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For starters, the term “anhydrous” means “without water.” Therefore, anhydrous compounds are compounds that do not contain any water elements. Anhydrous compounds exist in all three forms: solid, liquid, and gas. They are used for various purposes, the main one being to carry out certain chemical reactions in which water is not needed.
Definition of anhydrous compound
Anhydrous compounds are defined as compounds that do not contain water. On the other hand, the term anhydrous is often applied to a crystalline substance after the water of crystallization has been removed. These compounds exist in solid, liquid, or gaseous form.
In nature there are many hydrated substances, that is, they are made up of a certain amount of water molecules. To become an anhydrous compound, these water molecules are generally removed in heat or by means of a vacuum.
An anhydrous compound has the ability to soak up ambient water, which is why they are often used as drying agents, also known as desiccants. Some examples are silica gel packets found in some consumer goods and dehumidifier bags used in clothing closets.
We use some solvents in the anhydrous state for certain reactions where water is not required or, in other words, to avoid the production of certain undesirable products. Sometimes they can be synthesized simply by heating the compound. Other times you have to boil it in the presence of a material that absorbs moisture from the air, such as hygroscopic material.
Examples of anhydrous compounds
Anhydrous compounds can be present in known substances, but it is highly likely that we have not even heard of most of them. Let’s see some examples:
- We know the formula for acetic acid, which is CH 3 COOH. When the water molecules are removed from the two acetic acid molecules, the result is acetic anhydride: CH 3 COOCOCH 3 . Acetic anhydride is used as an acetylating agent, that is, to introduce acetyl groups through a chemical reaction. It is also used for the production of cellulose acetate, plastics or fibers.
- The common salt we use is known as anhydrous sodium chloride, that is, NaCl.
- When we heat copper (II) sulfate pentahydrate, ie CuSO 4 ⋅5H 2 O, we get copper sulfate in its anhydrous form, ie CuSO 4 .
Other examples of anhydrous compounds are:
- Sodium sulfate Na 2 SO 4
- Sodium carbonate Na 2 CO 3
- Sodium tetraborate NaB 4 O 7
- Calcium chloride CaCl 2
- Calcium nitrate Ca(NO 2 ) 2
- Calcium sulfate CaSO 4
- Copper (II) chloride CuCl 2
- succinic anhydride C₄H₄O₃
- Maleic anhydride C 4 H 2 O 3
anhydrous compounds
Some anhydrous materials are considered water-reactive materials, which means that they can release a large amount of heat, which in turn can cause a pressure or chemical explosion. However, anhydrous materials differ from anhydrides. An anhydride is a water-reactive compound that, when combined with water, gives an acid or a base. Anhydrides react irreversibly with the oxygen-hydrogen bonds in water to form new compounds, while anhydrous compounds simply form reversible adducts with water molecules.
When a substance that is not itself an acid or a base is exposed to substances of this nature, it is transformed into a similar one. However, this does not imply that water (such as from a fire hose) must be added. Many anhydrides react readily with moisture in the environment and even with water in the skin or lungs.
The reaction of anhydrous compounds with water is usually impetuous and exothermic (ie, it releases a lot of heat energy). Here are some examples of anhydrides and their reaction products with water:
- Sulfur trioxide SO 3 (g) + water H 2 O (l) = sulfuric acid H 2 SO 4 (aq).
- Barium oxide BaO (s) + water H 2 O (l) = barium hydroxide Ba(OH) 2 (aq)
water reactive substances
An anhydrous compound can become dangerous when in contact with water. This is due to the chemical reactions they undergo upon contact. This reaction can release a gas that is flammable or poses a toxic health hazard. In addition, the heat generated by the contact of water with these materials is often enough to cause spontaneous combustion or explosion of the item.
In simpler terms, substances that react with water are not compatible with it.
On the other hand, and under the UN classification system, hazardous materials when wet are classified as R2 and DOT Hazard Class 4.3 under 49 CFR. Therefore, special precautions, labeling, and proper training are required to transport, store, and use these materials.
Some examples of chemicals that react with water are alkali metals, alkaline earth metals, anhydrides, certain carbides, hydrides, sodium hydrosulfite, and other similar chemicals.
Sources
- Desiccating agents . University of Barcelona.
- JHH, Carlos. (2018). Dangerous chemical reactions of water with alkali metals .
- United Nations. (nd). United Nations Common Coding System .
- chempedia. (nd). anhydrous .
- I am your teacher. (2016). Oxides and anhydrides | inorganic formulation
- 49 CFR Parts 300-399
