If A Metal Changes Color Over Time From Exposure To Air, What Has Likely Occurred?
10.4: Physical and Chemical Changes
- Page ID
- 58840
Learning Outcomes
- Distinguish between physical and chemical changes.
- Requite examples of physical and chemic changes.
Physical Changes
Equally an ice cube melts, its shape changes equally information technology acquires the power to catamenia. However, its composition does not change. Melting is an example of a physical change. A physical modify is a change to a sample of matter in which some properties of the textile change, but the identity of the affair does not. Physical changes can further exist classified every bit reversible or irreversible. The melted water ice cube may be refrozen, so melting is a reversible physical alter. Physical changes that involve a alter of state are all reversible. Other changes of state include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolving is as well a reversible physical change. When table salt is dissolved into h2o, the table salt is said to take entered the aqueous state. The salt may exist regained past boiling off the water, leaving the table salt behind.
When a slice of wood is footing into sawdust, that alter is irreversible since the sawdust could not be reconstituted into the same piece of wood that it was before. Cutting the grass or pulverizing a rock would be other irreversible concrete changes. Firewood too represents an irreversible physical change since the pieces cannot be put back together to form the tree.
Chemical Changes
When exposed to air, an object made of iron volition eventually begin to rust (run across effigy beneath).
As the rust forms on the surface of the iron, it flakes off to expose more atomic number 26, which will continue to rust. Rust is clearly a substance that is different from iron. Rusting is an instance of a chemical change.
A chemical property describes the ability of a substance to undergo a specific chemical modify. A chemical property of atomic number 26 is that information technology is capable of combining with oxygen to class iron oxide, the chemical proper name of rust. A more than general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions of chemic changes are burn, rot, explode, and ferment. Chemic properties are very useful as a way of identifying substances. However, unlike physical properties, chemical properties can only be observed equally the substance is in the process of being changed into a different substance.
A chemical modify is also chosen a chemic reaction. A chemical reaction is a process that occurs when 1 or more substances are inverse into ane or more new substances. Zinc \(\left( \ce{Zn} \correct)\) is a silvery-grayness chemical element that can be ground into a pulverisation. If zinc is mixed at room temperature with powdered sulfur \(\left( \ce{S} \right)\), a bright yellow chemical element, the event will only be a mixture of zinc and sulfur. No chemic reaction occurs. However, if energy is provided to the mixture in the form of heat, the zinc will chemically react with the sulfur to course the compound zinc sulfide \(\left( \ce{ZnS} \correct)\). Pictured beneath are the substances involved in this reaction.
The reaction betwixt zinc and sulfur tin exist depicted in something called a chemical equation. In words, nosotros could write the reaction as:
\[\text{zinc} + \text{sulfur} \rightarrow \text{zinc sulfide}\]
A more convenient way to limited a chemic reaction is to apply the symbols and formulas of the substances involved:
\[\ce{Zn} + \ce{Due south} \rightarrow \ce{ZnS}\]
The substance(s) to the left of the pointer in a chemic equation are called reactants. A reactant is a substance that is present at the start of a chemic reaction. The substance(s) to the right of the pointer are chosen products. A product is a substance that is nowadays at the end of a chemical reaction. In the equation in a higher place, zinc and sulfur are the reactants that chemically combine to course zinc sulfide every bit a production.
Recognizing Chemical Reactions
How can y'all tell if a chemical reaction is taking place? Certain visual clues bespeak that a chemic reaction is probable (but not necessarily) occurring, including the following examples:
- A modify of color occurs during the reaction.
- A gas is produced during the reaction.
- A solid product, chosen a precipitate, is produced in the reaction.
- A visible transfer of energy occurs in the course of light equally a upshot of the reaction.
When zinc reacts with hydrochloric acrid, the reaction bubbling vigorously equally hydrogen gas is produced (encounter effigy beneath). The production of a gas is as well an indication that a chemical reaction may exist occurring.
When a colorless solution of lead (Two) nitrate is added to a colorless solution of potassium iodide, a yellow solid called a precipitate is instantly produced (run into effigy below). A precipitate is a solid product that forms from a reaction and settles out of a liquid mixture. The formation of a precipitate may also signal the occurrence of a chemical reaction.
\[\ce{Pb(NO_3)_2} \left( aq \correct) + 2 \ce{KI} \left( aq \right) \rightarrow \ce{PbI_2} \left( due south \correct) + two \ce{KNO_3} \left( aq \right)\]
Contributors and Attributions
-
Allison Soult, Ph.D. (Department of Chemistry, Academy of Kentucky)
Source: https://chem.libretexts.org/Courses/University_of_Kentucky/UK%3A_CHE_103_-_Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_10%3A_Nuclear_and_Chemical_Reactions/10.4%3A_Physical_and_Chemical_Changes
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