In chemistry, a physical change is a change to the form or structure of a chemical compound, but not to its chemical composition. An object undergoes a physical change when there is some alteration of its physical structure or arrangements, but not its chemical composition. A physical change is opposed to a chemical change, which involves the breaking and formation of new chemical bonds. Most of the time, a physical change results in a change in the physical properties and behavior of an object, such as its shape, size, color volume, density, and texture. Most of the time, a physical change is the result of the spatial rearrangement of atoms that make up the object.
A simple example of a physical change is an ice cube melting. When an ice cube melts, its constituent molecules change their arrangement and gain some properties, like flow, or lose some properties, like definite shape. The chemical composition of water molecules (H2O) do not change; they are still made out of 2 hydrogen atoms and 1 oxygen atom. but their physical arrangement does. The change in the water’s properties is a consequence of the change in the arrangement of its molecules. The water becomes fluid and loses its definite shapes because its molecules are no longer fixed in a rigid spatial arrangement anymore.
“Physical changes take place continuously, while chemical changes take place discontinuously. Physics deals chiefly with continuous varying quantities, while chemistry deals chiefly with whole numbers.” — Max Planck
Contrast this with a chemical change, like electrolysis, where one molecule of water is split into its atomic components, oxygen and hydrogen. Such a change is chemical, as it involves the breaking or forming of chemical bonds. Chemical changes involve an alteration of the chemical composition of the substance.
Physical changes that can be undone are called reversible. An ice cube that has melted can be frozen again, so melting is a reversible process. Most physical changes are reversible to some degree, though the reversibility of a change is not a condition for it being a physical change; some chemical changes are reversible as well.
10 Examples Of Physical Changes
10. Mechanical Deformation
Mechanical deformation is probably the simplest example of physical change. Mechanical deformation involves a change in the spatial arrangement of atoms or molecules by the application of a mechanical force. Crumpling a piece of paper, shattering a wine glass, or denting a metal plate with a hammer are all examples of mechanical deformations that arise from the application of some external force.
Some objects and arrangements are much more resistant to mechanical deformation than others. In fact, this is just what we mean when we call something hard; it is resistant to having its molecular or atomic structure changed by a mechanical force. In the context of material sciences, the tendency for a material to permanently deform under mechanical stress is called creep. Most deformations are reversible; a dent in a metal can be undone, but whether they actually can be undone is a practical matter that depends on the context.
9. Heating And Cooling
The kinetic molecular theory of heat states that the phenomenon of heat is identical to molecular motion. That is, the temperature of a substance is directly related to the average kinetic energy of its constituent particles. It follows that a change in temperature corresponds to a change in the average kinetic energy of the particles. Since kinetic energy is the result of the motion of particles, it follows that an increase in temperature means that the particles are moving faster. So, heating is an object is a kind of physical change that is characterized by the object’s particles moving faster and faster. Similarly, the colder an object is, the slower its molecules are moving.
8. Phase Changes
“We might expect that as we come close upon living nature the characters of our old records would grow legible and clear; but just when we begin to enter on the history of the physical changes going on before our eyes, and in which we ourselves bear a part, our chronicle seems to fail us: a leaf has been torn out from Nature’s book, and the succession of events is almost hidden from our eyes.” — Adam Sedgwick
A phase is a physically distinct form of matter, like a solid, liquid, or a gas. A substance can change between states of matter depending on the temperature and pressure. These changes are called a phase change. All phase changes are physical changes. They are physical because they involve a change in the arrangement of molecules or atoms and not a change in chemical bonds. For the basic three states of matter; solid, liquid, and gas, the names for the transitions are: evaporation (liquid to gas), condensation (gas to liquid), melting (solid to liquid), freezing (liquid to solid), deposition (gas to solid) and sublimation (solid to gas). There is also the state of matter plasma, a superheated gas of delocalized charged particles. The phase change from gas to plasma is called ionization, and the phase change from plasma to gas is called recombination. All phase changes are reversible processes.
Each state of matter is associated with a few characteristic properties. Solids tend to have definite shapes and volumes and are resistant to deformation. Liquids are fluid, have no definite shape, and are incompressible. Gases are also fluid and have neither a definite shape or volume. Gases can be easily compressed and expanded, unlike solids and liquids.
A mixture refers to a physical combination of two or more distinct chemical substances. In a mixture, the different chemical substances retain their identity, so a mixture is the result of the comingling of the molecules of different substances. Mixtures can be homogeneous (evenly distributed) or heterogeneous (unevenly distributed).
A mixture can result in the physical change of the two substances. For example, pigments are chemicals that produce colors by reflecting specific wavelengths of light. Two pigments can be physically mixed together to change the range of light waves they reflect, thus creating a new pigment with a distinct color. Red paint and blue paint will combine to form purple paint. The fact that the purple paint is a different color signifies a physical change.
A solution is the result of the dissociation of one ionic compound, the solvent, into another, the solvent. In cases where the solvent is water, the solution is called an aqueous solution. In most cases, the solute is an ionic compound that is capable of dissociating into ions.
Dissolving an ionic compound in a solvent is an example of a physical change. In some ways, a solution is a special kind of mixture, one that is homogeneous and immune to mechanical filtration. Most solutions are liquid-liquid solutions, but gas-liquid, liquid-solid, and gas-solid solutions are possible. Dissolving a solute in a solvent often involves a physical change in the properties of the solvent. Dissolving salt in water, for example, has the effect of decreasing the boiling point of that water. The change in properties is a result of the spatial intermingling of the particles.
Crystallization is the process of the solidification of a substance into a highly ordered structure known as a lattice structure. Lattice structures are highly ordered periodic arrangement of atoms into geometric cells. Crystals can form from phase changes like freezing and deposition, or from high temperature and pressures.
Most precious gems and other minerals deposits are formed via the crystallization of organic and inorganic compounds in the Earths crust. Diamonds, for example, are a form of crystallized carbon atoms that have taken on a highly ordered lattice structure. This lattice structure gives diamond its unique physical properties; e.g. its hardness, clarity, transparency, and color.
Alloying is the process of mixing two metals together evenly in certain proportions to make an alloy. Alloying is a special kind of mixing in that alloyed metals tend to synergistically adopt properties of the mixed metals. Alloys are different from metal compounds, as the constituent metals of an alloy are not chemically bonded together. They are evenly dispersed within one another, though the exact concentration of the alloy may differ from point-to-point. Alloying is used to reduce the overall cost of materials while still maintaining desirable properties like strength or ductility, and also as a way to impart desirable properties on a quantity of metal.
Steel is a simple kind of metallic alloy that is made from treating iron with carbon. The introduction of carbon among the allotropes of iron has a reinforcing effect, giving steel a higher tensile strength and less malleability than iron. The new physical properties of steel result from the physical mixing of carbon with iron.
All macroscopic objects contain electrons. One of the fundamental properties of electrons is that they create magnetic dipoles, regions of space that have a positive and negative end. In some materials, there tiny magnetic fields can become perfectly aligned and pull in the same direction, which manifests as a macroscopic magnetic field that acts on other magnetic dipoles. Materials that spontaneously arrange their magnetic fields into a larger one are called ferromagnetic and produce their own magnetic field. The kitchen magnets on your fridge are an example of a ferromagnetic material.
Some objects, when introduced to a magnetic field, will have their electrons rearranged so that they take on a temporary magnetic field. These materials that can rearrange to form temporary magnetic field are called paramagnetic. In general, most materials that are ferromagnetic or paramagnetic are metals, as metals tend to have numerous unoccupied orbitals for electrons to move around in.
2. Hydrogen Bonding
Hydrogen bonds are a kind of a misnomer. They are not true chemical bonds because they are not formed via the sharing or capture of an electron, like covalent or ionic bonds. Hydrogen bonds are the result of the electrostatic interaction between polar molecules that contain hydrogen and other molecules that contain electronegative elements. The positively charged hydrogen ends are drawn to the negative end of other molecules, creating a tight electrostatic attraction. Water is a substance that has hydrogen bonds. the presence of hydrogen bonds explains water’s unique properties, like its high boiling point and high specific heat capacity.
1. Bose-Einstein Condensates
“To think is to practice brain chemistry.” — Deepak Chopra
A Bose-Einstein condensate is a state of matter consisting of a gas cloud of bosons cooled to temperatures extremely close to absolute zero. Strictly speaking, the change of a gas from to a Bose-Einstein condensate is a type of phase change, but the nature of Bose-Einstein condensates is so strange it deserves special consideration.
All particles have wave-like properties. When particles are cooled, their wave-like properties become more pronounced. When particles are supercooled beyond a certain threshold (a fraction of a degree near absolute zero) the wave-like properties of particles become very pronounced and quantum phenomena becomes apparent on a macroscopic scale. Particles in a Bose-Einstein condensate essentially overlap and form one super-wave that all share the same state.
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