Diagram of Water

Since you need to figure out if water can exist in its liquid form on Planet Z, let's start by examining the phase diagram for water.

The x-axis is labeled 'T' for temperature, and the y-axis is labeled 'P' for pressure.

You'll notice the graph is separated into three parts: the solid, liquid and gas phases of the substance. Each line separating the different phases is where they are in equilibrium. For example, on the line separating liquid water and solid water, or ice, the water is partially liquid and partially solid. Both phases exist at this point. When you're not on the line, only one phase exists.

When you cross the line, there has been a phase change. For example, when you cross from solid to liquid, melting has occurred. Or when you cross from a gas to a liquid, condensation has occurred, but I think you get the idea now!

Before we compare water to other substances, let's take a closer look at a couple of points on the graph. First, take a look at the triple point. This is the point where water is at an equilibrium between solid, liquid and gas. In other words, all three phases occur at this point.

Triple Point
triple point on phase diagram

Next, check out the critical point. Above the critical point, a substance exists as a supercritical fluid. A supercritical fluid is a gas that is really compressed so it behaves like a gas with some properties of a liquid. You can't turn a supercritical fluid into a liquid without lowering the temperature. Each substance has a unique critical point and triple point.

Critical Point
critical point on phase diagram

Okay, so can Planet Z have liquid water? Well, first we need to figure out the temperature and pressure on the surface. From our futuristic instruments, we know the temperature is extremely cold and the pressure is really low. So if we look at the graph, it appears that if there is any water on Planet Z, it's probably in the solid, or ice, form.

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