Acids, Bases and Salts for 10th Class Chemistry — Complete Guide
Acids, bases and salts guide for 10th class board exam Pakistan — definitions, properties, reactions, pH scale, indicators, and important examples for SSC chemistry students.
Acids, Bases and Salts for 10th Class Chemistry — Complete Guide
Quick Answer: Acids are substances that donate protons (H⁺ ions) or produce H⁺ in water; bases accept protons or produce OH⁻ ions. The pH scale from 0-14 measures acidity/alkalinity — below 7 is acidic, 7 is neutral, above 7 is alkaline. Neutralization (acid + base → salt + water) is the most important reaction in this chapter.
Acids, bases, and salts is one of the most practically relevant chapters in 10th class chemistry. You encounter acids and bases every day — lemon juice, vinegar, baking soda, and soap are all acids or bases. Understanding this chapter not only helps you in board exams but explains real chemistry you experience daily.
Defining Acids and Bases
There are three definitions of acids and bases used in chemistry. For 10th class, the Arrhenius definition is most important, with Bronsted-Lowry as an extension.
Arrhenius Definition
Acid: A substance that produces hydrogen ions (H⁺) when dissolved in water. Examples: HCl → H⁺ + Cl⁻ (hydrochloric acid dissociates completely) H₂SO₄ → 2H⁺ + SO₄²⁻ (sulfuric acid)
Base: A substance that produces hydroxide ions (OH⁻) when dissolved in water. Examples: NaOH → Na⁺ + OH⁻ (sodium hydroxide — fully dissolves) KOH → K⁺ + OH⁻ (potassium hydroxide)
Bronsted-Lowry Definition (Extended Concept)
Acid: Proton (H⁺) donor Base: Proton (H⁺) acceptor
This is broader — it includes substances like NH₃ (ammonia) that act as bases without having OH⁻.
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (ammonia accepts proton from water → basic solution)
Properties of Acids
Physical properties:
- Often sharp or sour taste (lemon = citric acid, vinegar = acetic acid) — do not taste laboratory acids
- Corrosive — can damage skin and materials
- Conduct electricity in solution (electrolytes)
- Turn blue litmus paper red
Chemical properties:
- React with metals to produce hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂↑
- React with metal oxides to form salt + water: CuO + H₂SO₄ → CuSO₄ + H₂O
- React with carbonates: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
- Neutralize bases: HCl + NaOH → NaCl + H₂O
Common Acids and Their Uses
| Acid | Formula | Source/Use | |------|---------|-----------| | Hydrochloric acid | HCl | Stomach acid, cleaning metals | | Sulfuric acid | H₂SO₄ | Car batteries, fertilizer production (most important industrial acid) | | Nitric acid | HNO₃ | Fertilizers, explosives | | Ethanoic acid | CH₃COOH | Vinegar | | Citric acid | C₆H₈O₇ | Citrus fruits | | Carbonic acid | H₂CO₃ | Fizzy drinks |
Properties of Bases (Alkalis)
Physical properties:
- Bitter taste (do not taste laboratory bases)
- Soapy or slippery feel
- Turn red litmus paper blue
- Conduct electricity in solution
Chemical properties:
- React with acids (neutralization): NaOH + HCl → NaCl + H₂O
- React with ammonium salts to produce ammonia: 2NaOH + (NH₄)₂SO₄ → Na₂SO₄ + 2H₂O + 2NH₃↑
- Alkalis react with certain metal solutions to form precipitates
Alkali vs Base
All alkalis are bases, but not all bases are alkalis.
- Base: Any substance that reacts with an acid to form salt and water
- Alkali: A base that dissolves in water to form OH⁻ ions
Insoluble metal hydroxides (Cu(OH)₂, Fe(OH)₃) are bases but not alkalis.
Common Bases and Their Uses
| Base | Formula | Use | |------|---------|-----| | Sodium hydroxide | NaOH | Soap making, drain cleaner | | Calcium hydroxide | Ca(OH)₂ | Slaked lime, treating acidic soil | | Ammonia solution | NH₃(aq) | Cleaning products, fertilizers | | Magnesium hydroxide | Mg(OH)₂ | Antacid (Milk of Magnesia) | | Sodium carbonate | Na₂CO₃ | Washing soda, glass making | | Sodium bicarbonate | NaHCO₃ | Baking soda, antacid |
The pH Scale
The pH scale measures how acidic or basic a solution is.
pH Range and Meaning
- pH 0-6: Acidic (lower = more acidic; pH 0 is extremely strong acid)
- pH 7: Neutral (pure water)
- pH 8-14: Alkaline/basic (higher = more alkaline; pH 14 is extremely strong base)
pH of Common Substances
| Substance | Approximate pH | |-----------|---------------| | Battery acid (H₂SO₄) | 0-1 | | Lemon juice | 2 | | Vinegar | 3 | | Coffee | 5 | | Pure water | 7 | | Blood | 7.4 | | Baking soda | 8-9 | | Bleach | 12-13 | | Drain cleaner (NaOH) | 13-14 |
pH and H⁺ concentration: Each unit decrease in pH represents a 10-fold increase in H⁺ concentration. pH 4 is 10 times more acidic than pH 5, and 100 times more acidic than pH 6.
Indicators
Indicators change colour to show if a solution is acidic or alkaline.
| Indicator | In Acid | In Neutral | In Alkali | |-----------|---------|-----------|----------| | Litmus | Red | Purple | Blue | | Phenolphthalein | Colourless | Colourless | Pink/red | | Methyl orange | Red | Orange | Yellow | | Universal Indicator | Red → Orange → Yellow | Green | Blue → Purple |
Natural indicators in daily life:
- Red cabbage juice turns red in acid, green in alkali
- Turmeric (haldi) turns red in alkali
- These can be made into simple indicators at home
Neutralization and Salts
Neutralization
Definition: The reaction between an acid and a base to form salt and water. Acid + Base → Salt + Water
This is an exothermic reaction — heat is released.
Key neutralization reactions: HCl + NaOH → NaCl + H₂O (sodium chloride — table salt) H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O (potassium sulfate) HNO₃ + NH₃ → NH₄NO₃ (ammonium nitrate — fertilizer)
Types of Salts
Normal salt: Acid completely neutralized by base. All replaceable H⁺ replaced. Examples: NaCl, K₂SO₄, CaCO₃
Acid salt: Acid partially neutralized. Some H⁺ still present. Examples: NaHCO₃ (sodium hydrogen carbonate), NaHSO₄
Basic salt: Excess base present. Examples: Pb(OH)Cl (basic lead chloride)
Naming Salts
The name of a salt comes from:
- The metal from the base (first word)
- The acid radical (second word: -chloride from HCl, -sulfate from H₂SO₄, -nitrate from HNO₃, -carbonate from H₂CO₃)
Examples: HCl + NaOH → NaCl (sodium chloride); H₂SO₄ + CuO → CuSO₄ (copper sulfate)
Practical Applications in Pakistani Context
Antacids: When stomach produces excess acid (pH drops too low), antacids contain Mg(OH)₂ or Al(OH)₃ to neutralize it. This is why milk, which is slightly alkaline, also relieves acidity.
Agriculture: Pakistani soil in some areas is too acidic or too alkaline. Farmers add lime (calcium hydroxide) to acidic soil and sulfur to alkaline soil to adjust pH for better crops.
Water treatment: Waterworks in Pakistani cities add small amounts of lime to neutralize acidity in drinking water and reduce corrosion of pipes.
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Frequently Asked Questions
Q: What is the difference between strong and weak acids? A: Strong acids (HCl, H₂SO₄, HNO₃) fully dissociate into ions in water. Weak acids (CH₃COOH, citric acid) only partially dissociate. Strong acids have lower pH at the same concentration.
Q: How do I remember which indicator changes to which colour? A: For litmus: Red + Acid = stays Red (RR); Blue + Alkali = stays Blue (BB). Opposite indicates the other. For phenolphthalein: colourless in acid, PINK in alkali (remember: PINK → alkalINE).
Q: What is the most important reaction in this chapter for board exams? A: Neutralization (acid + base → salt + water) is the most versatile concept, appearing in MCQs, definitions, and reactions. Know multiple examples with balanced equations.
Q: How do I test if a solution is acidic or alkaline in the lab? A: Use litmus paper (red = acidic, blue = alkaline) or universal indicator (green = neutral, colours indicate pH). pH meter gives exact measurement.
Q: Is baking soda an acid or a base? A: Baking soda (NaHCO₃, sodium hydrogen carbonate) is a weak base — its solution has pH slightly above 7. This is why it is used to neutralize acidity in cooking and as an antacid.
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