phase diagram
Think of a weather map, but instead of showing where it is sunny or raining, it shows where a substance is solid, liquid, or gas. Set up a chart with pressure running up the side and temperature running across the bottom; then for every combination, mark which phase the substance settles into. That filled-in map is a phase diagram.
The map is carved into regions — one for solid, one for liquid, one for gas — separated by lines. Each line marks the conditions where two phases coexist in balance, so crossing a line means a phase transition. The lines meet at two special spots: the triple point, where all three phases coexist at once, and the critical point, where the liquid–gas line simply stops and the two become indistinguishable. Pick any pressure and temperature, find your spot on the map, and you can read off the phase.
Phase diagrams are the working maps of chemistry, geology, and materials science. They tell you the pressure needed to keep liquid carbon dioxide in a fire extinguisher, why ice skates glide, what conditions forge diamonds deep underground, and how to freeze-dry coffee. Water's diagram even has a famously tilted melting line — its solid is less dense than its liquid, which is why ice floats.
On water's phase diagram, the dot at 1 atmosphere and 25 °C lands inside the liquid region — which is just our everyday confirmation that a glass of water sits there as a liquid.
A point's location on the map tells you the phase; a line tells you a transition.
Water is unusual: its solid–liquid line leans slightly to the left, meaning higher pressure lowers the melting point. For most substances that line leans right. This quirk traces back to ice being less dense than liquid water.