Understanding the properties of hydrates is critical for successful execution of experiments in a variety of chemistry fields. Due to the fact water is essential to all life forms, and available in such abundant supply, hydrates appear in some form during almost every chemistry experiment imaginable. Knowing what they can be used to accomplish will also assist chemists in designing their own experiments.
Heating a hydrate leads to an endothermic reaction that produces a residue known as the anhydrous compound. This compound is different in structure, texture and even color in some cases, from its parent hydrate. Anhydrous compounds are highly soluble in water and the color of the hydrate is restored to the anhydrous compound when it is added to water. Most hydrates are stable at room temperature, but freezing points vary among compounds.
Efflorescent, Hygroscopic and Deliquescent
A few unique hydrates are not stable at room temperature and are affected by moisture in the atmosphere. Efflorescent hydrates, which include a variety of salts, lose water at room temperature, producing a powdery crust. Hygroscopic hydrates absorb water from the surrounding environment and are therefore often used as dessicants to dry out liquids and gases. Deliquescent hydrates, such as solid NaOH, cannot stop absorbing water from the atmosphere until they self-dissolve.
True hydrate reactions are always reversible. Water that is added can be removed, and the compound will retain its original properties. Likewise, water that has been removed can be replaced, and the results of the hydration reaction will be the same every time the experiment is conducted. Certain heat reactions cause water extraction through decomposition of the compound rather than the loss of water. For example, although carbohydrates release water when heated, they are not true hydrates, because the carbohydrates are decomposing to produce energy that is expended. Therefore, the hydration process is not reversible in the reaction because the energy produced cannot be replaced.
Every hydrate possesses a crystalline structure that contains a fixed number of water molecules. A hydrate will often seek water molecules from the atmosphere to fill an incomplete crystal, but too many water molecules surrounding a hydrate will lead to dissolution or clumping with other hydrates possessing similar properties. Most salts are hydrates, and many salt structures will remain dissolved in water at any temperature. As a result, these crystals are used in a variety of sports beverages, such as Gatorade, to provide essential hydration to athletes during practice and game performance.
- "Chemical Principles in the Laboratory"; Emil Solwinski; 2009
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