Density is a useful property in terms of describing another property of matter:  its phase, or state. There are four states of matter observable in everyday life: solid, liquid, gas, and plasma. You should already have an intuitive understanding of the first three.  For example, water can exist as a solid (ice), a liquid (water), and a gas (water vapor or steam).

Solids: In a solid the particles are closely packed together, and thus have a high density. Due to this tight packing, particles in a solid cannot move freely but can only vibrate. As a result, a solid has a stable, definite shape, and a definite volume. Solids can only change their shape by force, as when broken or cut.  There are two categories of solids:  crystalline and amorphous. In crystalline solids, the particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern. Sugar and diamond are examples of crystalline solids. In amorphous solids, the particles do not have a regular structure.  Styrofoam is an example of an amorphous solid.

Liquids: In a liquid, the particles are not as closely packed together as a solid, and thus have a lower density than solids.  Due to this lower density, the particles in liquid are free to move past each other.  As a result, a liquid has no definite shape, and can take the shape of the container it is placed in.  The density of a liquid, while lower than a solid, is still high enough for attractive forces to hold the particles of a liquid together.  Thus, liquids also have a definite volume.

Gases: In a gas, the particles are spread out very far, and thus, have a very low density.  Due to this very low density, the particles in a liquid are free to move past each other and the gas then has no definite shape.  Additionally, because the particles are so far apart, the force of attraction between is very weak.  Thus, gases do not have a definite volume.

The following table summarizes some of the properties of solids, liquids, and gases:

It was suggested that you have an intuitive understanding of solids, liquids, and gases through your understanding of water.  It is important to note that water is actually an exception to one characteristic of solids, liquids, and gases.  Water’s solid form is actually slightly less dense than its liquid form.  For this reason, solid ice can float in liquid water.  The reasons for this are due to hydrogen bonding, which will be discussed in the next lesson.

Plasma: One state or phase of matter with which you are likely not familiar is a plasma. Plasma is actually quite common on Earth, and the majority of people observe it on a regular basis without even realizing it. Fire, lightning, electric sparks, fluorescent lights, neon lights, plasma televisions, and the stars are all examples of illuminated matter in the plasma state. Like a gas, plasma does not have definite shape or volume. However, plasmas are different in structure at the atomic level, which you will learn more about in the next lesson. Due to these unique structural characteristics, plasmas are electrically conductive, produce magnetic fields and electric currents, and respond strongly to electromagnetic forces.

Properties of Matter: Solids

liquid and gas. Matter is the "stuff" of the universe, the atoms, molecules and ions that make up all physical substances. In a solid, these particles are packed closely together and are not free to move about within the substance. Molecular motion for the particles in a solid is confined to very small vibrations of the atoms around their fixed positions; therefore, solids have a fixed shape that is difficult to change. Solids also have a definite volume; that is, they keep their size no matter how you try to change them. Solids are divided into two main categories, crystalline solids and amorphous solids, based on how the particles are arranged. Crystalline solidsCrystalline solids, or crystals, are regarded as "true solids." Minerals are crystalline solids. Common table salt is one example of this kind of solid. In crystalline solids, the atoms, ions or molecules are arranged in an ordered and symmetrical pattern that is repeated over the entire crystal. The smallest repeating structure of a solid is called a unit cell, which is like a brick in a wall. Unit cells combine to form a network called a crystal lattice. There are 14 types of lattices, called Bravais lattices (named after Auguste Bravais, a 19th-century French physicist), and they are classified into seven crystal systems based on the arrangement of the atoms. The ChemWiki page at the University of California, Davis lists these systems as cubic, hexagonal, tetragonal, rhombohedral, orthorhombic, monoclinic and triclinic. AdvertisementAside from the regular arrangement of particles, crystalline solids have several other characteristic properties. They are generally incompressible, meaning they cannot be compressed into smaller shapes. Because of the repeating geometric structure of the crystal, all the bonds between the particles have equal strength. This means that a crystalline solid will have a distinct melting point, because applying heat will break all the bonds at the same time. Crystalline solids also exhibit anisotropy. This means that properties such as refractive index (how much light bends when passing through the substance), conductivity (how well it conducts electricity) and tensile strength (the force required to break it apart) will vary depending on the direction from which a force is applied. Crystalline solids also exhibit cleavage; when broken apart, the pieces will have planed surfaces, or straight edges. Types of crystalline solidsThere are four types of crystalline solids: ionic solids, molecular solids, network covalent solids and metallic solids. Ionic solidsIonic compounds form crystals that are composed of oppositely charged ions: a positively charged cation and a negatively charged anion.  Because of the strong attraction between opposite charges, it takes a lot of energy to overcome ionic bonds. This means that ionic compounds have very high melting points, often between 300 and 1,000 degrees Celsius (572 to 1,832 degrees Fahrenheit). While the crystals themselves are hard, brittle and nonconductive, most ionic compounds can be dissolved in water, forming a solution of free ions that will conduct electricity. They may be simple binary salts like sodium chloride (NaCl), or table salt, where one atom of a metallic element (sodium) is bonded to one atom of a nonmetallic element (chlorine). They may also be composed of polyatomic ions such as NH4NO3 (ammonium nitrate). Polyatomic ions are groups of atoms that share electrons (called covalent bonding) and function in a compound as if they constituted a single charged ion.Molecular solidsMolecular solids are composed of covalently bonded molecules attracted to each other by electrostatic forces (called van der Waals forces, according to the HyperPhysics website). Because covalent bonding involves sharing electrons rather than outright transfer of those particles, the shared electrons may spend more time in the electron cloud of the larger atom, causing weak or shifting polarity. This electrostatic attraction between the two poles (dipoles) is much weaker than ionic or covalent bonding, so molecular solids tend to be softer than ionic crystals and have lower melting points (many will melt at less then 100 C, or 212 F). Most molecular solids are nonpolar. These nonpolar molecular solids will not dissolve in water, but will dissolve in a nonpolar solvent, such as benzene and octane. Polar molecular solids, such as sugar, dissolve easily in water. Molecular solids are nonconductive.Examples of molecular solids include ice, sugar, halogens like solid chlorine (Cl2), and compounds consisting of a halogen and hydrogen such as hydrogen chloride (HCl). Fullerene "buckyballs" are also molecular solids.Network covalent solidsIn a network solid, there are no individual molecules. The atoms are covalently bonded in a continuous network, resulting in huge crystals. In a network solid, each atom is covalently bonded to all the surrounding atoms. Network solids have similar properties to ionic solids. They are very hard, somewhat brittle solids with extremely high melting points (higher than 1,000 C or 1,800 F). Unlike ionic compounds, they do not dissolve in water, nor do they conduct electricity.Examples of network solids include diamonds, amethysts and rubies.Metallic solidsMetals are opaque, lustrous solids that are both malleable and ductile. Malleable means they are soft and can be shaped or pressed into thin sheets, while ductile means they can be pulled into wires. In a metallic bond, the valence electrons are not donated or shared as they are in ionic and covalent bonding. Rather, the electron clouds of adjacent atoms overlap so that electrons become delocalized. The electrons move with relative freedom from one atom to another throughout the crystal. A metal may be described as a lattice of positive cations within a "sea" of negative electrons. This electron mobility means that metals are highly conductive of heat and electricity. Metals tend to have high melting points, though notable exceptions are mercury, which has a melting point of minus 37.84 degrees Fahrenheit (minus 38.8 Celsius), and phosphorous, with a melting point of 111.2 F (44 C).An alloy is a solid mixture of a metallic element with another substance. While pure metals can be overly malleable and heavy, alloys are more workable. Bronze is an alloy of copper and tin, while steel is an alloy of iron, carbon and other additives.Amorphous solidsIn amorphous solids (literally "solids without form"), the particles do not have a repeating lattice pattern. They are also called "pseudo solids." Examples of amorphous solids include glass, rubber, gels and most plastics. An amorphous solid does not have a definite melting point; instead, it melts gradually over a range of temperatures, because the bonds do not break all at once. This means an amorphous solid will melt into a soft, malleable state (think candle wax or molten glass) before turning completely into a liquid. Amorphous solids have no characteristic symmetry, so they do not have regular planes of cleavage when cut; the edges may be curved. They are called isotropic because properties such as refractive index, conductivity and tensile strength are equal regardless of the direction in which a force is applied.