Physics
Phase Transitions in States of Matter
Quick fact
Water can exist in all three states at the same time—called the triple point—at exactly 0.01°C and 611.73 pascals of pressure.
Why this is interesting
You know ice melts into water and water boils into steam—but have you ever wondered why dry ice disappears without melting, or why you can boil water at room temperature in a vacuum?
Read the full explanation
Understanding Phase Transitions in States of Matter
Think of matter as a crowd of particles. In a solid, they are packed tightly, only vibrating in place. As you add heat, they vibrate faster until some break free of their neighbors—that's melting. Continue heating, and eventually particles break away from the surface entirely—that's evaporation. The reverse happens when you remove heat: gas particles slow down, stick together (condensation), and then lock into a rigid structure (freezing). You might notice that boiling occurs throughout the liquid, not just at the surface—that's vaporization. Also, some substances, like dry ice, go directly from solid to gas—that's sublimation. Each transition requires or releases energy, which is called latent heat.
A deeper explanation
The driving force behind phase transitions is the competition between kinetic energy (the energy of particle motion) and intermolecular forces (the attractions between particles). When you heat a solid, you're adding energy that increases kinetic energy, eventually overcoming the attractive forces that hold particles in a lattice—melting. The temperature at which this happens (the melting point) depends on the strength of those forces; stronger forces mean higher melting points. Similarly, boiling occurs when vapor pressure equals external pressure—that's why water boils at lower temperatures on a mountain. The latent heat absorbed or released during a phase change is used to break or form bonds, not to change temperature. Phase diagrams summarize all these transitions, showing which state is stable at any given temperature and pressure and revealing special points like the critical point, beyond which a gas and liquid become indistinguishable.