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Physical change
Physical change
Physical change
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Physical changes are changes affecting the form of a chemical substance, but not its chemical composition. Physical changes are used to separate mixtures into their component compounds, but can not usually be used to separate compounds into chemical elements or simpler compounds.[1]

Physical changes occur when objects or substances undergo a change that does not change their chemical composition. This contrasts with the concept of chemical change in 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, salt dissolved in water can be recovered by allowing the water to evaporate.

A physical change involves a change in physical properties. Examples of physical properties include melting, transition to a gas, change of strength, change of durability, changes to crystal form, textural change, shape, size, color, volume and density.

An example of a physical change is the process of tempering steel to form a knife blade. A steel blank is repeatedly heated and hammered which changes the hardness of 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 are irreversible, and many physical changes are reversible, 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

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Heating and cooling

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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 as iodine and carbon dioxide go directly from solid to gas in a process called sublimation.

Magnetism

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

Crystalisation

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Many elements and compounds form crystals. Some such as carbon can form several different forms including diamond, graphite, graphene and fullerenes including buckminsterfullerene.

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 physical hammering, rolling and by heat

Mixtures

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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 make sandcastles. Neither the sand on its own nor the water on its own will make a sand-castle but by using physical properties of surface tension, the mixture behaves in a different way.

Solutions

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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 by fractional distillation.

Alloys

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The mixing of different metal elements is known as alloying. Brass is an alloy of copper and zinc. 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 where mercury is one of the metals can be separated physically by melting the alloy and boiling the mercury off as a vapour.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Physical change

View on Grokipedia
from Grokipedia
A physical change is a transformation in that alters its physical properties, such as shape, size, density, or , without affecting its or identity. These changes typically involve rearrangements of particles or transfers that do not break or form chemical bonds, allowing the original substance to be recovered unchanged. Common examples include the of into , the of into vapor, and the crushing or cutting of a solid object like wood. Unlike chemical changes, which produce new substances with different molecular structures through reactions, physical changes are generally reversible and do not involve alterations to the atomic or molecular makeup of the material. For instance, boiling represents a physical change because the H₂O molecules remain intact, merely transitioning from to gas phase, whereas burning wood initiates a chemical change by forming new compounds like and ash. Physical changes often occur in response to , , or mechanical force, and they play a fundamental role in natural processes, such as the , and industrial applications like material processing. Key characteristics of physical changes include the , no breaking or forming of chemical bonds (though energy may be absorbed or released due to intermolecular forces during phase transitions), and the retention of the substance's chemical identity, including traits like and conductivity. Examples extend to everyday phenomena, such as dissolving in —where the sugar can be recovered by —or the expansion of metals under , both of which highlight how physical changes facilitate phase transitions without altering the fundamental nature of the involved. Understanding physical changes is essential in fields like and , as they underpin phenomena from patterns to techniques.

Definition and Fundamentals

Definition

A physical change is a transformation in the form or physical properties of a substance that does not alter its chemical composition or identity. Such changes affect attributes like state, shape, size, or texture while preserving the fundamental makeup of the material. At its core, matter consists of atoms and molecules, the basic units that define a substance's properties and behavior. In a physical change, the atomic and molecular structure remains intact, ensuring no new substances are formed and the original identity of the matter is maintained. This preservation distinguishes physical alterations as rearrangements or reconfigurations at the macroscopic level without breaking or forming chemical bonds. Physical changes are often reversible, allowing the substance to return to its original form through appropriate conditions.

Key Characteristics

Physical changes are characterized by their reversibility, meaning that in most cases, the original state of the substance can be restored by reversing the conditions that induced the change, such as refreezing melted to return it to a solid form. This reversibility stems from the fact that physical changes involve alterations in the form or arrangement of without affecting its fundamental identity. A defining feature of physical changes is the and energy, where the total mass of the matter remains constant before and after the change, as no atoms are created or destroyed. This principle, rooted in Lavoisier's law of formulated in 1789, applies equally to physical processes, ensuring that the quantity of matter is preserved even as its physical state or structure varies. Energy similarly holds, with any absorbed or released energy facilitating the change without altering the material's composition. Unlike chemical changes, physical changes do not involve breaking or forming chemical bonds, thereby lacking an energy barrier that would modify the molecular composition of the substance. This absence of bond alterations means that the chemical identity of the persists, with changes limited to intermolecular interactions or spatial arrangements. These changes are observable and measurable through shifts in physical properties such as , , or texture, all while the underlying remains unchanged. For instance, the of a substance may increase during expansion due to heating, allowing direct quantification of the transformation without any alteration to its elemental makeup. Such metrics provide of the change's occurrence and extent, emphasizing the non-compositional nature of physical transformations.

Distinction from Chemical Change

Core Differences

A chemical change is defined as a process that alters the of a substance, resulting in the formation of new substances through the breaking and forming of chemical bonds. In contrast, a physical change does not modify the chemical identity of the substance, preserving its molecular structure while only altering its physical state or form. At the process level, physical changes primarily involve alterations to intermolecular forces, which are the attractions between molecules, such as those governing phase transitions or shape modifications without disrupting the internal bonding within molecules. Chemical changes, however, entail intramolecular reactions that affect the bonds within molecules, leading to a reconfiguration of atoms into distinct chemical entities. This distinction underscores that physical changes operate on weaker, reversible interactions between molecules, whereas chemical changes require overcoming stronger covalent or ionic bonds. Detection of physical changes typically relies on observing reversible shifts in properties like , , or appearance, which can often be undone by adjusting conditions such as or . For chemical changes, identification involves confirming the formation of irreversible new compounds, frequently through spectroscopic techniques that reveal alterations in molecular signatures, such as shifts in absorption or emission spectra. Regarding energy implications, physical changes commonly involve the absorption or release of associated with phase transitions, representing the energy needed to overcome intermolecular forces without initiating bond rearrangements. Chemical changes, by comparison, necessitate to surmount the energy barrier for bond breaking and forming, enabling the reaction pathway to proceed and often resulting in a net energy change reflected in the reaction .

Illustrative Comparisons

One illustrative comparison between physical and chemical changes is the melting of versus the burning of . When melts, it undergoes a physical change by transitioning from a solid to a state while retaining its as H₂O, and the process is reversible by freezing the back into . In contrast, burning represents a chemical change where the and other compounds in the react with oxygen to produce new substances such as , , and , resulting in an irreversible transformation. Another paired example is dissolving sugar in water compared to the reaction of sodium with water. Dissolving sugar creates a physical change, forming a uniform solution where sugar molecules disperse throughout the without altering their chemical identity, and the sugar can be recovered by evaporating the . Conversely, sodium reacting with is a chemical change, as the sodium metal vigorously displaces ions to produce and gas, forming entirely new compounds that cannot be reverted to the original sodium. A common misconception arises with processes like cooking, which may appear as mere physical alterations in texture or appearance but often involve chemical changes, such as the denaturation and cross-linking of proteins in an when heated, producing a substance with different chemical properties. To distinguish between physical and chemical changes in a setting, simple tests can be applied: for physical changes like mixtures, or can separate components without altering their identity, whereas chemical changes are indicated by reactivity tests, such as observing gas evolution or precipitate formation that signals new substance creation. In physical changes, the law of holds strictly, as the total mass remains unchanged before and after the process.

Types of Physical Changes

Phase Transitions

Phase transitions represent fundamental physical changes in which matter shifts between states—solid, liquid, or gas—without altering its . These transformations occur as a result of variations in and , which influence the strength and balance of intermolecular forces holding particles together. For instance, increasing provides to overcome these forces, allowing particles to move more freely and transition to a less ordered state. The primary types of phase transitions include , where a solid converts to a ; , the reverse process from to ; , or , transforming a to a gas; , shifting gas back to ; sublimation, direct conversion from to gas; and deposition, the opposite from gas to . Each type is reversible under appropriate conditions, maintaining the substance's molecular identity throughout. These processes are endothermic when absorbing to increase disorder (, , sublimation) and exothermic when releasing to form more ordered structures (, , deposition). A prominent example is the , where () occurs as liquid water absorbs solar energy and turns into , rising into the atmosphere without changing its H₂O composition. Subsequently, forms liquid water droplets in clouds as the vapor cools, again preserving the molecular structure. This cycle demonstrates how phase transitions drive natural phenomena while keeping chemical identity intact. Energy involvement in phase transitions is quantified through latent heat, the energy absorbed or released per unit during the change without a temperature shift. The QQ required is given by the equation Q=mLQ = m L where mm is the of the substance and LL is the specific latent heat (of fusion for solid-liquid transitions or for liquid-gas transitions). For , the latent heat of fusion is approximately 334 J/g, and that of is about 2260 J/g at standard conditions, highlighting the greater energy needed for gas formation due to stronger intermolecular disruption.

Mechanical and Structural Changes

Mechanical and structural changes encompass modifications to a material's form, dimensions, or internal organization while preserving its chemical identity and phase. These transformations occur through external forces or environmental conditions that reposition particles without inducing molecular-level reactions. Unlike phase transitions, they do not involve alterations in molecular motion driven by or ; instead, they focus on macroscopic or microscopic rearrangements that affect observable traits such as texture or geometry./03:_Matter_and_Energy/3.06:Changes_in_Matter-_Physical_and_Chemical_Changes) Key types of these changes include cutting, crushing, , , and . Cutting divides a solid into smaller segments using , as seen when is sliced into strips, resulting in pieces that retain the original composition. Crushing applies to reduce particle size, exemplified by pulverizing rock salt into finer grains, where the formula remains unchanged. deforms materials like metal rods under flexural load, altering without compositional shift, while elongates elastic substances such as rubber through tensile pull, increasing but not molecular bonds. forms highly ordered lattices from supersaturated solutions or melts, such as precipitating into cubic crystals from evaporated , organizing ions into a periodic without new substances./Physical_Properties_of_Matter/Solutions_and_Mixtures/Case_Studies/RECRYSTALLIZATION) The underlying processes rely on mechanical force application for shape alterations, which disrupts intermolecular arrangements but spares covalent or ionic bonds, requiring only for repositioning rather than for reactions. In , controlled cooling or solvent evaporation promotes and growth of facets, driven by thermodynamic favorability for lower-energy ordered states. These mechanisms ensure reversibility in many cases, such as reshaping bent wire or redissolving crystals, underscoring the absence of involvement. Illustrative examples highlight retention of identity: breaking a glass pane shatters it into shards that are chemically identical silica-based fragments, useful for demonstrating increased surface area without loss of material essence. Alloy formation via non-reactive mixing, like blending molten copper (88%) and tin (12%) to create bronze, yields a homogeneous solid solution that enhances mechanical strength through atomic substitution rather than compound formation. Such changes influence physical properties, including expanded surface area from cutting or crushing—which can accelerate dissolution rates in subsequent processes—and altered density from stretching, yet the substance remains recoverable by physical means, with total mass conserved as per fundamental principles./06%3A_Structures_and_Energetics_of_Metallic_and_Ionic_solids/6.07%3A_Alloys_and_Intermetallic_Compounds/6.7A%3A_Substitutional_Alloys)

Dissolution and Mixtures

Dissolution refers to the process by which a solute disperses uniformly throughout a to form a homogeneous , known as a solution, without undergoing a . This physical change occurs primarily through intermolecular attractions between solute and solvent particles, where the solute-solute and solvent-solvent interactions are overcome by stronger or comparably strong solute-solvent forces, leading to the solute particles becoming surrounded by solvent molecules. For instance, when (NaCl) is added to , the ionic bonds in the salt lattice break due to interactions with water's polar molecules, resulting in Na⁺ and Cl⁻ ions dispersing evenly, but the ions retain their identity without forming new substances. Mixtures formed through dissolution or simple combination can be classified as homogeneous or heterogeneous based on the uniformity of their composition. Homogeneous mixtures, or solutions, appear uniform throughout and include examples like saltwater, where the solute is fully dissolved and not visible as separate particles. In contrast, heterogeneous mixtures display distinct phases or components that are not uniformly distributed, such as sand mixed with water, where the sand settles and can be observed separately. These mixtures do not involve chemical bonding between components, allowing the original substances to be recovered unchanged through physical means. Solubility, the maximum amount of solute that can dissolve in a given under specific conditions, is a key factor in dissolution and is significantly influenced by and . For most solid solutes, solubility increases with rising as more facilitates the breaking of solute-solute attractions, whereas for gases, solubility typically decreases with higher temperatures but increases with greater according to . In the case of NaCl(aq), the dissolved state represents a physical dispersion of ions, with no covalent bonds forming or new compounds created, emphasizing that dissolution is reversible and non-reactive. Heterogeneous and homogeneous mixtures can be separated using physical techniques that exploit differences in properties like , volatility, or , without altering the chemical identity of the components. effectively separates insoluble solids from liquids in heterogeneous mixtures, such as removing from by passing the through a filter. For homogeneous mixtures, separates components based on boiling point differences, vaporizing and condensing the more volatile substance, while divides mixtures by differential adsorption onto a stationary phase as they move through a mobile phase, useful for complex solutions. These methods underscore the reversible nature of physical changes in mixtures.

Applications and Examples

Everyday Phenomena

Physical changes are integral to numerous routine activities encountered in daily life, where alters its form or state without changing its fundamental composition. For instance, when cubes are added to a beverage, they gradually melt into due to the surrounding warmth, transitioning from a solid to a phase while remaining H₂O throughout the process. This is observable as the drink cools initially before reaching equilibrium, illustrating a reversible physical change that enhances refreshment without altering the drink's chemical makeup. Another common mechanical physical change occurs when tearing a sheet of , which simply alters its size and shape by breaking the physical bonds holding the fibers together, without producing any new substances. Similarly, stirring cream into creates a homogeneous , where the cream disperses evenly through the , changing the appearance and texture to a , blended consistency, yet the components— and cream—retain their original identities and can theoretically be separated. Sensory experiences often highlight these transformations, such as the visible lightening of color when diluting with , where the particles spread out in a larger volume, reducing intensity without any molecular rearrangement. This dilution effect is purely physical, as the paint's composition stays the same, allowing for easy reversal by concentrating it again. Recognizing such changes fosters an understanding of non-compositional alterations in , which supports safer home practices by identifying reversible processes like these over those involving potential hazards. Seasonal weather patterns provide further relatable examples, including the melting of accumulated into as temperatures rise, a phase change that replenishes without chemical alteration. Likewise, in roadside puddles evaporates under , shifting from to vapor and disappearing from view, yet the water molecules persist in the atmosphere, ready to condense elsewhere. These natural occurrences underscore the ubiquity of physical changes, connecting abstract concepts to tangible environmental shifts.

Industrial and Scientific Uses

In industrial applications, physical changes play a central role in processes like for oil refining, where crude oil is separated into fractions through phase transitions involving and based on differing points. This atmospheric distillation, the primary step in most refineries, heats crude oil to around 350–400°C, allowing lighter hydrocarbons to vaporize and rise in a tower, where they condense at various heights to yield products such as , , and diesel without altering the of the molecules. Similarly, mechanical changes occur in metal , a technique that deforms solid metals under compressive forces from hammers, presses, or rolls to achieve desired shapes, enhancing grain structure and strength through plastic deformation rather than chemical alteration. This process is widely used for producing durable components like automotive parts and fittings, minimizing material waste compared to subtractive methods. In scientific contexts, crystallization serves as a key physical change for purifying compounds, as seen in production where supersaturated solutions are cooled to form pure , separating the from impurities like based on differences. This reversible process allows for high-purity output in the , with harvested via and the mother liquor recycled, avoiding chemical reagents. In , physical mixtures enable drug formulations by blending active pharmaceutical ingredients with excipients such as binders or diluents, creating uniform tablets or capsules without chemical reactions, which ensures stability and controlled release while leveraging properties for . The efficiency of these physical changes stems from their reversibility, which reduces waste and costs in industrial operations by allowing material recovery and reuse, unlike irreversible chemical processes that generate byproducts requiring disposal. For instance, in refining and purification, recycled streams lower energy demands and environmental impact, contributing to sustainable manufacturing practices. In modern technologies, cooling via refrigeration cycles exploits phase transitions in the vapor-compression system, where refrigerants undergo evaporation to absorb heat and condensation to release it, enabling efficient temperature control in food preservation and HVAC without chemical decomposition. Alloy mixing further demonstrates this through physical blending of metals in solid-state processes like mechanical alloying, which refines microstructure to yield stronger materials—such as high-strength steels—for aerospace applications, preserving elemental integrity without reactions.

Specialized Cases

One specialized case of physical change involves the temporary of ferromagnetic materials, such as exposed to an external . In this process, the magnetic domains within the iron align with the applied field, resulting in a net that attracts the filings into patterns visualizing the field lines; this alignment is reversible upon removal of the field or through demagnetization techniques like heating or mechanical shock, without altering the material's . Changes in represent another category of specialized physical changes, exemplified by the fogging of glass surfaces due to . When humid air contacts a cold glass, undergoes a to droplets, increasing the optical and to create a translucent or opaque appearance; this effect is purely physical, as the water molecules retain their identity and can evaporate back to vapor upon warming. Similarly, the sublimation of (solid ) demonstrates a direct solid-to-gas transition under atmospheric conditions, producing a visible of condensed from the surrounding air without melting into a intermediate, altering only the phase while preserving molecular structure. Certain alloys exhibit physical through domain alignment without chemical alteration, such as in iron-nickel compositions where arises from the collective spin orientation of electrons in the lattice, enabling applications like temporary electromagnets. In gem formation, crystallization processes further illustrate specialized physical changes, where minerals like (ruby and ) or solidify from cooling magmas or hydrothermal solutions into ordered crystal lattices, enhancing optical and mechanical properties through structural organization alone. These specialized cases remain classified as physical changes provided there is no alteration in or between atoms, distinguishing them from chemical reactions that involve bond formation or breakage. For instance, while magnetic domain alignment in alloys may involve electron spin reorientation, it does not result in new substances, maintaining the original atomic makeup.

Historical and Conceptual Development

Early Observations

The earliest recognitions of physical changes emerged in ancient philosophical frameworks, where observable transformations in matter were interpreted through qualitative observations rather than systematic experimentation. In the 4th century BCE, Aristotle proposed a theory of four elements—earth, water, air, and fire—each characterized by pairs of qualities such as hot/cold and wet/dry, to explain natural phenomena including changes in state. He described evaporation as a process where water, being cold and moist, could be heated by the sun to transform into air-like vapor, as detailed in his Meteorology, where moisture from the earth is drawn upward and contributes to atmospheric cycles without altering the fundamental substance. These ideas framed physical changes as alterations in elemental qualities rather than distinct processes, influencing Western thought for centuries. During the medieval period, Islamic scholars refined these concepts within alchemical traditions, distinguishing observable mixtures and dissolutions from the elusive goal of transmutation. In the , (Ibn Sina) critiqued alchemical claims of metallic transformation while documenting practical changes, such as the dissolution of substances into their "first matter" through processes like involving heat and moisture. He noted that mixing perfect bodies like with imperfect ones does not elevate the latter to , emphasizing instead reversible dissolutions where substances could be reduced and reformed without essential change, as in the conjunction of body and spirit leading to a spiritual state that could be recongealed. These observations highlighted distinctions between mere mixing—preserving component identities—and purported transmutations, laying groundwork for empirical scrutiny of physical alterations. Pre-modern metallurgical practices provided hands-on evidence of physical changes through repeated observations of phase transitions in . In societies around 3000–1200 BCE, artisans noted the melting of and tin s in crucibles at high temperatures, followed by controlled cooling to achieve solidification into tools and ornaments, as evidenced by remnants and casting molds from sites across and the . These processes involved pouring molten metal, which solidified upon cooling—termed "freezing" in later terminology—demonstrating reversible state changes without loss of material identity, a key insight in early production. observations occurred in fluxing liquids used to purify ores, where and aided separation, underscoring practical utility of such changes in refining techniques. The transition to modern science in the marked a shift toward , explicitly separating physical changes from alchemical through controlled experiments. , in his 1661 work , rejected traditional alchemical principles like the tria prima (salt, , mercury) in favor of a corpuscular , arguing that changes like dissolution or were mechanical rearrangements of particles rather than qualitative transmutations. Influenced by the Royal Society's emphasis on observation and replication, Boyle's experiments—such as sublimating from solid to gas and back—demonstrated reversible physical alterations, paving the way for chemistry as a distinct empirical discipline. This empiricist approach formalized the recognition of physical changes as predictable, non-destructive processes, distinct from the speculative goals of .

Modern Understanding

In the late , formulated the law of , demonstrating that the total mass remains unchanged during physical processes such as , , or dissolution, as no matter is created or destroyed. This principle, verified through precise weighings in closed systems, laid a foundational quantitative basis for distinguishing physical changes from speculative alchemical transformations. Building on this, John Dalton's atomic theory in the early provided a microscopic explanation, asserting that atoms are indivisible units that retain their identity in physical changes, involving only rearrangements of position or state without bond formation or breakage. Dalton's framework solidified the core differences between physical and chemical changes by emphasizing atomic integrity. The brought sophisticated thermodynamic models to analyze phase transitions, central to physical changes like solidification or . introduced the in , expressed as F=C−P+2F = C - P + 2, where FF represents the (variables like and that can be independently varied), CC the number of independent components, and PP the number of coexisting phases in equilibrium. This invariant relation predicts the constraints on multi-phase systems, such as the invariant of where solid, liquid, and vapor coexist at a fixed and . Such models enabled precise mapping of phase diagrams, informing and theoretical predictions of stability under varying conditions. Advancements in the late 20th and early 21st centuries have leveraged to achieve controlled mechanical changes, such as reversible deformations in without chemical alteration. For example, carbon nanotubes and composites exhibit tunable elasticity and strength at the nanoscale, allowing engineered physical responses like shape recovery in response to external stimuli. Concurrently, simulations have elucidated the kinetics of dissolution at the atomic scale, modeling how solute molecules or ions separate and hydrate in solvents through intermolecular forces alone. These computations, often using force fields to track trajectories over picoseconds, reveal mechanisms like the stepwise disassembly of salt nanocrystals in , confirming dissolution as a purely physical dispersion. Physical changes intersect with in contemporary climate modeling, where phase transitions like glacial ice melt drive global impacts. Thermodynamic simulations of ablation under warming oceans quantify mass loss rates, projecting contributions to from processes such as surface melting and basal sliding. This integration highlights how physical changes amplify feedback loops, such as reduced accelerating further melt in polar regions.

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