Acids, Bases and Salts
Acids, Bases and Salts – Complete Solutions
2.1 Acids
Definition:
An acid is a substance that produces H⁺ ions (or H₃O⁺ ions) when dissolved in water. According to Arrhenius definition, acids are proton donors.
Properties of Acids:
- Taste: Sour (e.g., lemon, vinegar)
- Litmus Paper: Turn blue litmus paper red
- Electrical Conductivity: Good conductors of electricity
- Reactions with Metals: Produce hydrogen gas
- Reactions with Carbonates: Produce CO₂ gas
- pH: pH < 7
Examples of Acids:
Mineral Acids:
- Hydrochloric acid (HCl) – stomach acid
- Nitric acid (HNO₃) – used in manufacturing fertilizers
- Sulfuric acid (H₂SO₄) – used in car batteries
Organic Acids:
- Acetic acid (CH₃COOH) – vinegar
- Citric acid – in citrus fruits
- Oxalic acid – in spinach, rhubarb
- Formic acid – bee sting venom
2.2 Bases
Definition:
A base is a substance that produces OH⁻ ions when dissolved in water. According to Arrhenius definition, bases are proton acceptors.
Properties of Bases:
- Taste: Bitter
- Litmus Paper: Turn red litmus paper blue
- Electrical Conductivity: Good conductors of electricity
- Solubility: Many bases are insoluble in water
- Reactions: React with acids to form salt and water
- pH: pH > 7
- Feel: Soapy, slippery feel
Examples of Bases:
- Sodium hydroxide (NaOH) – caustic soda, used in cleaning
- Potassium hydroxide (KOH) – used in soap making
- Calcium hydroxide (Ca(OH)₂) – slaked lime, used in whitewashing
- Ammonia (NH₃) – pungent smell, used as cleaner
- Magnesium hydroxide (Mg(OH)₂) – milk of magnesia, antacid
2.3 Salts
Definition:
A salt is an ionic compound formed from the neutralization reaction between an acid and a base. It consists of a cation (positive ion) from the base and an anion (negative ion) from the acid.
General Reaction:
Acid + Base → Salt + Water
Examples:
- HCl + NaOH → NaCl + H₂O
- H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
- HNO₃ + KOH → KNO₃ + H₂O
Common Salts and Their Uses:
- Sodium Chloride (NaCl): Table salt, food preservative, de-icing roads
- Calcium Carbonate (CaCO₃): Chalk, limestone, used in toothpaste
- Potassium Nitrate (KNO₃): Fertilizer, used in food preservation
- Ammonium Nitrate (NH₄NO₃): Fertilizer
- Copper Sulfate (CuSO₄): Fungicide, used in electroplating
2.4 pH Scale
Definition:
pH is a measure of acidity or basicity (alkalinity) of a solution. It is defined as the negative logarithm of hydrogen ion concentration.
Formula: pH = -log[H⁺]
pH Scale (0-14):
| pH Value | Nature | Example |
|---|---|---|
| 0-6 | Acidic | Lemon juice (pH 2), Vinegar (pH 3) |
| 7 | Neutral | Pure water (pH 7) |
| 8-14 | Alkaline/Basic | Baking soda (pH 8.3), Soap solution (pH 12) |
Important Relation:
pH + pOH = 14 (at 25°C)
pH = 7 → Neutral
pH < 7 → Acidic
pH > 7 → Basic/Alkaline
2.5 Indicators
Indicators are substances that change color in acidic and basic solutions to show whether a solution is acidic or basic.
| Indicator | Color in Acid | Color in Base |
|---|---|---|
| Litmus | Red | Blue |
| Methyl Orange | Red | Yellow |
| Phenolphthalein | Colorless | Pink |
| Methyl Red | Red | Yellow |
| Universal Indicator | Red-Green | Green-Blue |
2.6 Neutralization
Neutralization: The reaction between an acid and a base to form a salt and water is called neutralization.
Acid + Base → Salt + Water
Example:
HCl + NaOH → NaCl + H₂O
In Ionic Form:
H⁺ + Cl⁻ + Na⁺ + OH⁻ → Na⁺ + Cl⁻ + H₂O
Net Ionic Equation:
H⁺ + OH⁻ → H₂O
2.7 Solved Examples
Example 1: What is the pH of a solution having H⁺ concentration of 10⁻³ M?
Solution:
pH = -log[H⁺] = -log(10⁻³) = 3
The solution is acidic.
Example 2: Write the equation for neutralization of H₂SO₄ with NaOH
Solution:
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Example 3: Name three natural sources of acids
Solution:
1. Citric acid – found in citrus fruits (lemons, oranges)
2. Acetic acid – found in vinegar
3. Formic acid – found in bee venom, ant bite