Condensing and freezing

    If you cool a gaseous or liquid substance, you can watch the changes of state, or phase changes, that occur. Here are the phase changes that happen as substances lose energy:

Condensation: When a substance condenses, it goes from a gas to a liquid state. Gas particles have a high amount of energy, but as they’re cooled, that energy decreases. The attractive forces now have a chance to draw the particles closer together, forming a liquid. The particles are now in clumps, as is characteristic of particles in a liquid state.

Freezing: A substance freezes when it goes from a liquid to a solid. As energy is removed by cooling, the particles in a liquid start to align themselves, and a solid forms.

  The temperature at which this occurs is called the freezing point (fp) of the substance. You can summarize the process of water changing from a gasto a solid in this way:

   Here, the (l) stands for liquid, the (g) stands for gas, and (s) stands for solid.

Melting and boiling

   As a substance heats, it can change from a solid to a liquid to a gas. For water, you represent the change like this:

From solid to liquid

   When a substance melts, it goes from a solid to a liquid state.  Here’s what happens: If you start with a solid, such as ice, and take temperature readings while heating it, you find that the temperature of the solid begins to rise as the heat causes the particles to vibrate faster and faster in the crystal lattice.

  After a while, some of the particles move so fast that they break free of the lattice, and the crystal lattice (which keeps a solid solid) eventually breaks apart. The solid begins to go from a solid state to a liquid state — a process called melting.   The temperature at which melting occurs is called the melting point (mp) of the substance. The melting point for ice is 32°F, or 0°C.

   During changes of state, such as melting, the temperature remains constant — even though a liquid contains more energy than a solid. So if you watch the temperature of ice as it melts, you see that the temperature remains steady at 0°C until all the ice has melted.  The melting point (solid to a liquid) is the same as the freezing point (liquid to a solid).

From liquid to gas

   The process by which a substance moves from the liquid state to the gaseous state is called boiling.   If you heat a liquid, such as a pot of cool water, the temperature of the liquid rises and the particles move faster and faster  as they absorb the heat. The temperature rises until the liquid reaches the next change of state — boiling. As the particles heat up and move faster and faster, they begin to break the attractive forces between each other and move freely as a gas, such as steam, the gaseous form of water.

  The temperature at which a liquid begins to boil is called the boiling point (bp). The bp depends on atmospheric pressure, but for water at sea level, it’s 212°F, or 100°C. The temperature of a boiling substance remains constant until all of it has been converted to a gas.