Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chemistry topic in the MP TET Varg-2 Science section. This chapter connects everyday experiences—like the sour taste of lemon, the slippery feel of soap, or the use of antacids—to core chemical concepts. Understanding this topic helps future teachers explain chemical reactions, safety precautions, and real-world applications to upper-primary students.

For the exam, expect questions on identification of acids and bases using indicators, pH scale interpretation, neutralisation reactions, and properties of common salts. The topic frequently appears in both direct knowledge questions and application-based problems involving daily life examples. Mastery requires knowing definitions, distinguishing properties, understanding the pH scale numerically, and recognising common acids, bases and salts used in laboratories and households.

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Key Concepts

• **Acids** are substances that release hydrogen ions (H⁺) when dissolved in water. They taste sour, turn blue litmus red, and react with metals to produce hydrogen gas.

• **Bases** are substances that release hydroxide ions (OH⁻) in water. They taste bitter, feel soapy/slippery, and turn red litmus blue.

• **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).

• **Indicators** are substances that show different colours in acidic and basic solutions. Common indicators include litmus, phenolphthalein, and methyl orange.

• **pH Scale** measures the strength of acidity or basicity on a scale of 0 to 14. pH 7 is neutral; below 7 is acidic; above 7 is basic.

• **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 neutralisation. They consist of a cation from the base and an anion from the acid.

• **Strong vs Weak Acids/Bases**: Strong acids (HCl, H₂SO₄) completely dissociate in water; weak acids (CH₃COOH, H₂CO₃) partially dissociate. Similarly for bases.

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Formulas / Key Facts

| Formula/Fact | Context | |--------------|---------| | pH = -log[H⁺] | Mathematical definition of pH (concept level for upper-primary) | | pH 7 = Neutral | Pure water has pH 7 | | pH < 7 = Acidic | Lower pH means stronger acid | | pH > 7 = Basic | Higher pH means stronger base | | HCl + NaOH → NaCl + H₂O | Classic neutralisation reaction | | 2HCl + Zn → ZnCl₂ + H₂↑ | Acid reacts with metal to release hydrogen gas | | NaHCO₃ + HCl → NaCl + H₂O + CO₂↑ | Baking soda neutralises acid, releases CO₂ | | CaO + H₂O → Ca(OH)₂ | Quick lime reacts with water to form slaked lime |

**Common Acids:**

• Hydrochloric acid (HCl) — found in stomach, used in cleaning
• Sulphuric acid (H₂SO₄) — "king of chemicals," used in batteries
• Acetic acid (CH₃COOH) — present in vinegar
• Citric acid — found in citrus fruits

**Common Bases:**

• Sodium hydroxide (NaOH) — caustic soda, used in soap making
• Calcium hydroxide (Ca(OH)₂) — slaked lime, used in whitewashing
• Magnesium hydroxide (Mg(OH)₂) — milk of magnesia, antacid
• Ammonium hydroxide (NH₄OH) — used in fertilizers

**Common Salts:**

• Sodium chloride (NaCl) — common salt
• Sodium bicarbonate (NaHCO₃) — baking soda
• Sodium carbonate (Na₂CO₃) — washing soda
• Calcium sulphate (CaSO₄·2H₂O) — gypsum, Plaster of Paris

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Worked Examples

### Example 1: Identifying Acid or Base Using Litmus

**Question:** A solution turns blue litmus red but does not change red litmus. What is the nature of the solution?

**Solution:**

• Blue litmus turning red indicates acidic nature
• Red litmus remaining unchanged confirms it is not basic
• **Answer:** The solution is acidic

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### Example 2: pH Scale Interpretation

**Question:** Arrange the following in increasing order of acidic strength: Gastric juice (pH 1.2), Lemon juice (pH 2.4), Tomato (pH 4.2), Pure water (pH 7)

**Solution:**

• Lower pH = stronger acid
• Increasing acidic strength means arranging from highest pH to lowest pH
• **Answer:** Pure water (7) < Tomato (4.2) < Lemon juice (2.4) < Gastric juice (1.2)

Note: The question asks for increasing acidic strength, so the most acidic (lowest pH) comes last.

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### Example 3: Neutralisation Reaction

**Question:** What happens when dilute sulphuric acid reacts with sodium hydroxide? Write the balanced equation and name the salt formed.

**Solution:**

• This is a neutralisation reaction
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• **Salt formed:** Sodium sulphate (Na₂SO₄)
• **Type of reaction:** Neutralisation (exothermic—releases heat)

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Common Mistakes

| Wrong Thinking | Correct Understanding | |----------------|----------------------| | "All bases are alkalis" | Only water-soluble bases are alkalis. Cu(OH)₂ and Fe(OH)₃ are bases but not alkalis because they don't dissolve in water. | | "pH 0 means no acidity" | pH 0 indicates extremely strong acidity, not absence of acid. The scale runs 0–14, with 0 being most acidic. | | "Neutralisation always produces neutral solution (pH 7)" | Not always. Salt of strong acid + weak base gives acidic solution; salt of weak acid + strong base gives basic solution. | | "All acids are dangerous/corrosive" | Weak acids like citric acid (in oranges) and acetic acid (in vinegar) are safe in dilute forms and consumed daily. | | "Indicators only include litmus" | Multiple indicators exist: phenolphthalein (colourless in acid, pink in base), methyl orange (red in acid, yellow in base), turmeric (yellow in acid, reddish-brown in base). |

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Quick Reference

• **Acid → H⁺ ions; Base → OH⁻ ions; Salt → formed by neutralisation**

• **pH Scale: 0–14; pH 7 neutral; <7 acidic; >7 basic**

• **Litmus test: Blue→Red = Acid; Red→Blue = Base**

• **Neutralisation: Acid + Base → Salt + Water (exothermic)**

• **Turmeric is a natural indicator: yellow in acid/neutral, reddish-brown in base**

• **Antacids (Mg(OH)₂) neutralise excess stomach acid (HCl)**

• **Baking soda (NaHCO₃) used in fire extinguishers—releases CO₂ with acid**