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Oxides

CSCA Oxides study guide organized around the publicly available CSCA syllabus. Practice Chemistry questions on aicsca.com.

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This study guide is organized around the publicly available CSCA syllabus for international undergraduate applicants.

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Tutorial Content

Oxides

1. Core Classification System

Oxides are not just "Oxygen + Another Element." In chemistry, we classify them based on their **reaction behavior with acids or bases**. This is the logical starting point for solving inorganic inference problems.

| Category | Definition | Key Property | Examples |

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

| **Acidic Oxide** | Reacts with base to form salt & water | Most react with water to form acid.<br>**Exception**: $SiO_2$ is insoluble and does not react with water. | $CO_2, SO_2, SO_3, P_2O_5, SiO_2$ |

| **Basic Oxide** | Reacts with acid to form salt & water | Only a few (Active Metals) react with water to form alkali.<br>**Note**: $CuO, Fe_2O_3$ do NOT react with water. | $Na_2O, CaO, MgO, CuO, Fe_2O_3$ |

| **Amphoteric Oxide** | Reacts with **BOTH** acid and strong base | Insoluble in water. | **$Al_2O_3$**, $ZnO$ |

| **Neutral Oxide** | Does not react with acid or base to form salt | Usually does not react with water. | $CO, NO$ |

2. Key Points & Traps

**Point 1: Concept Distinction**

This is the most confusing part of the CSCA exam. You must distinguish between "Element Category" and "Oxide Category."

* **Acidic Oxide $\neq$ Non-metal Oxide**

* Most acidic oxides are non-metal oxides, **BUT**:

* $Mn_2O_7$ is a metal oxide, yet it is an acidic oxide.

* $CO, NO$ are non-metal oxides, yet they are neutral oxides.

* $NO_2$ reacts with water to form $HNO_3$ and $NO$, but it is NOT an acidic oxide (because the reaction involves redox, and products are not just salt and water).

* **Basic Oxide $\neq$ Metal Oxide**

* Most basic oxides are metal oxides, **BUT**:

* $Al_2O_3$ is a metal oxide, yet it is an amphoteric oxide.

* $Na_2O_2$ (Sodium Peroxide) is a metal oxide, but it is a peroxide (produces $O_2$).

**Point 2: The Uniqueness of Silica ($SiO_2$)**

$SiO_2$ is an acidic oxide and reacts with alkali solutions:

$$SiO_2 + 2NaOH \rightarrow Na_2SiO_3 + H_2O$$

* **Lab Safety Warning**:

Reagent bottles containing alkali solutions (like $NaOH, Na_2CO_3$) **must NOT use glass stoppers**; rubber stoppers must be used. Since glass contains $SiO_2$, the reaction produces sodium silicate ($Na_2SiO_3$), a mineral glue (Water Glass), which will cement the stopper to the bottle neck.

**Point 3: Amphoteric Oxide ($Al_2O_3$)**

$Al_2O_3$ is the raw material for industrial aluminum smelting. Its "amphoteric nature" is a must-know.

* With Acid: $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$

* **With Sodium Hydroxide** (Focus): $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$

*(Note: The product can also be written as sodium tetrahydroxyaluminate $Na[Al(OH)_4]$, but $NaAlO_2$ is commonly used in concise tutorials)*

3. Typical Example Analysis

**Example: Purification**

How to remove a small amount of $SO_2$ from $CO_2$?

* **Wrong Method**: Pass through $NaOH$ solution. (Reason: Both $CO_2$ and $SO_2$ are acidic oxides and will be absorbed).

* **Correct Method**: Pass through **Saturated $NaHCO_3$ solution**, then dry.

* Principle: $SO_2 + 2NaHCO_3 \rightarrow Na_2SO_3 + H_2O + 2CO_2\uparrow$

* Analysis: Uses the principle that strong acids displace weak acids. $H_2SO_3$ is stronger than $H_2CO_3$.