What Happens During a Chemical Change?

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Alterations to the electron-based ionic or covalent bonds between ions or atoms happen during a chemical change. Such changes result in one or more new substances with different physical and chemical properties. This type of change stands in contrast to physical changes which, despite sometimes closely resembling chemical changes, only produce a change in physical properties.

One example of chemical and physical changes that appear almost identical, both in the initial processes and their reversal, is the dissolution of salt and sugar in water. Dissolving salt is a chemical change, while dissolving sugar is a physical change due to the nature of salt versus sugar.

Sugar molecules are composed of highly covalent bonds, with each constituent atom bonded to another via shared electrons. Each sugar molecule is polar, however, and when these molecules encounter water, the polar water molecules pull individual sugar molecules out into solution. However, while the solution is breaking up the sugar crystal and dispersing it through the water, no bonds are being broken, and each molecule is unchanged.

Table salt, also known as sodium chloride or NaCl, is an ionic compound. This means that salt crystals are held together not by shared electrons, but by the opposite charges of the sodium and chloride ions. When polar water molecules interact with salt, they pull off not sodium chloride molecules, but individual sodium and chloride ions. This breaks up the crystal in both its physical and chemical structure.