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Chemical reactions

Rainwater slowly dissolving a statue's face Rainwater slowly dissolving a statue's face A chemical reaction takes place when two or more different substances join together to form one or more new substances. The change involves the movement of electrons in both the forming and breaking of bonds between atoms. The changes may be described by chemical equations, e.g. 2HCl + 2Na → 2NaCl + H2 (hydrochloric acid reacts with sodium to form sodium chloride and hydrogen). Chemical reactions are taking place all around us. Some happen naturally. For example, the food we eat is chemically changed inside our bodies to produce energy; certain gases in the air react with rainwater to form acids that, over time, may dissolve rocks such as limestone. Other reactions can be made to take place artificially, for example in the manufacture of medicinal drugs.

A cake's ingredients before bakingA cake's ingredients before baking

Chemical and physical change

Baking a cake is an example of using chemical reactions to make things. The molecules that make up a cake’s ingredients, flour, eggs, butter and others, are re-arranged using energy (the heat of the oven). The resulting cake is chemically different from its ingredients. Most (but not all) chemical changes are irreversible: the baked cake cannot be turned back into its separate ingredients.

In the Middle Ages people called alchemists studied chemical reactions. They tried (and failed) to convert ordinary metals like lead into precious metals such as gold.

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