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. ExamplesHeating and coolingMany 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. MagnetismFerro-magnetic materials can become magnetic. The process is reversible and does not affect the chemical composition. CrystalisationMany 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 MixturesMixtures 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. SolutionsMost 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. AlloysThe 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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The difference between a physical reaction and a chemical reaction is composition. In a chemical reaction, there is a change in the composition of the substances in question; in a physical change there is a difference in the appearance, smell, or simple display of a sample of matter without a change in composition. Although we call them physical "reactions," no reaction is actually occurring. In order for a reaction to take place, there must be a change in the elemental composition of the substance in question. Thus, we shall simply refer to physical "reactions" as physical changes from now on. Physical changes are limited to changes that result in a difference in display without changing the composition. Some common changes (but not limited to) are: Physical properties include many other aspects of a substance. The following are (but not limited to) physical properties.
Any change in these physical properties is referred to as a physical change. For further information, please refer to Properties of Matter. Chemical changes, on the other hand, are quite different. A chemical change occurs when the substance's composition is changed. When bonds are broken and new ones are formed a chemical change occurs. The following are indicators of chemical changes:
Note: When two or more reactants are mixed and a change in temperature, color, etc. is noticed, a chemical reaction is probably occurring. These are not definite indicators; a chemical reaction may not be occurring. A change in color is not always a chemical change. If one were to change the color of a substance in a non-chemical reaction scenario, such as painting a car, the change is physical and not chemical. This is because the composition of the car has not changed. Proceed with caution.
The texture of a substance can differ with a physical change. For example, if a piece of wood was sanded, waxed, and polished, it would have a very different texture than it initially had as a rough piece of wood.  (left) Rough plank boardwalk, Quebec City, Canada (right) Finished mountain ash floor. (CC BY-SA 4.0; WikiPedant and CC BY-SA 2.5; MarkAnthonyBoyle, respectively). As you can see, the texture of the finished wood is much smoother than the initial grainy wood.
The changing of color of a substance is not necessarily an indicator of a chemical change. For example, changing the color of a metal does not change its physical properties. However, in a chemical reaction, a color change is usually an indicator that a reaction is occurring. Painting the metal car does not changing the composition of the metallic substance. Robotic arm applying paint on car parts. Image use with permission (CC BY-SA 4.0l RoboGuru).
Although we cannot see temperature change, unless if a change of state is occurring, it is a physical change. Hot metalwork. (CC BY-SA-NC 2.0; flagstaffotos.com.au) One cannot see the pan physically changing shape, color, texture, or any of the other physical properties. However, if one were to touch the pan, it would be incredibly hot and could cause a burn. Sitting idle in a cupboard, this pan would be cold. One cannot assess this change only through visual exposure; the use of a thermometer or other instrument is necessary.
The shape of an object can be changed and the object will still remain true to its chemical composition. For example, if one were to fold money, as shown by the figure below, the money is still chemically the same. Origami Money
The change of state is likewise a physical change. In this scenario, one can observe a number of physical properties changing, such as viscosity and shape. As ice turns into water, it does not retain a solid shape and now becomes a viscous fluid. The physical "reaction" for the change of ice into liquid water is: \[H_2O_{(s)} \rightarrow H_2O_{(l)}\] The following are the changes of state:
The luster of an element is defined as the way it reacts to light. Luster is a quality of a metal. Almost all of the metals, transition metals, and metalloids are lustrous. The non-metals and gases are not lustrous. For example, oxygen and bromine are not lustrous. Shown below is are lustrous paper clips. Lustrous Paperclips
Malleability is also a quality of metals. Metals are said to be malleable. This means that the metals can deform under an amount of stress. For example, if you can hit a metal with a mallet and it deforms, it is malleable. Also, a paperclip can be shaped with bare hands. Bent Paperclip The image shows the malleability of a certain metal as stress is applied to it.
In materials science, this property is called ductility. For example, raw copper can be obtained and it can be purified and wrapped into a cord. Once again, this property is characteristic of mainly metals, nonmetals do not possess this quality. Copper Wire
The density of an object is its mass divided by its volume (d=m/v). A substance will have a higher density if it has more mass in a fixed amount of volume. For example, take a ball of metal, roughly the size of a baseball, compressed from raw metal. Compare this to a baseball made of paper. The baseball made of metal has a much greater weight to it in the same amount of volume. Therefore the baseball made out of metal has a much higher density. The density of an object will also determine whether it will sink or float in a particular chemical. Water for example has a density of 1g/cm3. Any substance with a density lower than that will float, while any substance with a density above that will sink. Oil Sinking in a Glass of Water
Viscosity is defined to be the resistance to deformation of a particular chemical substance when a force is applied to it. In the example below, one can see two cubes falling into two different test tubes. The upper substance shows a violent reaction to the dropping of the cube. The lower substance simply engulfs it slowly without much reaction. The upper substance has a lower viscosity relative to the lower substance, which has very high viscosity. One may even think of viscosity in terms of thickness. The substance with more thickness has higher viscosity than a substance that is deemed "thin." Water has a lower viscosity than honey or magma, which have relatively high viscosities. Viscosity of Fluids
The follow are all indicators of chemical reactions. For further information on chemical reactions, please refer to Chemical Reactions.
A change in temperature is characteristic of a chemical change. During an experiment, one could dip a thermometer into a beaker or Erlenmeyer Flask to verify a temperature change. If temperature increases, as it does in most reactions, a chemical change is likely to be occurring. This is different from the physical temperature change. During a physical temperature change, one substance, such as water is being heated. However, in this case, one compound is mixed in with another, and these reactants produce a product. When the reactants are mixed, the temperature change caused by the reaction is an indicator of a chemical change. As an example of a exothermic reaction, if \(Fe_2O_3\) is mixed with Al and ignighted (often with burning Mg), then the thermite reaciton is initiated \[Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + \text{Heat}\] This reaction generates heat as a product and is (very) exothermic. However, physical changes can be exothermic or endothermic. The melting of an ice cube, which is endothermic, is a change in a physical property and not composition. Thus, it is a physical change.
A change in color is also another characteristic of a chemical reaction taking place. For example, if one were to observe the rusting of metal over time, one would realized that the metal has changed color and turned orange. This change in color is evidence of a chemical reaction. However, one must be careful; sometimes a change in color is simply the mixing of two colors, but no real change in the composition of the substances in question. Metal Rusting The reaction above is that of the rusting of iron. \[4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3\]
When two or more compounds or elements are mixed and a scent or odor is present, a chemical reaction has taken place. For example, when an egg begins to smell, (a rotten egg) a chemical reaction has taken place. This is the result of a chemical decomposition. Spoiled Egg
The formation of a precipitate may be one of the most common signs of a chemical reaction taking place. A precipitate is defined to be a solid that forms inside of a solution or another solid. Precipitates should not be confused with suspensions, which are solutions that are homogeneous fluids with particles floating about in them. For instance, when a soluble carbonate reacts with Barium, a Barium Carbonate precipitate can be observed. Test Tube Reaction: \[Ba^{2+}_{(aq)} + CO^{2-}_{3\;(aq)} \rightarrow BaCO){3\;(s)}\] For further information, please refer to Classification of Matter.
The formation of bubbles, or rather a gas, is another indicator of a chemical reaction taking place. When bubbles form, a temperature change could also be taking place. Temperature change and formation of bubbles often occur together. For example, in the following image, one can see a gas spewing. This is the formation of a gas. Gas Formation However, most reactions are much more subtle. For instance, if the following reaction occurs, one may notice Carbon Dioxide bubbles forming. If there is enough Hydrochloric Acid, bubbles are visible. If there isn't, one can't readily notice the change: \[Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\] References
All images are courtesy of http://www.sxc.hu, which provides royalty free images that are free to be copied without restrictions. The viscosity image is also free to be duplicated as per permission of author on Wikipedia.com.
1. Which of the following is a chemical reaction?
2. Which of the following is a physical reaction?
3. Which of the following is a chemical reaction?
4. Which of the following is a physical reaction?
5. Write C for Chemical Reaction or P for Physical Reaction.
1. D 2. A 3. B 4. C 5. a) C b) P c) P d) C e) Neither. This is one of the gray areas of chemical change and physical change. Although the salt has dissociated into Sodium and Chloride ions, it is still salt in water. Salt, initially is actually just a conglomerate of sodium and chloride ions and by dissociating them, just the arrangement of the ions has changed. Please click here for more information. |
Which process is best classified as a physical change?
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