Chemical changes occur when a substance combines with another to form a new substance, called synthesis or, alternatively, decomposes 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 reaction.
Chemists categorize chemical changes into three main classes: inorganic chemical changes, organic chemical changes and biochemical changes.
An inorganic change describes the reactions of elements and compounds that, in general, do not involve carbon. The changes typically take place in laboratories or industries. Typical types of change include neutralization, and oxidization including combustion.
Organic changes are 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 of pharmaceutical, paints, detergents, cosmetics and fuels. Typical examples of organic chemical changes include cracking heavy hydrocarbons at an oil refinery to create more gasoline from crude oil.
Biochemical change 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. This chemical change 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 and bacteria and other micro-organisms that is involved. Typical types of change include all the process involved in photosynthesis. This is a process where carbon dioxide and water are changed into sugars and oxygen by plants.
There are several ways by which the evidence can be seen that a chemical change has taken place. Change of odor and color reveals change. Change in temperature or the energy level of materials reveals that a chemical change has occurred. When there is a change of composition, where light or heat is produced, or a formation of gases is produced, all are evidence that a chemical change has taken place. Some examples of everyday chemical changes include rusting iron, burning wood, cooking an egg, baking a cake, explosion of fireworks, rotting bananas, or grilling hamburgers.
Chemical changes are reactions involve combining different substances. The chemical reaction produces a new substance with new and different physical and chemical properties. Matter is never destroyed or created in chemical change. The particles of one substance are rearranged to form a new substance. The same number of particles that exist before the reaction exist after the reaction.
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