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Bases (Alkalis)

CSCA Bases (Alkalis) study guide organized around the publicly available CSCA syllabus. Practice Chemistry questions on aicsca.com.

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Bases (Alkalis)

1. Core Definitions & Classification Logic

In the CSCA exam, the focus on Bases is the impact of **Solubility** on reactions and the distinction between **Strong and Weak Bases**.

* **Definition**: Compounds whose anions generated during ionization are **all** hydroxide ions ($OH^-$).

* *Counter-example*: Basic copper carbonate $Cu_2(OH)_2CO_3$ produces both $OH^-$ and $CO_3^{2-}$, so it is a **Basic Salt**, not a Base.

* **Solubility Rules** (Crucial for Metathesis Reactions):

* **Soluble Strong Bases**: $KOH, NaOH, Ba(OH)_2$ (Completely ionize).

* **Slightly Soluble Strong Base**: $Ca(OH)_2$ (Limewater).

* **Soluble Weak Base**: $NH_3\cdot H_2O$ (Ammonia water, partial ionization).

* **Insoluble Bases**: $Cu(OH)_2$ (Blue), $Fe(OH)_3$ (Red-brown), $Mg(OH)_2$ (White), $Al(OH)_3$ (White).

* **Key Deduction**: **Insoluble bases** do NOT change indicator colors and do NOT react with salt solutions.

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2. General Properties of Bases

The properties of bases are essentially the properties of $OH^-$. Compare this with "Properties of Acids":

1. **Indicators**:

* Litmus $\rightarrow$ **Blue**

* Phenolphthalein $\rightarrow$ **Red**

2. **Acidic Oxides**:

* General: $Base + Acidic \ Oxide \rightarrow Salt + Water$

* **Remove $CO_2$**: $2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O$ (No visible change)

* **Test for $CO_2$**: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$ (Turns cloudy)

* 3. **Acids**:

* **Neutralization**: $OH^- + H^+ \rightarrow H_2O$ (Exothermic).

4. **Salts**:

* General: $Base(Soluble) + Salt(Soluble) \rightarrow New \ Base + New \ Salt$

* **Prep Insoluble Base**: $2NaOH + CuSO_4 \rightarrow Cu(OH)_2\downarrow + Na_2SO_4$

* **Test for Ammonium ($NH_4^+$)**: $NaOH + NH_4Cl \xrightarrow{\Delta} NaCl + NH_3\uparrow + H_2O$

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3. Three High-Frequency Substances

#### A. Sodium Hydroxide (NaOH)

* **Common Names**: Caustic Soda, Lye.

* **Physical Properties**:

* White solid, **Deliquescent** $\rightarrow$ Must be weighed in glass containers (e.g., beakers), not on paper.

* Dissolving in water is **strongly exothermic**.

* **Storage Trap**:

* $NaOH$ reacts with $SiO_2$ in glass to form sodium silicate (adhesive), causing glass stoppers to get stuck. Therefore, **rubber stoppers must be used**.

#### B. Calcium Hydroxide ($Ca(OH)_2$)

* **Common Names**: Slaked Lime, Hydrated Lime.

* **Preparation**: $CaO + H_2O \rightarrow Ca(OH)_2$ (Exothermic, boiling egg experiment).

* **Application**: Neutralizing acidic soil, making Bordeaux mixture.

#### C. Ammonia Water ($NH_3\cdot H_2O$)

* **Weak Base**: $NH_3\cdot H_2O \rightleftharpoons NH_4^+ + OH^-$

* **Instability**: Decomposes upon heating to form Ammonia gas ($NH_3$).

* **Special Reaction**: $Al(OH)_3$ is insoluble in ammonia water (used for lab preparation of Aluminum Hydroxide) but dissolves in $NaOH$.

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4. Typical Examples & Tips

**Question: Purification**

How to remove a small amount of $Na_2CO_3$ from $NaOH$ solution?

* **Wrong Method**: Add dilute HCl. (Reason: HCl will neutralize NaOH first).

* **Correct Method**: Add appropriate amount of **$Ca(OH)_2$ solution**, then filter.

* Equation: $Na_2CO_3 + Ca(OH)_2 \rightarrow CaCO_3\downarrow + 2NaOH$

* Advantage: Introduces no new impurities (Generates the target substance $NaOH$).

**Question: Identification**

Distinguish between $NaOH$ and $Ca(OH)_2$ solutions?

* **Method**: Bubble $CO_2$ or add $Na_2CO_3$ solution.

* **Result**: White precipitate indicates $Ca(OH)_2$; no reaction indicates $NaOH$.