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Acids

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

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Acids

1. Core Definitions & Microscopic Essence

Acids are not just substances that "taste sour." In the CSCA exam, we must define them precisely from the perspective of **Ionization**:

* **Definition**: A compound that produces **only Hydrogen ions ($H^+$)** as cations when ionized in aqueous solution.

* **Core Distinction: Strong vs. Weak Acids**

This is the key criterion for writing **Ionic Equations**.

* **Strong Acid**: **Completely ionizes** in water. E.g., $HCl, H_2SO_4, HNO_3$.

* *Rule*: Split into ions (e.g., $H^+ + Cl^-$).

* **Weak Acid**: **Partially ionizes** in water; reversible equilibrium exists. E.g., $CH_3COOH$ (Acetic Acid), $H_2CO_3$.

* *Rule*: **Keep as molecule**, do NOT split (e.g., write as $CH_3COOH$).

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2. The "Five Fingers" of Acid Properties

The chemical properties of acids are essentially the properties of $H^+$. We can memorize this using a "Star" model:

1. **Indicators**: Turns purple litmus **Red**, colorless phenolphthalein remains **Colorless**.

2. **Active Metals**:

* General: $Metal + Acid \rightarrow Salt + H_2\uparrow$

* *Condition*: Metal is before $H$; **Acid must be non-oxidizing** (e.g., dilute $HCl$, dilute $H_2SO_4$).

* *Exception*: $HNO_3$ and Concentrated $H_2SO_4$ are oxidizing acids; they **do NOT produce Hydrogen gas** with metals.

3. **Metal Oxides**:

* General: $Basic \ Oxide + Acid \rightarrow Salt + Water$

* App: Rust removal (e.g., $Fe_2O_3 + 6HCl \rightarrow 2FeCl_3 + 3H_2O$).

4. **Bases**:

* **Neutralization**: $H^+ + OH^- \rightarrow H_2O$

5. **Salts**:

* Condition: Produces gas ($CO_2$) or precipitate ($AgCl, BaSO_4$).

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3. Exam Traps for the Three Strong Acids

| Acid | Properties | CSCA Exam Focus |

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

| **Hydrochloric** ($HCl$) | **Volatile** | **White Mist** when bottle opens. Test for $Cl^-$ using $AgNO_3$ (White precipitate). |

| **Sulfuric** ($H_2SO_4$) | **Hygroscopic, Dehydrating, Oxidizing** | **Dilution Safety**: Must pour **"Acid into Water"** while stirring!<br>**Test**: Use $BaCl_2$ solution (White precipitate $BaSO_4$, insoluble in acid). |

| **Nitric** ($HNO_3$) | **Strong Oxidizer, Unstable** | Decomposes in light (Store in brown bottles). Reaction with Copper:<br>Conc. $HNO_3$ $\rightarrow NO_2$ (Red-brown gas)<br>Dilute $HNO_3$ $\rightarrow NO$ (Colorless gas) |

### 4. Typical Examples & Common Mistakes

**Common Mistake: Writing Ionic Equations**

* **Question**: Reaction between Acetic Acid solution and Calcium Carbonate.

* **Wrong**: $2H^+ + CaCO_3 \rightarrow Ca^{2+} + H_2O + CO_2\uparrow$

* **Why**: Acetic acid is a **Weak Acid** and cannot be split into $H^+$.

* **Correct**: $2CH_3COOH + CaCO_3 \rightarrow Ca^{2+} + 2CH_3COO^- + H_2O + CO_2\uparrow$

**Thinking Question**: How to distinguish dilute HCl from dilute H2SO4?

* **Method**: Add $BaCl_2$ or $Ba(NO_3)_2$.

* **Result**: White precipitate indicates $H_2SO_4$ ($BaSO_4$); no reaction indicates $HCl$.