The Taste Behind Food: Acids & Bases
Ever wondered why lemons taste sour or soap feels bitter? It’s due to the presence of acids and bases in them.
Acids
- Acids are substances that release hydrogen ions (H⁺) when dissolved in water.
- They usually taste sour.
- They turn blue litmus red, a key test for acids.
- Common examples: Hydrochloric Acid (HCl), Sulphuric Acid (H₂SO₄), Citric Acid.
Common Reactions of Acids:
- Acid + Metal → Salt + Hydrogen Gas
Example:
Mg + H2SO4 → MgSO4 + H2 - Acid + Carbonate → Salt + Water + Carbon Dioxide
Example:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2 - Acid + Bicarbonate → Salt + Water + Carbon Dioxide
Example:
NaHCO3 + HCl → NaCl + H2O + CO2
Types of Acids:
- Organic Acids: Naturally found in plants or animals (e.g., Citric acid in citrus fruits).
- Mineral Acids: Inorganic and strong (e.g., HCl, H₂SO₄).
- Strong Acids: Completely dissociate in water (e.g., HCl, HNO₃).
- Weak Acids: Partially dissociate (e.g., Acetic Acid).
- Dilute vs Concentrated:
- Dilute Acid: Lower concentration in water.
- Concentrated Acid: High concentration.
⚠ Always add acid to water, not the other way around. Adding water to acid releases excessive heat which can be dangerous.
- Based on H⁺ Ion Count:
- Monoprotic Acid: Releases one H⁺ per molecule (e.g., HCl)
- Diprotic Acid: Releases two H⁺ (e.g., H₂SO₄)
Bases
- Bases release hydroxide ions (OH⁺) in water.
- Alkalis are water-soluble bases.
- They taste bitter and turn red litmus blue.
- Examples: NaOH, KOH, NH₄OH.
Reactions of Bases:
- Base + Metal → Salt + Hydrogen Gas
Example:
2NaOH + Zn → Na2ZnO2 + H2 - Base + Acid → Salt + Water
Neutralisation Reaction
KOH + HCl → KCl + H2O - Base + Non-Metallic Oxide → Salt + Water
Example:
2NaOH + CO2 → Na2CO3 + H2O
Types of Bases:
- Strong Base: Fully ionizes (e.g., NaOH)
- Weak Base: Partial ionization (e.g., NH₄OH)
- Dilute vs Concentrated Base:
- Dilute: Low concentration
- Concentrated: High concentration
pH Scale: Measuring Acidity & Alkalinity
- pH indicates the concentration of hydrogen ions.
- pH = 7: Neutral (e.g., pure water)
- pH < 7: Acidic
- pH > 7: Basic
⚡ “pH” stands for “potenz”, meaning “power” in German.
Real-Life Importance of pH:
- Human blood: pH ~ 7.4
- Stomach acid: pH ~ 2 (for digestion)
- Tooth decay: Begins below pH 5.5 due to acid from sugar breakdown.
- Bee sting: Acidic → relieved with baking soda (base)
- Wasp sting: Alkaline → relieved with vinegar (acid)
Soil pH & Plant Growth:
- Plants prefer soil close to pH 7.
- Acidic soil: Treated with quicklime or slaked lime.
- Alkaline soil: Treated with compost or organic matter.
Salts
- Salts form when acid neutralises a base.
- Reaction:
Acid + Base → Salt + Water
Types of Salts:
- Neutral Salt (Strong Acid + Strong Base)
NaOH + HCl → NaCl + H2O
- Acidic Salt (Strong Acid + Weak Base)
HCl + NH4OH → NH4Cl + H2O
- Basic Salt (Weak Acid + Strong Base)
CH3COOH + NaOH → CH3COONa + H2O
Indicators
- Indicators change colour to detect acidic or basic nature.
- Examples:
- Litmus: Blue → Red (acid), Red → Blue (base)
- Phenolphthalein: Colourless (acid), Pink (base)
- Methyl Orange, Methyl Red
Difference Between Natural and Synthetic Indicators:
| Property | Natural Indicators | Synthetic Indicators |
|---|---|---|
| Origin | Derived from plants | Man-made |
| Precision | Less precise | More precise |
| Range | Narrower range | Wider range |
| Stability | Less stable | More stable |
| Uses | Traditional/alternative medicine and food preparation | Scientific settings such as laboratories |
| Availability | May be difficult to obtain and vary in quality | More readily available and consistent |
⚠ Important Compounds & Uses
| Compound | Preparation | Uses |
|---|---|---|
| NaCl (Salt) | From seawater, mining, or acid-base neutralisation | Used in food, industry, de-icing roads |
| NaOH (Caustic Soda) | Electrolysis of brine (NaCl solution) | Soaps, detergents, paper, refining oil |
| NaHCO3 (Baking Soda) | NaCl + NH3 + CO2 + H2O | Antacid, baking, fire extinguishers |
| Na2CO3·10H2O (Washing Soda) | Heating baking soda | Glass making, softening water, cleaning |
| CaOCl2 (Bleaching Powder) | Ca(OH)2 + Cl2 | Bleaching, water treatment, disinfectant |
| CaSO4·½H2O (Plaster of Paris) | Heating gypsum below 100°C | Casts, sculptures, fireproofing |