Aluminum Hydroxide, chemically represented as Al(OH)3, is fundamentally a solid compound. This inorganic substance is classified as a metal hydroxide and is naturally found in the mineral gibbsite and its three less common forms, known as polymorphs. The compound’s white, odorless nature confirms its identity as a distinct material with a defined structure.
The Direct Answer: State and Form
Aluminum hydroxide exists as a solid due to its strong internal structure, characteristic of many ionic compounds. The aluminum ion, with its positive charge of +3, forms strong electrostatic bonds with the hydroxide ions, resulting in a stable crystal lattice. This strong bonding network provides high thermal stability, with decomposition beginning around 180°C and converting entirely to aluminum oxide by 300°C.
Confusion about its state arises because the public often encounters it in liquid-like preparations, such as antacid suspensions. These preparations are not the pure, dry compound, but rather a fine white powder or gelatinous precipitate suspended in water. The solid particles are so small and dispersed that the mixture appears and flows like a liquid, but the Al(OH)3 itself remains in its solid, undissolved form.
Defining Characteristics and Solubility
In its pure state, aluminum hydroxide is typically a white, amorphous powder or fine granules. Its solid density is approximately 2.42 g/cm3. A defining characteristic of this solid is its extremely low solubility in water, with only about 0.0001 grams dissolving in 100 mL of water at room temperature.
This negligible solubility allows it to function effectively in liquid preparations without dissolving away. The compound is known for its amphoteric nature, meaning it can react with both strong acids and strong bases. When a strong acid is present, such as hydrochloric acid in the stomach, the solid aluminum hydroxide acts as a base to neutralize the acid. Conversely, in the presence of a strong base, it can act as an acid.
Real-World Applications
The solid nature and amphoteric reactivity of aluminum hydroxide are responsible for its widespread use in medicine and industry. The most common medical application is as an antacid, where the solid particles neutralize excess stomach acid without significantly raising the stomach’s pH level. The low solubility ensures that the acid neutralization is localized and prevents a rebound effect of overproducing acid.
The compound is also a frequently used adjuvant in many vaccines, including those for hepatitis and diphtheria-tetanus-pertussis. In this role, the particles act as a micro-platform, adsorbing the vaccine’s antigen to enhance the immune response. The solid particles create a depot effect at the injection site, which stimulates the body’s innate immune system. Furthermore, the compound is used as a flame retardant filler in plastics because it decomposes upon heating, absorbing thermal energy and releasing water vapor.