When Paper Turns Yellow-brown Upon Exposure To Sunlight, What Type Of Change Is Likely Taking Place?
10.4: Physical and Chemical Changes
- Page ID
- 58840
Learning Outcomes
- Distinguish between concrete and chemical changes.
- Give examples of physical and chemic changes.
Physical Changes
Every bit an water ice cube melts, its shape changes as it acquires the ability to menstruum. However, its composition does not change. Melting is an example of a physical modify. A physical modify is a change to a sample of matter in which some backdrop of the material change, but the identity of the matter does non. Physical changes can further be classified as reversible or irreversible. The melted ice cube may be refrozen, then melting is a reversible concrete change. Physical changes that involve a change of state are all reversible. Other changes of land include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolving is besides a reversible physical change. When common salt is dissolved into h2o, the salt is said to have entered the aqueous state. The salt may exist regained by boiling off the h2o, leaving the common salt behind.
When a slice of wood is ground into sawdust, that change is irreversible since the sawdust could not be reconstituted into the aforementioned slice of wood that information technology was before. Cutting the grass or pulverizing a rock would be other irreversible physical changes. Firewood also 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 fe volition eventually begin to rust (see figure below).
Equally the rust forms on the surface of the iron, information technology flakes off to expose more iron, which will continue to rust. Rust is clearly a substance that is different from atomic number 26. Rusting is an instance of a chemic modify.
A chemical property describes the ability of a substance to undergo a specific chemical modify. A chemic holding of iron is that it is capable of combining with oxygen to course fe oxide, the chemical name of rust. A more general term for rusting and other like processes is corrosion. Other terms that are unremarkably used in descriptions of chemical changes are burn, rot, explode, and ferment. Chemic properties are very useful as a way of identifying substances. However, unlike physical properties, chemical backdrop tin only exist observed every bit the substance is in the procedure of being changed into a different substance.
A chemic change is too called a chemical reaction. A chemical reaction is a process that occurs when ane or more substances are changed into one or more than new substances. Zinc \(\left( \ce{Zn} \right)\) is a silver-gray chemical element that can be ground into a pulverisation. If zinc is mixed at room temperature with powdered sulfur \(\left( \ce{S} \correct)\), a brilliant yellow element, the event will simply exist a mixture of zinc and sulfur. No chemic reaction occurs. All the same, if energy is provided to the mixture in the form of heat, the zinc volition chemically react with the sulfur to form the compound zinc sulfide \(\left( \ce{ZnS} \correct)\). Pictured beneath are the substances involved in this reaction.
The reaction between zinc and sulfur tin be depicted in something called a chemical equation. In words, we could write the reaction every bit:
\[\text{zinc} + \text{sulfur} \rightarrow \text{zinc sulfide}\]
A more convenient manner to express a chemical reaction is to use the symbols and formulas of the substances involved:
\[\ce{Zn} + \ce{S} \rightarrow \ce{ZnS}\]
The substance(southward) to the left of the pointer in a chemic equation are chosen reactants. A reactant is a substance that is present at the start of a chemical reaction. The substance(due south) to the right of the arrow are called products. A product is a substance that is present at the cease of a chemic reaction. In the equation higher up, zinc and sulfur are the reactants that chemically combine to form zinc sulfide as a product.
Recognizing Chemical Reactions
How can you lot tell if a chemical reaction is taking place? Sure visual clues point that a chemical reaction is likely (simply not necessarily) occurring, including the following examples:
- A modify of color occurs during the reaction.
- A gas is produced during the reaction.
- A solid production, called a precipitate, is produced in the reaction.
- A visible transfer of free energy occurs in the class of lite every bit a result of the reaction.
When zinc reacts with hydrochloric acid, the reaction bubbles vigorously equally hydrogen gas is produced (see figure beneath). The production of a gas is also an indication that a chemical reaction may be occurring.
When a colorless solution of pb (II) nitrate is added to a colorless solution of potassium iodide, a yellowish solid called a precipitate is instantly produced (see 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.
\[\ce{Pb(NO_3)_2} \left( aq \correct) + 2 \ce{KI} \left( aq \right) \rightarrow \ce{PbI_2} \left( south \right) + 2 \ce{KNO_3} \left( aq \correct)\]
Contributors and Attributions
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Allison Soult, Ph.D. (Department of Chemical science, Academy of Kentucky)
Source: https://chem.libretexts.org/Courses/University_of_Kentucky/UK:_CHE_103_-_Chemistry_for_Allied_Health_%28Soult%29/Chapters/Chapter_10:_Nuclear_and_Chemical_Reactions/10.4:_Physical_and_Chemical_Changes
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