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Excellence in Science Education
ACIDS
Properties of Acids
- Acids have sour taste (e.g., citric acid in lemons)
- They turn blue litmus paper to red
- They release hydrogen ions (H⁺) in aqueous solutions
- They react with active metals to produce hydrogen gas
- They have pH values less than 7
Types of Acids
Strong Acids
• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)
Weak Acids
• Acetic acid (CH₃COOH)
• Oxalic acid
• Lactic acid
Concentration
Concentrated: High acid-to-water ratio
Dilute: Low acid-to-water ratio
Characteristic Reactions of Acids
1. Neutralization Reaction
Acid reacts with base to form salt and water:
Acid + Base → Salt + Water
KOH + HCl → KCl + H₂O
2. Reaction with Metals
Acid + Metal → Salt + Hydrogen gas
Mg + 2HCl → MgCl₂ + H₂↑
3. Reaction with Metal Oxides
Metal oxide + Acid → Salt + Water
CuO + 2HCl → CuCl₂ + H₂O
4. Reaction with Carbonates/Bicarbonates
Acid + Carbonate → Salt + CO₂ + H₂O
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
Electrical Conductivity
Acidic solutions conduct electricity due to the presence of hydronium ions (H₃O⁺). Free hydrogen ions (H⁺) immediately combine with water molecules:
H⁺ + H₂O → H₃O⁺
Acid Strength
The strength of an acid is measured using the pH scale. Strong acids have pH values much less than 7, while weak acids are closer to neutral (pH 7).
BASES
Properties of Bases
- Bases have bitter taste and feel slippery
- They turn red litmus paper to blue
- They release hydroxide ions (OH⁻) in solution
- They react with acids to form salt and water
- They have pH values greater than 7
Classification of Bases
Strong Bases
• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)
Weak Bases
• Ammonium hydroxide (NH₄OH)
Alkalis
Water-soluble bases (e.g., NaOH, KOH)
Reaction with Metals
Base + Metal → Salt + Hydrogen gas
2NaOH + Zn → Na₂ZnO₂ + H₂↑
Note: Only certain reactive metals (like Zn, Al) react with bases.
Bases in Aqueous Solution
Bases dissociate in water to produce hydroxide ions (OH⁻). Soluble bases (alkalis) form strongly alkaline solutions.
NaOH → Na⁺ + OH⁻
Reaction with Non-metal Oxides
Non-metal oxide + Base → Salt + Water
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
This demonstrates that non-metal oxides are typically acidic in nature.
Electrical Conductivity
Basic solutions conduct electricity due to the presence of mobile hydroxide ions. Dissolving bases in water is highly exothermic.
Base Strength
Base strength is indicated by pH values above 7. Strong alkalis have pH values approaching 14.
THE pH SCALE
- Quantitative measure of acidity/basicity from 0 (strong acid) to 14 (strong base)
- pH 7 represents neutral solutions (pure water at 25°C)
- Each pH unit represents a 10-fold difference in H⁺ concentration
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Acidic Solutions
pH < 7
Higher H⁺ concentration
Neutral Solutions
pH = 7
[H⁺] = [OH⁻] = 10⁻⁷ M
Basic Solutions
pH > 7
Higher OH⁻ concentration
Practical Applications of pH
1) Treating Insect Stings
Bee stings (acidic) can be neutralized with baking soda (NaHCO₃), while wasp stings (alkaline) may require vinegar.
2) Agricultural Soil Management
Most crops thrive at pH 6-7.5. Lime (CaO) is added to acidic soils, while sulfur treats alkaline soils.
3) Human Digestive System
Stomach pH is ~1.5-3.5 for digestion. Antacids (like Mg(OH)₂) neutralize excess acid during indigestion.
4) Dental Health
Tooth enamel dissolves below pH 5.5. Fluoride in toothpaste forms more acid-resistant fluorapatite.
5) Swimming Pool Maintenance
Ideal pool pH is 7.2-7.8. Acids or bases are added to maintain this range.
SALTS
Salts are ionic compounds formed when acids and bases neutralize each other:
Acid + Base → Salt + Water
Common table salt (NaCl) results from hydrochloric acid and sodium hydroxide neutralization:
NaOH + HCl → NaCl + H₂O
Salt Properties:
• Strong acid + weak base → Acidic salt (pH < 7)
• Weak acid + strong base → Basic salt (pH > 7)
• Strong acid + strong base → Neutral salt (pH ≈ 7)
Important Salt Varieties
1. Rock Salt
Naturally occurring crystalline halite (NaCl) with mineral impurities, often pink or gray.
(काला नमक/ सेंधा नमक)
2. Common Salt (Table Salt)
Refined NaCl, often iodized to prevent deficiencies. Essential for nerve function and fluid balance.
(टाटा नमक 😊)
Industrial Salt Processes
Chlor-Alkali Process
Electrolysis of brine produces three important chemicals:
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
- Chlorine gas (Cl₂): Collected at anode - used for PVC, disinfectants
- Hydrogen gas (H₂): Collected at cathode - fuel, ammonia production
- Sodium hydroxide (NaOH): Near cathode - important industrial base
Important Salt Compounds
1. Bleaching Powder (CaOCl₂)
Preparation:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Applications:
- Textile bleaching and paper industries
- Disinfectant for water treatment
- Oxidizing agent in chemical synthesis
- Production of chloroform (CHCl₃)
- Swimming pool sanitation
2. Baking Soda (Sodium Bicarbonate - NaHCO₃)
Production (Solvay Process):
NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
Uses:
- Leavening agent in baking (with acidic components)
- Antacid for acid indigestion
- Fire extinguishers (soda-acid type)
- Odor neutralizer and mild abrasive cleaner
- pH buffer in pools and aquariums
3. Washing Soda (Sodium Carbonate Decahydrate - Na₂CO₃·10H₂O)
Preparation:
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
Applications:
- Water softening by removing Ca²⁺/Mg²⁺ ions
- Manufacture of glass, paper, and detergents
- Precursor for other sodium compounds
- Industrial cleaning agent
- pH regulator in textile processing
4. Plaster of Paris (Calcium Sulfate Hemihydrate - CaSO₄·½H₂O)
Production:
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
Properties and Uses:
- White powder that hardens when mixed with water
- Used for casts, molds, and construction materials
- Sets via rehydration: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
- Fire-resistant wall coatings
- Dental impressions and surgical splints
THP Coaching Classes
Chemistry Notes | Mohit Sir (MSc in Neuroscience)