Phase change is a key model transformation to help us build a general thermodynamic understanding of the transformations that matter undergoes in chemistry.

Matter is made up of minuscule particles which, governed by the thermodynamic conditions, will interact to form one of several possible macroscopic phases: solid, liquid or gas. Although technically, the phases of matter may also include such variations as plasmas and Bose-Einstein condensates among others, in general chemistry our discussion will be concerned with the three phases common on the earth. Solids have a fixed volume and shape. Liquids have a fixed volume but take the shape of the portion of the container they occupy, and gases assume the shape and volume of the container they occupy. Our concern will be with the intrinsic properties of each phase and the thermodynamic factors underlying the transformation of one state of matter to another in phase change processes.

Phase equilibria diagrams, the behavior of real gases, and the vapor pressure related phenomena are perenial MCAT favorites. With liquids, surface tension and capillary action have both shown up on many exams. Occasionally, an MCAT will expect students at least to be familiar with the terminology of crystal structure in solids. In other words, as core general chemistry knowledge, you should expect to see questions from this material on your exam.

WikiPremed Resources

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States of Matter Practice Items
Problem set for States of Matter in PDF format

The States of Matter Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for The States of Matter
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals

Proficiency

Make sure you have achieved the learning goals for Ideal Gases including fully understanding the gas laws and the kinetic theory of gases.

Be capable of problem solving using Dalton's Law of Partial Pressures.

Understand the foundation in kinetic theory of Graham's Law of Effusion and be prepared to solve problems using these concepts.

Be prepared to describe how real gas behavior deviates from ideal gas behavior. Be able to interpret the Van der Waals Equation.

Grasp the intermolecular / thermodynamic basis of surface tension.

Be able to explain capillary action.

For crystalline solids, be ready to discuss the basic vocabulary of crystal lattice structure. Comprehend the technique of x-ray diffraction.

Grasp the concept of vapor pressure in thermodynamic terms.

Understand how to read phase diagrams. Be ready to discuss the triple point and the criticial point.

Suggested Assignments

Review the terminology for states of matter using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Perform the WikiPremed practice items for states of matter. Here is the answer key for the practice items.

Read pp. 166-169 and read pp. 171-174 in ExamKrackers Chemistry. Perform practice items 105-112 on pg 170 and practice items 113-120 on pp. 175-176

Conceptual Vocabulary for The States of Matter

State of matter
A state of matter is one of the many ways that matter can interact with itself to form a macroscopic, homogenous phase.
Solid
A solid object is in the states of matter characterized by resistance to deformation and changes of volume.
Liquid
A liquid is a fluid that can freely form a distinct surface at the boundaries of its bulk material.
Boiling point
The boiling point of a substance is the maximum temperature at which a liquid can remain a liquid at a given pressure.
Enthalpy of vaporization
The enthalpy of vaporization is the energy required to transform a given quantity of a substance into a gas, measured at the boiling point of the substance.
Enthalpy of fusion
The enthalpy of fusion is the amount of thermal energy which must be absorbed or evolved at the melting point for 1 mole of a substance to change states from a solid to a liquid or vice versa.
Melting point
The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid.
Dew point
The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water.
Surface tension
Surface tension is an effect within the surface layer of a liquid that causes that layer to behave as an elastic sheet.
Crystalline solid
Crystalline solids are a class of solids that have regular or nearly-regular structures, meaning that the atoms in these solids are arranged in an orderly manner
Freezing
Freezing is the process whereby a liquid turns to a solid.
Evaporation
Evaporation is the process by which molecules in a liquid state spontaneously become gaseous without being heated to boiling point.
Vapor pressure
Vapor pressure is the pressure of a gaseous phase in equilibrium with its non-gaseous phases.
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume.
Condensation
Condensation is the change in matter of a substance to a denser phase, such as a gas to a liquid.
Amorphous solid
An amorphous solid is a solid in which there is no long-range order of the positions of the atoms.
Capillary action
Capillary action is the ability of a substance to draw another substance into it.
Meniscus
A meniscus is a curve in the surface of a liquid and is produced in response to the surface of the container or another object.
Plasma
A plasma is typically an ionized gas, considered to be a distinct state of matter, apart from gases, because of its unique properties.
Phase
A phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties
Vapor
Vapor is the gas phase component present along with a solid or liquid sample of matter which does not completely fill its container.
Phase diagram
A phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases.
Triple point
The triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.
Crystal structure
A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice.
Critical point
A critical point, also called a critical state, specifies the conditions (temperature, pressure) at which the liquid state of the matter ceases to exist.
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point.
Nucleation
Nucleation is the onset of a phase transition in a small region such as with the formation of a bubble or of a crystal from a liquid.
Cooling curve
A cooling curve is a line graph that represents the change of phase of matter, typically from either a gas to a solid or from a liquid to a solid.
Saturation vapor pressure
The saturation vapor pressure is the static pressure of a vapor when the vapor phase of some material is in equilibrium with the liquid phase of that same material.
Lattice constant
The lattice constant refers to the constant distance between unit cells in a crystal structure.
Close-packing
Close-packing of spheres is the arranging of a lattice of spheres so that they take up the greatest possible fraction of a 3-dimensional space.
Vapor-Liquid Equilibrium
Vapor-liquid equilibrium is a condition or state where the rate of evaporation equals the rate of condensation.
Superheating
Superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its standard boiling point, without actually boiling.
Supercooling
Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid.
Ceramic
A ceramic is an inorganic non-metallic materials whose formation is due to the action of heat.
Space group
The space group of a crystal is a mathematical description of the symmetry inherent in the crystalline structure.
Aerogel
An aerogel is a low-density solid-state material derived from gel in which the liquid component of the gel has been replaced with gas.
Cubic crystal system
The cubic crystal system is a crystal system where the unit cell is in the shape of a cube.
Crystal system
A crystal system is a category of space groups, which characterize symmetry of structures in three dimensions with translational symmetry in three directions, having a discrete class of point groups.
Advanced terms that may appear in context in MCAT passages
Kapustinskii equation
The Kapustinskii equation calculates the Lattice Energy for an ionic crystal in cases where it is experimentally difficult to determine.
Bravais lattice
In geometry and crystallography, a Bravais lattice is an infinite set of points generated by a set of discrete translation operations.
Clausius-Clapeyron relation
The Clausius-Clapeyron relation is a way of characterizing the phase transition between two states of matter, such as solid and liquid, giving the slope of the coexistance curve separating the two phases on a pressure-temperature diagram.
Atomic packing factor
The atomic packing factor or packing fraction is the fraction of volume in a crystal structure that is occupied by atoms.
Kepler conjecture
The Kepler conjecture is a conjecture about sphere packing in three-dimensional Euclidean space which says that no arrangement of equal spheres filling space has a greater average density than that of the cubic close packing and hexagonal close packing arrangements.
Leidenfrost effect
The Leidenfrost effect is a phenomenon in which a liquid, in near contact with a mass significantly hotter than its boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly.
Epitaxy
The term epitaxy describes an ordered crystalline growth on a monocrystalline substrate.
Orthorhombic
Orthorhombic lattices result from stretching a cubic lattice along two of its lattice vectors by two different factors, resulting in a rectangular prism with a rectangular base.
Rhombohedral
In the rhombohedral crystal system, the crystal is described by vectors of equal length, of which all three are not mutually orthogonal.
Schoenflies notation
The Schoenflies notation is one of two conventions commonly used to describe crystallographic point groups. The other convention is the Hermann-Mauguin notation, also known as the International notation.
Hermann-Mauguin notation
Hermann-Mauguin notation is used to represent the symmetry elements in point groups, plane groups and space groups. This notation is sometimes called international notation.