How do temperature and pressure affect whether matter is a solid, liquid, or gas?

The state of matter—solid, liquid, or gas—is determined by the balance between the kinetic energy of a substance's particles and the forces of attraction acting between them. These states are mapped using a phase diagram, which illustrates how a substance transitions between phases based on changes in temperature and pressure.12

• Temperature: Increasing temperature adds heat, providing particles with the energy needed to overcome attractive forces. Heating a solid allows particles to move into a liquid state, while further heating provides the energy for liquid particles to escape into a gaseous state. Conversely, cooling a gas slows particle motion, allowing attractive forces to coalesce them into a liquid and eventually a solid.1
• Pressure: Pressure influences the density and spacing of particles. Higher pressure generally favors the more dense phase. For example, increasing pressure can force a gas to condense into a liquid.1324

At specific temperature and pressure combinations, two phases can exist in equilibrium, represented by lines on a phase diagram. The triple point is the unique condition where all three phases coexist.23