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Chemical change

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A chemical change is a process in which reactants are changed into one or more different products. A chemical change occurs whenever compounds are formed or decomposed. During this reaction, there is a rearrangement of atoms that makes or breaks chemical bonds. This change is usually not reversible.

In short, chemical change occurs when a substance is changed into another subtance (i.e. a new substance is formed), while 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; dissolving salt in water is also a physical change because the salt water formed is still salt and water.)

There are several different types of chemical change. These include synthesis, decomposition, single displacement, double displacement, neutralization, precipitation and redox.

An elementary example of a chemical change is the combustion of methane to produce carbon dioxide and water:

CH₄ + 2O₂ → CO₂ + 2H₂O

Other examples of chemical changes are:

Burning a log of wood
Mixing an acid with a base, producing water and a salt
Melting a piece of plastic

Changes in state but not chemical composition are not considered chemical changes. For example, while boiling water involves a change in temperature and the release of a gas (water vapor), a chemical change did not take place.

The following can indicate that a chemical change took place, although this evidence is not conclusive:

Change of color (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).

In a chemical reaction bonds 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. When new substances are formed, a chemical change has occurred, and a chemical reaction has taken place. Note that, in a few cases, 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.