Acids, Bases and Salts

Overview

Acids, bases and salts form the backbone of chemical understanding in upper-primary science. This topic connects everyday observations—sour taste of lemon, slippery feel of soap, table salt in food—to fundamental chemistry concepts. For JTET Paper II, questions typically test identification of acidic and basic substances, pH scale interpretation, neutralisation reactions and common salts with their uses.

Mastery here requires understanding the defining properties of acids and bases, the pH scale as a quantitative measure of acidity/alkalinity, and how neutralisation produces salts. Expect direct questions on indicators, pH values of common substances, chemical formulae of important acids/bases/salts, and their applications in daily life and industry. The tribal and rural context of Jharkhand makes questions on agricultural uses (soil pH, fertilisers) particularly relevant.

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.

• **Bases** are substances that release hydroxide ions (OH⁻) in water, taste bitter, feel slippery, and turn red litmus blue. Water-soluble bases are called **alkalis**.

• **pH scale** measures the concentration of H⁺ ions on a scale of 0 to 14. pH 7 is neutral; below 7 is acidic; above 7 is basic.

• **Indicators** are substances that show different colours in acidic and basic solutions. Litmus, phenolphthalein, methyl orange and turmeric are common indicators.

• **Neutralisation** is the reaction between an acid and a base to form salt and water: Acid + Base → Salt + Water.

• **Salts** are ionic compounds formed by replacing the hydrogen of an acid with a metal or ammonium ion. They have varied properties depending on the parent acid and base.

• **Strong acids/bases** ionise completely in water (HCl, NaOH); **weak acids/bases** ionise partially (acetic acid, NH₄OH).

• The **pH of soil** affects plant growth—most crops prefer pH 6 to 7. Acidic soil is treated with lime (calcium oxide); basic soil with organic matter.

Formulas / Key Facts

**Important Acids:** | Acid | Formula | Source/Use | |------|---------|------------| | Hydrochloric acid | HCl | Gastric juice, cleaning | | Sulphuric acid | H₂SO₄ | King of chemicals, batteries | | Nitric acid | HNO₃ | Fertilisers, explosives | | Acetic acid | CH₃COOH | Vinegar | | Citric acid | C₆H₈O₇ | Citrus fruits |

**Important Bases:** | Base | Formula | Use | |------|---------|-----| | Sodium hydroxide | NaOH | Soap making, caustic soda | | Calcium hydroxide | Ca(OH)₂ | Whitewashing, slaked lime | | Ammonium hydroxide | NH₄OH | Cleaning agents | | Magnesium hydroxide | Mg(OH)₂ | Antacid (milk of magnesia) |

**Important Salts:** | Salt | Formula | Use | |------|---------|-----| | Sodium chloride | NaCl | Table salt, food preservation | | Sodium bicarbonate | NaHCO₃ | Baking soda, antacid | | Sodium carbonate | Na₂CO₃ | Washing soda, glass making | | Calcium carbonate | Cite | Chalk, marble, cement |

**Key Reactions:**

• Neutralisation: HCl + NaOH → NaCl + H₂O
• Metal + Acid: Zn + 2HCl → ZnCl₂ + H₂↑
• Carbonate + Acid: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

**pH Values to Remember:**

• Gastric juice: 1–2 (strongly acidic)
• Lemon juice: 2.2
• Vinegar: 2.5–3
• Pure water: 7 (neutral)
• Blood: 7.4 (slightly basic)
• Milk of magnesia: 10
• Soap solution: 9–10

Worked Examples

**Example 1:** A solution turns blue litmus red and has pH = 3. Identify if it is acidic or basic. Name one such substance.

*Solution:*

• Blue litmus turning red indicates an acid.
• pH = 3 is less than 7, confirming acidic nature.
• Example: Vinegar (acetic acid) or lemon juice.

**Example 2:** Write the balanced equation when dilute sulphuric acid reacts with sodium hydroxide.

*Solution:*

• This is a neutralisation reaction.
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• Products: Sodium sulphate (salt) and water.

**Example 3:** A farmer finds his soil has pH = 4.5. What treatment should he apply?

*Solution:*

• pH 4.5 is acidic (below 7).
• Acidic soil is neutralised by adding a base.
• Treatment: Add quicklime (CaO) or slaked lime (Ca(OH)₂) to raise pH toward neutral.

Common Mistakes

• **Confusing pH direction** → Students think higher pH means more acidic. *Correct:* Lower pH = more acidic; higher pH = more basic. pH 2 is more acidic than pH 5.

• **Mixing up litmus colour changes** → Forgetting which litmus changes in which solution. *Correct:* "Acid turns blue litmus red" — remember "A-B-R" (Acid, Blue to Red).

• **Assuming all bases are alkalis** → Not all bases dissolve in water. *Correct:* Only water-soluble bases (like NaOH, KOH) are called alkalis. Cu(OH)₂ is a base but not an alkali.

• **Writing salt formula incorrectly** → Students write NaCl₂ instead of NaCl. *Correct:* Balance charges—Na⁺ combines with Cl⁻ in 1:1 ratio to give NaCl.

• **Ignoring dilution before use** → Forgetting that concentrated acids must always be diluted by adding acid to water, not water to acid. *Correct:* Add acid to water slowly with stirring to avoid violent reaction.

Quick Reference

• pH < 7 = Acidic | pH = 7 = Neutral | pH > 7 = Basic

• Acid + Base → Salt + Water (Neutralisation)

• Blue litmus → Red = Acid | Red litmus → Blue = Base

• Baking soda (NaHCO₃) is used as antacid because it neutralises excess stomach acid

• Strong acid + Strong base = Neutral salt (pH ≈ 7)

• Turmeric turns red in basic solution—used as natural indicator in Indian kitchens