Physical science, which includes chemistry and physics, is usually thought of as the study of the nature and properties of matter and energy in non-living systems. Matter is the “stuff” of the universe — the atoms, molecules and ions that make up all physical substances. Matter is anything that has mass and takes up space.

Energy is the capacity to cause change. Energy cannot be created or destroyed; it can only be conserved and converted from one form to another. "Potential energy" is the energy stored in an object due to its position — for example, a bucket of water balanced over a doorway has the potential to fall. "Kinetic energy" is energy that is in motion and causing changes. Any object or particle that is in motion has kinetic energy based on its mass and speed. Kinetic energy can be converted into other forms of energy, such as electrical energy and thermal energy.

Five phases

There are five known phases, or states, of matter: solids, liquids, gases, plasma and Bose-Einstein condensates. The main difference in the structures of each state is in the densities of the particles.


In a solid, particles are packed tightly together so they are unable to move about very much. Particles of a solid have very low kinetic energy. The electrons of each atom are in motion, so the atoms have a small vibration, but they are fixed in their position. Solids have a definite shape. They do not conform to the shape of the container in which they are placed. They also have a definite volume. The particles of a solid are already so tightly packed together that increasing pressure will not compress the solid to a smaller volume. [Related: Properties of Matter: Solids]


In the liquid phase, the particles of a substance have more kinetic energy than those in a solid. The liquid particles are not held in a regular arrangement, but are still very close to each other so liquids have a definite volume. Liquids, like solids, cannot be compressed. Particles of a liquid have just enough room to flow around each other, so liquids have an indefinite shape. A liquid will change shape to conform to its container. Force is spread evenly throughout the liquid, so when an object is placed in a liquid, the liquid particles are displaced by the object. [Related: Properties of Matter: Liquids]

The magnitude of the upward buoyant force is equal to the weight of the fluid displaced by the object. When the buoyant force is equal to the force of gravity pulling down on the object’s mass, the object will float. This principle of buoyancy was discovered by the Greek mathematician Archimedes who, according to legend, sprang from his bath and ran naked through the streets shouting "Eureka!"

Particles of a liquid tend to be held by weak intermolecular attraction rather than moving freely as the particles of a gas will. This cohesive force pulls the particles together to form drops or streams.

Scientists reported in April 2016 they had created a bizarre state of matter, one that had been predicted to exist but never seen in real life. Though this type of matter could be held in one's hand as if it were a solid, a zoom-in on the material would reveal the disorderly interactions of its electrons, more characteristic of a liquid. In the new matter, called a Kitaev quantum spin liquid, the electrons enter into a sort of quantum dance in which they interact or "talk" to one another. Usually when matter cools down the spin of its electrons tends to line up. But in this quantum spin liquid, the electrons interact so that they affect how the others are spinning and never align no matter how cool the material gets. The material would behave as if its electrons, considered indivisible, had broken apart, the researchers reported April 4, 2016, in the journal Nature Materials.


Gas particles have a great deal of space between them and have high kinetic energy. If unconfined, the particles of a gas will spread out indefinitely; if confined, the gas will expand to fill its container. When a gas is put under pressure by reducing the volume of the container, the space between particles is reduced, and the pressure exerted by their collisions increases. If the volume of the container is held constant, but the temperature of the gas increases, then the pressure will also increase. Gas particles have enough kinetic energy to overcome intermolecular forces that hold solids and liquids together, thus a gas has no definite volume and no definite shape. [Related: Properties of Matter: Gases]


Plasma is not a common state of matter here on Earth, but may be the most common state of matter in the universe. Plasma consists of highly charged particles with extremely high kinetic energy. The noble gases(helium, neon, argon, krypton, xenon and radon) are often used to make glowing signs by using electricity to ionize them to the plasma state. Stars are essentially superheated balls of plasma. [Related: Properties of Matter: Plasma]

Bose-Einstein condensates

In 1995, technology enabled scientists to create a new state of matter, the Bose-Einstein condensate (BEC). Using a combination of lasers and magnets, Eric Cornell and Carl Weiman cooled a sample of rubidium to within a few degrees of absolute zero. At this extremely low temperature, molecular motion comes very close to stopping altogether. Since there is almost no kinetic energy being transferred from one atom to another, the atoms begin to clump together. There are no longer thousands of separate atoms, just one “super atom.” A BEC is used to study quantum mechanics on a macroscopic level. Light appears to slow down as it passes through a BEC, allowing study of the particle/wave paradox. A BEC also has many of the properties of a superfluid — flowing without friction. BECs are also used to simulate conditions that might apply in black holes. [Related: Properties of Matter: Bose-Einstein Condensate]

Going through a phase

Adding energy to matter causes a physical change — matter moves from one state to another. For example, adding thermal energy — heat — to liquid water causes it to become steam or vapor — a gas. Taking away energy also causes physical change, such as when liquid water becomes ice — a solid — when heat is removed. Physical change also can be caused by motion and pressure.

Melting and freezing

When heat is applied to a solid, its particles begin to vibrate faster and tend to move farther apart. When the substance, at standard pressure, reaches a certain point — called the melting point — the solid will begin to turn into a liquid. The melting point of a pure substance can often be determined to within 0.1 degrees C, the point at which the solid and liquid phases are in equilibrium. If you continue to apply heat to the sample, the temperature will not rise above the melting point until the entire sample has been liquefied. The heat energy is being used to convert the solid into the liquid form. Once the entire sample has become a liquid the temperature will begin to rise again. Compounds that are otherwise very similar can have different melting points, so melting point can be a useful way to distinguish among them. For example, sucrose has a melting point of 367 F (186.1 C) while the melting point of glucose is 294.8 F (146 C). A solid mixture, such as a metal alloy, can often be separated into its constituent parts by heating the mixture and extracting the liquids as they reach their different melting points.

The freezing point is the temperature at which a liquid substance is cooled enough to form a solid. As the liquid is cooled, particle motion slows. In many substances, the particles align in precise, geometric patterns to form crystalline solids. Most liquids contract as they freeze. One of the important characteristics of water is that it expands when it freezes, so ice floats. If ice didn’t float, there would be no liquid water underneath a frozen body of water and many forms of aquatic life would be impossible.

The freezing point is often nearly the same temperature as the melting point, but is not considered to be characteristic of a substance, as several factors can alter it. For example, adding dissolved substances, or solutes, to a liquid will depress the freezing point. An example of this is using salt slurry to lower the temperature at which water freezes on our roads. Other liquids can be cooled to temperatures well below their melting point before they begin to solidify. Such liquids are said to be “super cooled” and often require the presence of a dust particle or “seed crystal” to start the process of crystallization.


When a solid is converted directly into a gas without going through a liquid phase, the process is known as sublimation. Sublimation occurs when kinetic energy of the particles is greater than atmospheric pressure surrounding the sample. This may occur when the temperature of the sample is rapidly increased beyond the boiling point (flash vaporization). More commonly, a substance can be "freeze dried" by cooling it under vacuum conditions so that the water in the substance undergoes sublimation and is removed from the sample. A few volatile substances will undergo sublimation at normal temperature and pressure. The best known of these substances is CO2 or “dry ice.”


Vaporization is the conversion of a liquid to a gas. Vaporization can occur through either evaporation or boiling.

Because the particles of a liquid are in constant motion they frequently collide with each other, transferring energy when they do so. This energy transference has little net effect beneath the surface, but when enough energy is transferred to a particle near the surface; it may gain enough energy to be knocked completely away from the sample as a free gas particle. This process is called evaporation and it continues as long as liquid remains. It is interesting to note that a liquid cools as it evaporates. The energy transferred to surface molecules, which causes their escape, is carried away from the remaining liquid sample.

When enough heat is added to a liquid that vapor bubbles form below the surface of the liquid, we say that the liquid is boiling. The temperature at which a liquid boils is variable. Boiling point is dependent upon the pressure the substance is under. A liquid under higher pressure will require more heat before vapor bubbles can form within it. At high altitudes, there is less atmospheric pressure pressing down on the liquid, so it will boil at a lower temperature. The same amount of liquid at sea level is under a greater atmospheric pressure and will boil at a higher temperature.

Condensation and deposition

Condensation is when a gas transforms into a liquid. Condensation occurs when a gas has been cooled or compressed to the point where kinetic energy of the particles can no longer overcome the intermolecular forces. An initial cluster of particles initiates the process which tends to further cool the gas so that condensation continues. When the gas transforms directly into a solid, without going through the liquid phase, it is called deposition or desublimation. An example of this occurs when subfreezing temperatures convert water vapor in the atmosphere into frost or ice. Frost tends to outline solid blades of grass and twigs because the air touching these solids cools faster than air that is not touching a solid surface.


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