Phase Diagram of Water

Phase changes

Fig. 1: The different States of Matter

• In a solid, the particles are close together and can only vibrate.
• In a liquid, the particles are slightly farther apart and can swap places with each other.
• In a gas, the particles are very far apart and can move freely.

Temperature is a measure of kinetic energy (energy of motion), so the hotter something is, the faster its particles are moving. Because of this, when a species is at a certain amount of energy/heat, it will change phase.

The phase changes caused by gaining heat are:

• Melting (solid --> liquid).
• Boiling (liquid --> gas).
• Subliming (solid --> gas).

Basically, the species has gained enough energy, so its particles can move more freely and therefore break free from, or weaken the interactions between them.

The phase changes caused by losing heat are:

• Freezing (liquid --> solid).
• Condensation (gas --> liquid).
• Deposition (gas --> solid).

When we place a species in an environment colder than it, the environment will "soak up" the species' heat energy, causing said species to cool.

When a species cools, its particles are moving slower, so the interactions between particles are getting stronger and the particles themselves are getting closer together.

Phase diagram definition

In a phase diagram, pressure is shown on the y-axis and temperature is shown on the x-axis. Following along the diagram, we can see how changes in pressure and temperature affect what state of matter a substance will be.

While we normally think of phase changes happening due to temperature, a change in pressure can also cause a phase change. When pressure increases, the particles are compressed closer together. Because of this, an increase in pressure is similar to a decrease in temperature in terms of phase change (i.e. when a liquid's pressure is increased enough, it will become a solid, similar to how a liquid freezes if at a low enough temperature).

How to read a phase diagram of water

Now that we know what a phase diagram is, let's look at the one for water.

Here's what the basic phase diagram of water looks like:

Fig. 2: The basic phase diagram for water

There are some key points here that we need to know:

• The triple point
  • The triple point is where all three phases can exist in equilibrium
  • Depending on how the pressure/temperature is changed, any phase change can occur here
• The critical pont
  • At this point, we can no longer distinguish between the liquid and gas phase
  • We consider the substance to be a supercritical fluid both at and past this point
• The sublimation/deposition curve
  • The curve at the bottom left between the gas and solid to the triple point
  • Shows us the different temps/pressures that a gas can covert to a solid and vice-versa
• The boiling/condensing curve
  • The curve from triple point to the critical point
  • Shows us the different temps/pressures that a liquid can convert to a gas and vice-versa
• The melting/freezing curve
  • The curve from the triple point upward between the solid and liquid phases
  • Shows us the different temps/pressures that a solid can convert to a liquid and vice-versa

Phase diagram of water practice questions

Now that we've learned how to read a phase diagram, let's work on some practice problems!

Importance of a Phase Diagram

Phase diagrams are a very important reference tool. It shows us the relationship between temperature, pressure, and phase. For example, have you ever cooked/baked something and noticed there were instructions for high altitudes? This is because at high altitudes, the pressure is lower, so different species like water are going to behave differently.

It also helps us understand what state certain species will be in at different temperatures and pressure, which can be very helpful for experimenting.