The phases or states of matter are those forms in which matter can exist.
1. Solids
2. Liquids
3. Gases
4. Plasma
The first three (solids, liquids, gases), commonly occur on Earth. The fourth, plasma, is only found under certain conditions such as those occurring on stars.
The phase in which a substance exists depends on several things, including the temperature of the substance, the pressure present, etc.
Our concept of the phases (states) in which matter can exist is based on our assumption of the Kinetic Theory of Matter which states that all matter is composed of extremely small particles that are in motion and collisions between these particles are perfectly elastic. There are 3 common phases and 1 additional phase. The phase at which a substance exists is dependent on the substance’s temperature.
The most familiar phase may be solids. Solids have definite shapes and definite volumes. They may be described as crystalline (having a regular arrangement of particles) or amorphous (having a random particle arrangement).
Solids have particles that are tightly packed together. Although the particles of solids are still moving, they are held in relatively fixed positions by the binding forces within molecules. They do, however, vibrate about their fixed positions.
Solids may exhibit the characteristics of diffusion (the penetration of one type of particle into a mass consisting of a second type of particle), cohesion (the force of attraction between molecules), adhesion (the force of attraction between molecules of different kinds), tensile strength (the force per unit cross-sectional area applied perpendicularly to the cross section that is required to break a rod or wire of that material), ductility (can be drawn into wires), malleability (can be hammered or rolled into sheets), and elasticity (the ability of an object to return to its original size or shape when external forces are removed; includes stress ratio of the internal force that occurs when the shape of a substance is changed to the area over which the force acts [stress = F/A] and strain relative amount of deformation produced in a body under stress)
Liquids are that phase of matter that have a specific volume but take the shape of their container. Liquids also diffuse (much more readily than do solids). Liquids also exhibit cohesion and adhesion. Their adhesiveness is much more easily seen than that of solids. If you pour water (or many other liquids) into a narrow glass tube, you will see that the surface of the water is not level but is slightly concave. This is because the adhesion between the water and the tube is stronger than the cohesion between water molecules. This crescent-shaped surface of a liquid column is called the meniscus.
If you use liquid mercury instead of water, you find that the surface is slightly convex because cohesion between mercury molecules is greater than adhesion between mercury and glass.
Cohesion is also responsible for viscosity, the ratio of shear stress to the rate of change of shear strain in a liquid or gas. The viscosity of a fluid (liquid or gas) determines the rate of flow of the fluid and is dependent on temperature. As temperature increases, the viscosity of liquids decreases, but the viscosity of gases increases.
All liquids also show surface tension . For instance, many objects such as razor blades or needles will float on the surface of water if balanced correctly. This is because the molecules at the surface of the liquid act as though they form a thin, flexible surface film. Liquids tend to have as small a surface area as possible (this is what causes them to exhibit surface tension). Surface tension produces contraction forces in liquid films and causes a free liquid to assume a spherical shape.
Some liquids, such as water, also show capillarity. They will not rise equally in tubes of small diameters. This elevation or depression in small diameter tubes is called capillarity and depends on both adhesion and surface tension. This has led to 4 statements of behavior for liquids:
1. Liquids rise in capillary tubes they wet (strong adhesion) and are depressed in tubes they do not wet (stronger cohesion).
2. Elevation or depression is inversely proportional to the diameter of the tube.
3. The elevation or depression decreases as the temperature increases.
4. The elevation of depression depends on the surface tension of the liquid.
Gases have enough EK (kinetic energy) that they have overcome the forces holding the molecules/atoms together. A gas has neither a definite volume nor a definite shape. It will expand to fill its container. Gas molecules move independently of each other at high speeds. Gases exhibit these characteristics:
1. Expansion – no definite shape or volume; expands to fill container; indicates that gas molecules are independent and they have inertia.
2. Pressure – the gas molecules exert pressure on the walls of the container as a result of continual bombardment.
3. Diffusion – exhibit rapid diffusion, through other gases, liquids, and solids (important in living things).
Plasma consists of a gas that is capable of conducting electricity. Gases do not ordinarily conduct electricity, but when heated to high temperatures or when they collide vigorously, they form electrically charged particles called ions. These ions give plasma the ability to conduct an electric current.
On Earth, under normal conditions, matter exists only as a solid, liquid or gas. However, on the sun and other stars, where extremely high temperature prevail, the constituent matter exists almost entirely in the plasma phase. Therefore, plasma is the most common phase of matter in the universe.
As is obvious to anyone who has ever paid the least bit of attention to the world around him or her, matter changes phases.
1. Melting – change in phase from solid to liquid (increase in temperature, molecules move farther apart)
2. Freezing – change in phase from liquid to solid (decrease in temperature, molecules move closer together)
3. Vaporization – production of a gas or vapor from matter in another phase
A. Evaporation – vaporization of liquids
B. Sublimation – vaporization of solids
4. Boiling – rapid vaporization that occurs when the vapor pressure of a liquid equals the pressure on its surface
A phase diagram is a graph of temperature versus pressure and details which phase exists under which conditions. It also shows the triple point – the temperature at which all 3 common states of matter exist in equilibrium. And it indicates the critical point which is the intersection of temperature and pressure above which molecules are unable to liquefy.
The following is an image of a phase diagram:
(source)