HiSET Science: Phase Transitions

States of Matter
The four states of matter are solid, liquid, gas, and plasma. Solids have a definite shape and a definite volume. Because solid particles are held in fairly rigid positions, solids are the least compressible of the four states of matter. Liquids have definite volumes but no definite shapes. Because their particles are free to slip and slide over each other, liquids take the shape of their containers, but they still remain fairly incompressible by natural means. Gases have no definite shape or volume. Because gas particles are free to move, they move away from each other to fill their containers. Gases are compressible. Plasmas are high-temperature, ionized gases that exist only under very high temperatures at which electrons are stripped away from their atoms.


The following table shows similarities and differences between solids, liquids, and gases:



Six Different Types of Phase Change
A substance that is undergoing a change from a solid to a liquid is said to be melting. If this change occurs in the opposite direction, from liquid to solid, this change is called freezing. A liquid which is being converted to a gas is undergoing vaporization. The reverse of this process is known as condensation. Direct transitions from gas to solid and solid to gas are much less common in everyday life, but they can occur given the proper conditions. Solid to gas conversion is known as sublimation, while the reverse is called deposition.


Phase Diagram and Critical Point
A phase diagram is a graph or chart of pressure versus temperature that represents the solid, liquid, and gaseous phases of a substance and the transitions between these phases. Typically, pressure is located on the vertical axis, and temperature is located along the horizontal axis. The curves drawn on the graph represent points at which different phases are in an equilibrium state. These curves indicate at which pressure and temperature the phase changes of sublimation, melting, and boiling occur. Specifically, the curve between the liquid and gas phases indicates the pressures and temperatures at which the liquid and gas phases are in equilibrium.
The curve between the solid and liquid phases indicates the temperatures and pressures at which the solid and liquid phases are in equilibrium. The open spaces on the graph represent the distinct phases solid, liquid, and gas. The point on the curve at which the graph splits is referred to as the critical point. At the critical point, the solid, liquid, and gas phases all exist in a state of equilibrium.

Lettered Regions of a Phase Diagram

A—Solid phase: This is a region of high pressure and low temperature where the substance always exists as a solid.
B—Liquid phase: This is a region of pressure and temperature where the substance is in the liquid phase.
C—Gas phase: This is a region of pressure and temperature where the substance is in the gaseous phase.
D—Sublimation point: The portion of the curve that contains point D shows all the combinations of pressure and temperature at which the solid phase is in equilibrium with the gaseous phase.
E—Critical point: The point at which the solid, liquid, and gaseous phases are all in equilibrium.
F—Boiling point: The line that contains point F indicates all the combinations of pressure and temperature at which the liquid phase and gas phase are in equilibrium.