The key differences between aluminum oxide (Al₂O₃) powder and aluminum hydroxide (Al(OH)₃) powder lie in their chemical composition, structure, thermal stability, reactivity, and primary applications:

1.  Chemical Composition & Structure:

*   Aluminum Oxide (Al₂O�₃): An oxide of aluminum. It consists of aluminum atoms bonded to oxygen atoms in a very stable, dense crystalline lattice (e.g., alpha-alumina, gamma-alumina).

*   Aluminum Hydroxide (Al(OH)₃): A hydroxide of aluminum. It consists of aluminum atoms bonded to hydroxyl groups (OH⁻). Its structure is layered.

2.  Thermal Stability & Behavior:

*   Aluminum Oxide (Al₂O₃):Extremely thermally stable. It has a very high melting point (>2000°C / 3632°F). It does not decompose when heated (below its melting point) and retains its structure.

*   Aluminum Hydroxide (Al(OH)₃):Thermally decomposes when heated. Starting around 180°C (356°F), it loses water molecules in an endothermic reaction to form aluminum oxide and water vapor:

`2Al(OH)₃ → Al₂O₃ + 3H₂O`

This decomposition is a key property for its use as a flame retardant.

3.  Reactivity:

*   Aluminum Oxide (Al₂O₃):Highly inert and unreactive under most conditions. It is resistant to acids and bases (though strong acids/bases at high temperatures can attack it). It's very stable and non-toxic.

*   Aluminum Hydroxide (Al(OH)₃):Amphoteric - meaning it can react with both strong acids and strong bases. It's relatively stable but less inert than alumina.

*   With acids: `Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O` (used in antacids)

*   With bases: `Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O`

4.  Physical Properties (Powder Form):

*   Both are typically white, fine powders.

*   Aluminum Oxide: Generally much harder (9 on Mohs scale), denser, and more abrasive.

*   Aluminum Hydroxide: Softer, less dense, and less abrasive.

5.  Primary Applications:

*   Aluminum Oxide (Al₂O₃):

*   Abrasives: Sandpaper, grinding wheels, blasting media.

*   Refractories: High-temperature furnace linings, kiln furniture.

*   Ceramics: Advanced technical ceramics, spark plugs, substrates, wear-resistant components.

*   Electrical Insulators: Substrates for electronics, insulators.

*   Catalyst Supports: High surface area gamma-alumina.

*   Aluminum Hydroxide (Al(OH)₃):

*   Flame Retardant & Smoke Suppressant: The dominant use (60-70% of production). Its endothermic decomposition cools the material and releases water vapor, which dilutes flammable gases. Used in polymers, rubber, carpets, cables.

*   Precursor for Alumina:** The starting material for producing most aluminum oxide via calcination.

*   Filler: In plastics, paper, paints, adhesives (improves whiteness, flame retardancy, other properties).

*   Antacid: Neutralizes stomach acid in medications (e.g., Maalox, Mylanta).

*   Coagulant/Flocculant  In water treatment (alum is made from it).

In essence:  Aluminum oxide is the final, highly stable, inert ceramic material prized for hardness and high-temperature performance. Aluminum hydroxide is a precursor material with useful reactivity (amphoteric nature, flame retardant decomposition) and is softer, often used as a functional filler or chemical feedstock.