Chemistry — Acids, Bases and Salts
Overview
Acids, Bases and Salts form a foundational chapter in upper-primary chemistry, appearing consistently in KAR TET Paper II. This topic bridges everyday experiences (lemon sourness, soap slipperiness, baking soda in cooking) with core chemical concepts. Questions typically test identification of acids and bases using indicators, understanding neutralisation reactions, and recognising common salts along with their uses.
For the TET examination, mastery requires three things: recognising the characteristic properties that distinguish acids from bases, understanding how indicators work to identify these substances, and knowing the products and applications of neutralisation reactions. The topic also connects to environmental science (acid rain, soil pH) and biology (digestion, pH of blood), making it a cross-curricular favourite for examiners.
Students must move beyond rote memorisation to understand why acids taste sour, why bases feel soapy, and why neutralisation produces salt and water. This conceptual clarity helps answer application-based questions that form a significant portion of TET science pedagogy.
Key Concepts
- Acids are substances that release hydrogen ions (H⁺) in water, taste sour, turn blue litmus red, and react with metals to produce hydrogen gas. Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), citric acid, acetic acid (vinegar).
- Bases are substances that release hydroxide ions (OH⁻) in water, taste bitter, feel soapy/slippery, and turn red litmus blue. Examples: sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂), magnesium hydroxide.
- Alkalis are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis (e.g., copper hydroxide is a base but not an alkali as it is insoluble).
- Indicators are substances that show different colours in acidic and basic solutions. Natural indicators include litmus, turmeric, red cabbage juice, and china rose. Synthetic indicators include phenolphthalein and methyl orange.
- pH Scale measures the strength of acids and bases on a scale of 0–14. pH < 7 indicates acid, pH = 7 indicates neutral, pH > 7 indicates base. Lower pH means stronger acid; higher pH means stronger base.
- Neutralisation is the reaction between an acid and a base to form salt and water: Acid + Base → Salt + Water. This reaction is exothermic (releases heat).
- Salts are ionic compounds formed by neutralisation. They consist of a cation from the base and an anion from the acid. Salts can be acidic, basic, or neutral depending on the parent acid and base.
- Strong vs Weak acids/bases: Strong acids (HCl, H₂SO₄, HNO₃) dissociate completely in water. Weak acids (acetic acid, carbonic acid) dissociate partially. Similarly for bases—NaOH is strong; NH₄OH is weak.
Formulas / Key Facts
| Fact/Formula | Context |
|---|---|
| Acid + Base → Salt + Water | General neutralisation reaction |
| Acid + Metal → Salt + Hydrogen gas | E.g., Zn + H₂SO₄ → ZnSO₄ + H₂↑ |
| Acid + Metal carbonate → Salt + Water + CO₂ | E.g., CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ |
| Acid + Metal bicarbonate → Salt + Water + CO₂ | E.g., NaHCO₃ + HCl → NaCl + H₂O + CO₂↑ |
| pH of pure water = 7 | Neutral reference point |
| pH of gastric juice ≈ 1–2 | Strongly acidic |
| pH of blood ≈ 7.35–7.45 | Slightly basic |
| Litmus: Red in acid, Blue in base | Most common indicator test |
| Turmeric: Yellow in acid/neutral, Red-brown in base | Natural indicator from kitchen |
| Phenolphthalein: Colourless in acid, Pink in base | Synthetic indicator |
Common Salts and Uses:
- Sodium chloride (NaCl) — table salt, food preservation
- Sodium bicarbonate (NaHCO₃) — baking soda, antacid
- Sodium carbonate (Na₂CO₃) — washing soda, glass manufacture
- Calcium sulphate hemihydrate (CaSO₄·½H₂O) — Plaster of Paris, casts
- Bleaching powder (CaOCl₂) — disinfection, bleaching
Worked Examples
Example 1: Indicator-based identification
A student tests an unknown solution with litmus paper. Blue litmus turns red; red litmus remains red. What is the nature of the solution?
Solution:
- Blue litmus turning red indicates an acidic solution.
- Red litmus remaining red confirms it is not basic.
- Therefore, the solution is acidic.
Example 2: Writing a neutralisation equation
Write the balanced equation when sodium hydroxide reacts with hydrochloric acid.
Solution:
- Reactants: NaOH (base) + HCl (acid)
- Products: Salt (NaCl) + Water (H₂O)
- Balanced equation: NaOH + HCl → NaCl + H₂O
Example 3: Identifying pH
Arrange the following in increasing order of pH: gastric juice, pure water, blood, lemon juice.
Solution:
- Gastric juice: pH ≈ 1–2 (most acidic)
- Lemon juice: pH ≈ 2–3
- Pure water: pH = 7 (neutral)
- Blood: pH ≈ 7.4 (slightly basic)
- Increasing order: Gastric juice < Lemon juice < Pure water < Blood
Common Mistakes
- Confusing "all bases are alkalis" → Correction: Only water-soluble bases are alkalis. Insoluble bases like Fe(OH)₃ are not alkalis.
- Thinking neutral pH means "no ions" → Correction: Neutral pH (7) means equal concentration of H⁺ and OH⁻ ions, not absence of ions.
- Assuming all salts are neutral → Correction: Salts of strong acid + weak base are acidic (e.g., NH₄Cl); salts of weak acid + strong base are basic (e.g., Na₂CO₃).
- Believing litmus paper measures exact pH → Correction: Litmus only indicates acidic or basic nature, not the exact pH value. pH paper or universal indicator shows the numerical pH.
- Writing hydrogen gas as H instead of H₂ → Correction: Hydrogen exists as diatomic molecule H₂ in gaseous state.
Quick Reference
- Acids = sour, H⁺ donors, turn blue litmus red
- Bases = bitter, OH⁻ donors, turn red litmus blue
- pH scale: 0–14; below 7 = acid, 7 = neutral, above 7 = base
- Neutralisation: Acid + Base → Salt + Water (always)
- Baking soda (NaHCO₃) = antacid; Washing soda (Na₂CO₃) = cleaning
- Turmeric turns red-brown in bases — quick kitchen test for alkalinity