phase
/ fayz /
Pour oil into water and let it settle: you see two clear layers, oil floating on water, each uniform throughout and separated by a sharp boundary. Each of those layers is a phase. A phase is a region of matter that is the same all the way through — same composition, same density, same properties — so that within it you cannot tell one spot from another.
More precisely, a phase is a physically distinct, uniform portion of a system with its own definite properties, marked off from neighboring phases by a boundary across which those properties change abruptly. Solid, liquid, and gas are the everyday examples, but a single substance can have several solid phases (ice has more than a dozen crystal forms), and a mixture can split into several liquid phases. Importantly, phase is not the same as state of matter: oil and water are both liquids, yet they are two separate phases.
Why bother with the word? Because phases are what physical chemistry tracks as conditions change. Heat, pressure, and composition decide which phases appear and how much of each. Knowing the number of phases present is the first thing you need before you can count how many things you are free to vary — the starting point for the phase rule and the phase diagram.
A glass of iced soda has three phases: solid ice, liquid soda, and the gas bubbles fizzing up through it.
Three phases of the same substance coexisting in one glass.
Phase and state of matter overlap but are not identical. There are three common states (solid, liquid, gas), but a system can contain many phases — two immiscible liquids are two phases yet one state, and a rock can hold several solid mineral phases at once.