From the sour taste of lemon juice and vinegar to the slippery feel of soap and the soothing effect of antacids, Acids, Bases and Salts form the foundation of chemical interactions in nature, industry, and everyday life. Understanding how these chemical families interact is essential for mastering CBSE Class 10 Chemistry Chapter 2.
In this comprehensive guide, we examine the chemical behavior of acids and bases, test indicators, the pH scale, real-world applications of pH, neutralization reactions, and the industrial synthesis of key sodium and calcium compounds.
- 1. Understanding Acids and Bases — Definitions
- 2. Acid-Base Indicators — Litmus, Synthetic & Olfactory
- 3. Chemical Properties & Reactions of Acids and Bases
- 4. The pH Scale & Strength of Acids/Bases
- 5. Importance of pH in Everyday Life
- 6. Important Chemical Compounds & Salts
- 7. Water of Crystallization
- 8. Solved Board Exam Questions
- 9. Frequently Asked Questions (FAQ)
1. Understanding Acids and Bases — Definitions
Acids and bases are classified according to their chemical behavior in water solution:
- Arrhenius Definition:
- Acid: A substance that dissociates in water to yield hydrogen ions, H⁺(aq) (or hydronium ions, H₃O⁺). E.g., HCl(aq) → H⁺(aq) + Cl¯(aq).
- Base: A substance that dissociates in water to yield hydroxide ions, OH¯(aq). E.g., NaOH(aq) → Na⁺(aq) + OH¯(aq).
- Alkalis: Bases that are soluble in water are specifically termed alkalis (e.g., NaOH, KOH, Ca(OH)₂). All alkalis are bases, but not all bases are alkalis (e.g., Cu(OH)₂ is insoluble in water).
2. Acid-Base Indicators — Litmus, Synthetic & Olfactory
An indicator is a dye or chemical substance that changes color or odor depending on whether it is placed in an acidic or basic medium.
| Indicator Type | Indicator Name | Color in Acidic Medium | Color in Neutral Medium | Color in Basic Medium |
|---|---|---|---|---|
| Natural | Litmus Paper / Solution | Red | Purple | Blue |
| Natural | Turmeric Paper | Yellow (No Change) | Yellow | Reddish-Brown |
| Natural | Red Cabbage Extract | Red / Pink | Purple | Green / Yellow |
| Synthetic | Phenolphthalein | Colorless | Colorless | Pink |
| Synthetic | Methyl Orange | Red / Pink | Orange | Yellow |
| Olfactory | Onion / Vanilla / Clove Oil | Retains characteristic odor | No change | Loses odor (Odor disappears) |
3. Chemical Properties & Reactions of Acids and Bases
Acid + Active Metal → Salt + Hydrogen Gas (↑)
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)↑ Test for H₂ gas: Brings a burning candle near the mouth of the test tube; it burns with a characteristic 'pop' sound.
2. Reaction with Metal Carbonates & Bicarbonates:
Metal Carbonate/Bicarbonate + Acid → Salt + Water + Carbon Dioxide (↑)
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)↑ NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)↑ Test for CO₂ gas: Turns lime water milky due to formation of insoluble CaCO₃. On passing excess CO₂, milkiness disappears as soluble Ca(HCO₃)₂ is formed.
3. Reaction with Metallic Oxides (Basic Oxides):
Metallic Oxide (Basic) + Acid → Salt + Water
CuO(s) [Black] + 2HCl(aq) → CuCl₂(aq) [Blue-Green] + H₂O(l)
Base + Metal → Salt + Hydrogen Gas (↑)
2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) [Sodium Zincate] + H₂(g)↑
2. Reaction with Non-Metallic Oxides (Acidic Oxides):
Non-Metallic Oxide (Acidic) + Base → Salt + Water
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ [Milky Precipitate] + H₂O(l)
3. Neutralization Reaction (Acid + Base):
Acid + Base → Salt + Water + Heat
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
4. The pH Scale & Strength of Acids/Bases
The pH scale (developed by Sørensen in 1909; p stands for German 'potenz' meaning power) measures the hydrogen ion concentration in a solution on a scale from 0 to 14.
- pH < 7: Acidic solution (Higher [H⁺] concentration)
- pH = 7: Neutral solution ([H⁺] = [OH¯])
- pH > 7: Basic / Alkaline solution (Higher [OH¯] concentration)
Strong vs. Weak Acids and Bases:
- Strong Acids: Completely ionize in water to yield high [H⁺] (e.g., HCl, H₂SO₄, HNO₃).
- Weak Acids: Partially ionize in water (e.g., CH₃COOH, H₂CO₃, Citric acid).
- Strong Bases: Completely ionize to yield high [OH¯] (e.g., NaOH, KOH).
- Weak Bases: Partially ionize in water (e.g., NH₄OH, Mg(OH)₂).
5. Importance of pH in Everyday Life
| Application Area | Scientific Phenomenon & Mechanism | Remedial / Practical Action |
|---|---|---|
| Human Digestive System | Stomach produces HCl (pH ~1.2 to 2.0) for digestion. Overeating causes hyperacidity and irritation. | Take antacids like Milk of Magnesia [Mg(OH)₂] or Baking Soda [NaHCO₃] to neutralize excess acid. |
| Tooth Decay Prevention | Bacteria in mouth produce acids from leftover food sugars. Tooth decay starts when mouth pH falls below 5.5 (enamel calcium phosphate corrodes). | Clean teeth with toothpaste (slightly basic) to neutralize excess acid. |
| Soil pH & Plant Growth | Plants require a specific pH range (close to 7) for optimal nutrient absorption. Excessive fertilizer turns soil too acidic or basic. | If soil is too acidic: add Quicklime (CaO) or Slaked Lime [Ca(OH)₂]. If too basic: add organic matter/manure. |
| Self Defense of Animals & Plants | Bee stings & ant bites inject Methanoic acid (Formic acid) causing pain. Nettle plant leaves have stinging hairs injecting methanoic acid. | Apply mild base like Baking soda paste on bee sting. Dock plant leaf rubbing neutralizes nettle sting. |
| Acid Rain Impact | Rainwater with pH < 5.6 due to SO₂ and NO₂ atmospheric pollution is called acid rain. Lowers river water pH. | Harms aquatic life; calcium carbonate added to lakes to neutralize acidity. |
6. Important Chemical Compounds & Salts
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O Key Uses:
• Bleaching cotton and linen in textile industry.
• Disinfectant for drinking water purification.
• Oxidizing agent in chemical industries.
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ Action of Heat:
2NaHCO₃ + Heat → Na₂CO₃ + H₂O + CO₂↑ Key Uses: Ingredient in baking powder (with tartaric acid), antacid, soda-acid fire extinguishers.
Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O Key Uses:
• Glass, soap, and paper industries.
• Manufacture of sodium compounds like Borax.
• Removing permanent hardness of water.
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O Rehydration (Setting):
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O [Hard Mass] Key Uses: Fractured bone plaster, toys, statues, smooth wall ceiling designs.
7. Water of Crystallization
Water of crystallization is the fixed number of water molecules present in one formula unit of a salt in its crystalline state.
- Hydrated Copper Sulphate: CuSO₄·5H₂O (Blue crystals). When heated, loses water and turns white anhydrous CuSO₄. Adding water restores the blue color!
- Gypsum: CaSO₄·2H₂O (Contains 2 water molecules).
- Washing Soda: Na₂CO₃·10H₂O (Contains 10 water molecules).
8. Solved Board Exam Questions
Options (a) 1, (b) 4, (c) 5 are all acidic (pH < 7). Option (d) 10 is basic.
The reaction produces Sodium Carbonate (Na₂CO₃), which is basic and tastes bitter. Tartaric acid is added to neutralize Na₂CO₃, forming pleasant-tasting sodium tartrate salt and releasing more CO₂ to make cakes fluffy and soft.
It must be stored in a moisture-proof container because in the presence of moisture, it absorbs water and slowly converts into a hard solid mass called Gypsum (CaSO₄·2H₂O), rendering it unusable.
Fizzing will occur much more vigorously in Test Tube A. HCl is a strong acid that completely ionizes to yield a high concentration of H⁺ ions. CH₃COOH is a weak acid that only partially ionizes, resulting in a lower H⁺ ion concentration and a slower reaction rate.
Explore Related CBSE Class 9 & 10 Science Guides
9. Frequently Asked Questions (FAQ)
Acids are substances that release hydrogen ions (H⁺ or H₃O⁺) in aqueous solution, taste sour, turn blue litmus red, and have a pH below 7. Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Citric acid (in lemons), and Acetic acid (in vinegar).
Bases are substances that release hydroxide ions (OH¯) in aqueous solution, taste bitter, feel slippery/soapy, turn red litmus blue, and have a pH above 7. Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), and Calcium hydroxide [Ca(OH)₂]. Water-soluble bases are called alkalis.
A neutralization reaction is a chemical reaction between an acid and a base that react together to form salt and water, accompanied by the evolution of heat.
General Formula: Acid + Base → Salt + Water + Heat
Chemical Equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Ionic Net Equation: H⁺(aq) + OH¯(aq) → H₂O(l)
The pH scale ranges from 0 to 14 and measures the concentration of H⁺ ions in a solution. pH < 7 is acidic, pH = 7 is neutral, and pH > 7 is basic.
• In Digestion: Stomach gastric juice has pH ~1.2–2.0 due to HCl. Excessive acid causes acidity, treated with antacids like Milk of Magnesia [Mg(OH)₂].
• In Tooth Decay: Mouth bacteria break down food sugars into acid. Tooth enamel (calcium phosphate) corrodes when mouth pH falls below 5.5. Toothpastes are basic to neutralize this acid.
Plaster of Paris (POP) is Calcium Sulphate Hemihydrate with the formula CaSO₄·½H₂O.
It is prepared by heating Gypsum (CaSO₄·2H₂O) at 373 K (100°C) in a kiln:
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
When mixed with water, POP rehydrates into a hard solid mass of Gypsum. Used for bone fracture alignment, decorative ceilings, and making statues.
Baking Soda: Sodium Hydrogen Carbonate — NaHCO₃. Prepared via Solvay process: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. Used as antacid and baking powder ingredient.
Washing Soda: Sodium Carbonate Decahydrate — Na₂CO₃·10H₂O. Prepared by recrystallization of sodium carbonate: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. Used in glass/soap industry and removing permanent water hardness.
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