Basic Anhydride
Definition and meaning of Basic Anhydride in chemistry.
A basic anhydride is a metal oxide that reacts with water to form a base. It is essentially a base that has had all of its water chemically removed. When you add water back to it, the original basic solution returns.
In more detail
The word anhydride literally translates from Greek to mean without water. Basic anhydrides are solid compounds made of a metal bonded to oxygen. You will usually find these active metals in Group 1 and Group 2 of the periodic table.
Common examples include sodium oxide, potassium oxide, and calcium oxide. When you drop one of these dry metal oxides into water, a chemical reaction happens immediately. The oxygen atom from the solid oxide grabs hydrogen from the water molecules.
This fast reaction creates free hydroxide ions in the surrounding liquid. A high concentration of hydroxide ions is exactly what makes a solution basic, or alkaline. The process of adding water back to an anhydride is called hydration.
Students often confuse basic anhydrides with acidic anhydrides, but they behave very differently. The key difference depends on the type of element bonded to the oxygen. Metals form basic anhydrides that raise the pH of water.
Nonmetals form acidic anhydrides that lower the pH of water. Understanding this periodic table pattern helps chemists predict what will happen when different oxide powders dissolve. These metal oxides are very important in making construction materials like cement and plaster.
Key facts
| General formula | Metal bonded to oxygen |
|---|---|
| Product when hydrated | Metal hydroxide (a base) |
| Common examples | Na2O, CaO, K2O |
| Typical metals | Group 1 and Group 2 elements |
| Opposite concept | Acidic anhydride (nonmetal oxides) |
| Field | Inorganic Chemistry |
Calcium oxide is a very common and highly reactive basic anhydride. People working in the construction industry often call it by the name quicklime. If you pour liquid water onto a pile of solid quicklime, it reacts violently. This aggressive reaction releases a massive amount of heat energy. The calcium oxide chemically absorbs the water molecules to form a new substance called calcium hydroxide. Builders use this basic calcium hydroxide mixture to make the strong mortar needed for bricklaying.
Frequently asked questions
What is the difference between a basic anhydride and an acidic anhydride?
Basic anhydrides are metal oxides that make water basic. Acidic anhydrides are nonmetal oxides that make water acidic.
Why does calcium oxide get hot when mixed with water?
The hydration reaction between calcium oxide and water is highly exothermic. This means it releases a large amount of trapped chemical energy as heat.
Can any metal oxide act as a basic anhydride?
Mostly, active metals from Group 1 and Group 2 form true basic anhydrides. Some transition metal oxides do not dissolve in water at all.