Chemical change 1326645 224338254 2008-07-08T12:15:56Z Itub 426390 rv [[Image:Streichholz.jpg|thumb|[[Combustion]], an example of a chemical change.]] A '''chemical change''' is a process in which [[reactant]]s are changed into one or more different [[product (chemistry)|products]]. A chemical change occurs whenever [[chemical compound|compounds]] are formed or decomposed. During this reaction, there is a rearrangement of [[atom]]s that makes or breaks [[chemical bond]]s. This change is usually not reversible, unlike [[physical changes]], which typically are reversible. In short, chemical change is a change of one substance into another substance (i.e. a new substance is formed). By contrast, no new substance is formed in a physical change (for example, the melting of ice is a physical change because when ice changes into water, no new substance is formed.) In a chemical reaction, [[chemical bond|bond]]s are broken and new bonds are formed between different atoms. This breaking and forming of bonds takes place when particles of the original materials collide with one another. Some [[exothermic]] reactions may be hot enough to cause certain chemicals to also undergo a change in state; for example in the case of aqueous solutions, bubbles may not necessarily be newly produced gas but instead water vapor. Chemical changes are happening very often. When wood burns it is a chemical change. There are several different types of chemical change. These include [[Chemical synthesis|synthesis]], [[Decomposition reaction|decomposition]], [[Single displacement reaction|single displacement]], [[Double displacement reaction|double displacement]], [[neutralization]], [[Precipitation (chemistry)|precipitation]] and [[redox]]. An elementary example of a chemical change is the [[combustion]] of [[methane]] to produce [[carbon dioxide]] and [[water]]: : {{chem|CH|4| + 2O|2| → CO|2| + 2H|2|O}} Other examples of chemical changes are: * Burning a [[wooden log|log]] of [[wood]]. * Mixing an [[acid]] with a [[base (chemistry)|base]], producing [[water]] and a [[salt (chemistry)|salt]]. * [[Photosynthesis]] - a process in which carbon dioxide and water are changed into sugars by plants. * [[Cracking (chemistry)|Cracking]] heavy hydrocarbons to create lighter hydrocarbons (part of the processs of refining oil). The following can indicate that a chemical change took place, although this evidence is not conclusive: * Change of colour (for example, silver to reddish-brown when [[iron]] rusts). * Change in temperature or [[energy]], such as the production ([[exothermic]]) or loss ([[endothermic]]) of [[heat]]. * Change of form (for example, burning paper). * [[Light]], heat, or [[sound]] is given off. * Formation of gases, often appearing as bubbles. * Formation of [[precipitate]] (insoluble particles). * The decomposition of [[organic matter]] (for example, rotting food). ==See also== * [[Physical change]] * [[Chemical reaction]] * [[Chemical process]] * [[Reactant]] == References == * {{Citation|last = Zumdahl|first = Steve S.|title = Chemical Principles|publisher = [[Houghton Mifflin]]|year = 2005|edition = 5|isbn = 0618372067}} == External links == * [http://wordnet.princeton.edu/perl/webwn?s=chemical+change Definition of 'chemical change', wordnet.princeton.edu] {{BranchesofChemistry}} [[Category:Chemistry]] [[Category:Chemical reactions]] [[simple:Chemical change]]