The fundamental difference between states of matter is the strength of the intermolecular forces. When the intermolecular forces are strongest, the substance is a solid. When the intermolecular forces are extremely weak, the substance is a gas.
Intermolecular forces are forces between molecules. They are not nearly as strong as the bonds that hold compounds together (called intramolecular forces).
Intermolecular forces dictate boiling points, melting points, surface tension, and many other physical properties.
In order from weakest to strongest:
If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force.
If one molecule is much larger (molar mass close to double the size), London dispersion forces will likely determine its physical properties.
A phase change is the conversion from one state of matter to another.
With a liquid sample, there is a minimum kinetic energy level required for a molecule to escape the sample and become a vapor. Although the average kinetic energy is below this, some molecules are above this level, and can escape into the space above the liquid.
These molecules in the air are sometimes recaptured by the liquid, which cycles molecules constantly. In an open container, molecules can drift away, so less molecules are recaptured. In humid conditions, more water vapor in the air causes water particles to bump back into the water more often, leading to more recapturing by the liquid water.
These molecules exert a pressure on the sample called vapor pressure.
The vapor pressure is higher when the sample is at a higher temperature, because more molecules will have sufficient kinetic energy to evaporate and join the molecules above the liquid (more molecules above = higher force pushing down on the liquid).
When the vapor pressure equals the external pressure, the liquid boils. At this point, the vapor pressure pushes against the atmospheric pressure above with equal force, allowing molecules within the liquid to also escape since they aren't pushed down anymore. This causes the bubbles you see when a liquid boils.
This also means that the boiling point of a liquid depends on atmospheric pressure, so boiling points change with altitude.
A phase diagram is a graph of pressure vs. temperature for a substance.
There are four types of solids (ranked* from weakest to strongest bonds): molecular solids, metallic solids, ionic solids, and network covalent solids.
*This ranking is approximate — properties vary, for example the ordering between metallic solids and ionic solids is not definite.
Therefore, higher boiling point = stronger intermolecular bonds.
Consist of atoms or molecules held together by weaker intermolecular forces (London dispersion, dipole-dipole, or hydrogen bonds).
The shape matters for some physical properties.
Since metals have few valence electrons and they're on high levels, the ionization energy is very low. This allows the electrons to form a "sea of electrons" around the cations (metals, but they don't have their electrons).
This model allows atoms to slide past each other without breaking.
An alloy is a material that contains more than one metal and has beneficial properties.
A substitutional alloy involves atoms of one metal replacing atoms of another metal. This is only possible if atoms of the two molecules are similar in size.
An interstitial alloy involves the atoms of a smaller metal occupying positions in the "holes" between the atoms of a larger metal. This generally makes the alloy less ductile, since the smaller atoms create covalent bonds with neighboring atoms.
The solid is comprised of alternatively charged ions.
Most favorable structures have cation-anion distances as close as possible, but cation-cation (repelling) distances are as far as possible.
These are quite hard but brittle. They're strong, but they're hit hard enough and ions shift, the repelling ions are facing each other and the entire substance shatters.
When identifying ionic solids, look for substances with a cation and an anion.
Atoms are covalently bonded over a large network with regular patterns of atoms.
Examples: diamond and graphite, silicon, quartz ().
Diamond forms a tetrahedral shape ( hybridization), while graphite forms sheets of hexagonal shapes ( hybridization) that are held together by London dispersion forces.
Both are made of carbon only, but since graphite has the weaker intermolecular bonds, it's weaker and comes apart in sheets (such as when writing with a pencil).
However, this means that graphite is a conductor, since each carbon has a free electron (it only made 3 bonds) that can move and conduct charges.