Physical change

Physical changesare changes affecting the form of achemical substance,but not itschemical composition.Physical changes are used to separatemixturesinto their componentcompounds,but can not usually be used to separate compounds intochemical elementsor simpler compounds.

Physical changes occur when objects or substances undergo a change that does not change their chemical composition. This contrasts with the concept ofchemical changein which the composition of a substance changes or one or more substances combine or break up to form new substances. In general a physical change is reversible using physical means. For example,saltdissolved in water can be recovered by allowing the water to evaporate.

A physical change involves a change inphysical properties.Examples of physical properties includemelting,transition to agas,change of strength, change ofdurability,changes tocrystal form,textural change,shape,size,color,volumeanddensity.

An example of a physical change is the process oftemperingsteel to form a knife blade. A steel blank is repeatedly heated and hammered which changes thehardnessof the steel, its flexibility and its ability to maintain a sharp edge.

Many physical changes also involve the rearrangement of atoms most noticeably in the formation of crystals. Many chemical changes areirreversible,and many physical changes arereversible,but reversibility is not a certain criterion for classification. Although chemical changes may be recognized by an indication such as odor, color change, or production of a gas, every one of these indicators can result from physical change.

Examples

Heating and cooling

Many elements and some compounds change from solids to liquids and from liquids to gases when heated and the reverse when cooled. Some substances such asiodineandcarbon dioxidego directly from solid to gas in a process calledsublimation.

Magnetism

Ferro-magneticmaterials can become magnetic. The process is reversible and does not affect the chemical composition.

Crystalisation

Many elements and compounds form crystals. Some such ascarboncan form several different forms includingdiamond,graphite,grapheneandfullerenesincludingbuckminsterfullerene.

Crystals in metals have a major effect of the physical properties of the metal including strength and ductility. Crystal type, shape and size can be altered by physicalhammering,rolling and by heat

Mixtures

Mixtures of substances that are not soluble are usually readily separated by physical sieving or settlement. However mixtures can have different properties from the individual components. One familiar example is the mixture of fine sand with water used to makesandcastles.Neither the sand on its own nor the water on its own will make a sand-castle but by using physical properties ofsurface tension,the mixture behaves in a different way.

Solutions

Most solutions of salts and some compounds such as sugars can be separated by evaporation. Others such as mixtures or volatile liquids such as low molecular weight alcohols, can be separated byfractional distillation.

Alloys

The mi xing of different metal elements is known asalloying.Brassis an alloy ofcopperandzinc.Separating individual metals from an alloy can be difficult and may require chemical processing – making an alloy is an example of a physical change that cannot readily be undone by physical means. Alloys wheremercuryis one of the metals can be separated physically by melting the alloy and boiling the mercury off as a vapour.

See also

• Chemical change
• Process (science)
• Physical property