1. Introduction to Bases
Bases are substances we commonly use in daily life, even if we don’t notice them. The bitter taste of baking soda, the slippery feel of soap, and household cleaners—all these involve bases.
In Chemistry, a base is understood through the ions it produces or accepts during reactions.
2. Scientific Definition of Bases
In Chemistry, a base is a substance that releases hydroxide ions \( OH^- \) in water, or accepts hydrogen ions \( H^+ \). These ions give bases their characteristic properties such as bitter taste, slippery touch, and ability to turn red litmus paper blue.
2.1. Arrhenius Definition
According to Arrhenius, a base is a substance that increases the concentration of hydroxide ions \( OH^- \) when dissolved in water.
Example: Sodium hydroxide (NaOH) dissociates as:
\( \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \)
2.2. Bronsted–Lowry Definition
Bronsted and Lowry defined a base as a proton (\( H^+ \)) acceptor. This means a base picks up hydrogen ions during a reaction.
2.3. Lewis Definition
According to Lewis, a base is a substance that donates an electron pair. This concept helps explain reactions more clearly, especially in organic chemistry.
3. Properties of Bases
Bases show certain physical and chemical properties that help us identify them in everyday life and laboratory experiments.
3.1. Physical Properties
- Bases taste bitter (never taste chemicals in the lab).
- They feel soapy or slippery to touch.
- They turn red litmus paper blue.
- Most bases dissolve in water (soluble bases are called alkalis).
3.2. Chemical Properties
- Reaction with Acids: Bases neutralise acids to form salt and water.
- Reaction with Ammonium Salts: Bases release ammonia gas when they react with ammonium salts.
- Reaction with Metals (Strong Bases Only): Some strong bases react with metals like aluminium and zinc to produce hydrogen gas.
3.2.1. Reaction with Acids
Neutralisation reaction:
\( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} \)
Example:
\( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2O \)
3.2.2. Reaction with Ammonium Salts
General reaction:
\( \text{Base} + \text{Ammonium Salt} \rightarrow \text{Salt} + \text{NH}_3 \uparrow + \text{H}_2O \)
3.2.3. Reaction with Certain Metals
Strong bases like sodium hydroxide react with aluminium to form sodium aluminate and hydrogen gas.
\( 2\text{Al} + 2\text{NaOH} + 6\text{H}_2O \rightarrow 2\text{NaAl(OH)}_4 + 3\text{H}_2 \uparrow \)
4. Common Examples of Bases
Bases can be naturally occurring or laboratory-prepared. Some commonly known bases include:
- Sodium Hydroxide (NaOH): Used in soaps and detergents.
- Calcium Hydroxide (Ca(OH)2): Used in whitewashing.
- Magnesium Hydroxide: Used in antacids.
- Ammonium Hydroxide (NH4OH): Present in window cleaning solutions.
- Baking Soda (NaHCO3): A mild base used in cooking.
5. Classification of Bases
Bases can be grouped into categories depending on their strength, solubility, and ion-releasing capacity.
5.1. Based on Strength
- Strong Bases: Completely dissociate in water (e.g., NaOH, KOH).
- Weak Bases: Partially dissociate in water (e.g., NH4OH).
5.2. Based on Solubility
- Alkalis: Soluble bases (NaOH, KOH).
- Insoluble Bases: Do not dissolve completely in water (e.g., CuO, Fe(OH)3).
5.3. Based on Number of Hydroxide Ions Released
- Monacidic Bases: Release one \( OH^- \) ion (NaOH).
- Diacidic Bases: Release two \( OH^- \) ions (Ca(OH)2).
- Triacidic Bases: Release three \( OH^- \) ions (Al(OH)3).
6. Uses of Bases in Daily Life
Bases play an essential role in households, industries, and our bodies.
- Soaps and detergents contain bases like NaOH.
- Milk of magnesia (Mg(OH)2) is used to relieve acidity.
- Calcium hydroxide is used in whitewashing walls.
- Cleaning liquids often contain ammonium hydroxide.
- Baking soda helps cakes and breads rise.
7. Safety Precautions While Handling Bases
- Wear protective gloves and goggles when dealing with strong bases.
- Do not touch bases with bare hands; they may cause skin burns.
- Store bases in tightly closed and labelled containers.
- Handle cleaning agents carefully as they may be corrosive.