So what is a chemical reaction? What is a chemical change, and what is a physical change? A chemical reaction occurs when compounds or substances undergo a chemical change to form different compounds or substances. Chemical equations express chemical reactions in terms of symbols and chemical formulas of the compounds involved in reactions. Let’s looks at some chemical change examples, and chemical reaction examples. Show
Chemical vs. Physical ChangesIn a physical change, a compound’s shape may change, but its chemical identity will not. For example, freezing or boiling water is a physical change. Once melted or boiled, the water may be in a different form (solid ice or gaseous water vapor), but it is still water, H2O, and it still has the same chemical composition. A chemical change happens when a substance’s chemical identity changes. An example of this is rusting. When an iron nail comes into contact with water and is then exposed to the air, it rusts, forming a brown-red substance. This process changes the chemical composition of the original substance. While some physical changes are easily reversible, such as re-freezing melted ice, reversing a chemical change requires another chemical reaction. Indications of a Chemical ReactionBased on its definition, a chemical reaction occurs when a substance undergoes a chemical change. So how can we tell if a chemical change has occurred? 1. Heat or Light EmissionsIf a reaction emits heat or light, that is an indication that energy has been released from the reaction. One example of this is a fire or spark, which is evidence of combustion. Because many physical changes also involve heat or light, this is not concrete, definitive evidence of a reaction. But it is a strong argument that one has occurred. 2. Change in ColorA change in color is a possible indication of a chemical reaction. 3. Gas FormationWhen substances react and bubbles or a vapor appears, this is an indication that a chemical reaction is occurring. An example of this is when baking soda and vinegar react violently to produce carbon dioxide gas in the famous volcano experiment. 4. Precipitate FormationA precipitate is a solid product that forms as a result of a chemical reaction between liquid reactants. An example of a precipitate is yellow cadmium sulfide, which is produced when water solutions of ammonium sulfide and cadmium nitrate are combined.  The most definitive way to confirm a chemical reaction is to analyze the chemical compositions of the reactants and products. Chemical Change ExamplesHere’s ten examples of chemical changes: Chemical Reaction Examples
Physical Change Examples
Further Reading – Physical Changes & Chemical ChangesThis article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages)
This article does not cite any sources. (September 2014) This article needs additional citations for verification. (January 2010) Chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. These processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. Some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. Understanding chemical changes is a major part of the science of chemistry.
When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated. An example of a chemical change is the reaction between sodium and water to produce sodium hydroxide and hydrogen. So much energy is released that the hydrogen gas released spontaneously burns in the air. This is an example of a chemical change because the end products are chemically different from the substances before the chemical reaction.
Chemists categorize chemical changes into three main classes: inorganic chemical changes, organic chemical changes and biochemical changes.
Inorganic chemistry describes the reactions of elements and compounds that, in general, do not involve carbon. The changes typically take place in laboratories, or on a larger scale in heavy industries.
Typical types of change include neutralization (mixing an acid with a base, resulting in water and salt), oxidization including combustion, redox reactions etc.
Organic chemistry is concerned with the chemistry of carbon and the elements and compound with which it reacts. These compounds include mineral oil and all of its products and much of the output of industries manufacturing pharmaceuticals, paints, detergents, cosmetics, fuels etc. Typical examples of organic chemical changes include cracking heavy hydrocarbons at an oil refinery to create more gasoline from crude oil, as gasoline is in higher demand than the heavier hydrocarbons, such as residual fuel oils. Other reactions include, methylation, condensation reactions, polymerisation, halogenation etc.
Biochemical changeBiochemistry deals with the chemistry of the growth and activity of living organisms. It is a chemistry where most reactions are controlled by complex proteins called enzymes and are moderated and limited by hormones. The chemistry is always highly complex and is still not fully understood. Decomposition of organic material is also within the scope of biochemistry although in this case it is the growth and activity of fungi, bacteria and other micro-organisms that is involved. Typical types of change include the processes involved in photosynthesis, a process in which carbon dioxide and water are changed into sugars and oxygen by plants, digestion in which energy rich materials are used by organisms to grow and move, the Krebs cycle which liberates energy from stored reserves, protein synthesis which enables organisms to grow using processes controlled by RNA, etc. The following can indicate that a chemical change has taken place, although this evidence is not conclusive:
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What experimental result would be evidence that a chemical reaction took place?
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