Pressure and Temperature Decide a Substance's Phase
Ice, water, and steam are not different substances. They are the very same H2O, just in different forms depending on pressure and temperature. Drag the point around on the phase diagram. Raise the temperature or lower the pressure and the same substance moves from solid to liquid, from liquid to gas. This map is the phase diagram. The horizontal axis is temperature, the vertical axis is pressure, and the plane is divided into regions of solid, liquid, and gas. On the lines that separate one region from another, two phases exist together, and there is even a single spot where all three regions meet. This one map holds everything about when a substance freezes and when it boils.
The phase diagram is a map with two axes: temperature across, pressure up. Drag the point around the plane. The region it lands in tells you which phase the substance is in under those conditions. The cold, high-pressure upper left is solid, the middle is liquid, and the hot, low-pressure lower right is gas. Raising the temperature alone melts a solid, and lowering the pressure alone can make a liquid boil. The key is that the phase depends on conditions, not on the kind of substance. The same water boils before reaching 100 degrees high on a mountain, because the low pressure up there shifts the boiling boundary on the diagram. A pressure cooker does the opposite: sealing the lid raises the inside pressure so water boils only above 100 degrees, and that hotter water cooks food faster.
Now focus on the lines that separate the regions. Use the buttons to pick a boundary one at a time. The line between solid and liquid is the fusion line; on it, ice and water exist together. Between liquid and gas is the vaporization line, where water and steam coexist. There is also a sublimation line where solid meets gas directly: that is dry ice turning straight to vapor without melting. On a boundary line, two phases sit in equilibrium together. And where all three lines meet at a single point is the triple point, where solid, liquid, and gas coexist all at once. For each substance, the temperature and pressure of this triple point are fixed to one exact value. The triple point of water is so exactly reproducible that sealed triple-point cells serve as a fixed reference for calibrating thermometers.
Now a P-v diagram, drawn with pressure and volume. See the rounded dome in the middle. Inside this dome is the saturation region, the stretch where liquid and vapor are mixed. Use the slider to increase the volume. Outside the dome on the left is compressed liquid, inside the dome is a mixture of liquid and vapor together, and outside on the right is superheated vapor. Inside the dome, the fraction taken up by vapor is called the quality. The left edge is quality 0, saturated liquid just beginning to boil, and the right edge is quality 1, saturated vapor that has fully evaporated. The top, where the left and right curves of the dome meet, is the critical point, which we save for the last scene. This dome and quality matter especially in power plants, where wet steam that still holds liquid droplets has a low quality and can erode the turbine blades, so engineers keep a close eye on exactly this quality.
Watch what happens during boiling. When water boils in a pot, no matter how high you turn up the flame, the water's temperature climbs no higher than 100 degrees. Use the slider to drag how far evaporation has progressed. The liquid turns into vapor bit by bit, yet the temperature and the pressure stay the same. The heat you add does not go into raising the temperature; it goes entirely into pulling molecules out of the liquid and into the gas. This is called latent heat. So while the phase is changing, the process is both isothermal and isobaric. Only the volume grows, a lot. On the P-v diagram this shows up as a flat line crossing the dome. This is exactly why the thermometer does not budge until the boiling water has all evaporated. This same latent heat is why sweat cools you: as it evaporates, it draws that latent heat out of your skin, leaving you cooler.
The last stop is the top of the dome, the critical point. Use the slider to raise the temperature. The higher it goes, the shorter the flat stretch crossing the dome becomes, the stretch where liquid and gas are distinct. At the critical temperature, that stretch shrinks to a single point. This point is the critical point. Go past this temperature and pressure and something remarkable happens: the distinction between liquid and gas disappears entirely. There is no boiling boundary, and you cannot say where liquid ends and gas begins. This state is called a supercritical fluid. It is dense like a liquid yet spreads freely like a gas. So the critical point is also where the boiling boundary on the phase diagram comes to an end. This supercritical state is put to real use: supercritical carbon dioxide seeps into coffee beans like a gas yet dissolves the caffeine like a liquid, which is how coffee is decaffeinated.