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).

**Summary Table:**

| Feature          | Aluminum Oxide Powder (Al₂O₃)        | Aluminum Hydroxide Powder (Al(OH)₃)   |

| :-------------- | :----------------------------------- | :------------------------------------ |

| **Type**        | Oxide                                | Hydroxide                             |

| **Thermal Stability** | Very High Melting Point (>2000°C), Does not decompose | Decomposes ~180°C to Al₂O₃ + H₂O |

| **Key Thermal Behavior** | Stable, retains structure          | Endothermic decomposition (absorbs heat) |

| **Reactivity**  | Very inert, unreactive (chemically resistant) | Amphoteric (reacts with strong acids & bases) |

| **Hardness**    | Very Hard (Mohs 9), Abrasive         | Softer, Less Abrasive                 |

| **Primary Uses** | Abrasives, Refractories, Ceramics, Electrical Insulators, Catalyst Supports | Flame Retardant/Smoke Suppressant, Filler, Antacid, Alumina Precursor, Water Treatment |

**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.