MP Board Class 10 Science Chapter 2: Acids, Bases & Salts Notes 2027 — Properties, pH Scale, Indicators & PYQs
Chapter 2: Acids, Bases & Salts is a high-weightage chapter in MP Board Class 10 Science, contributing 10–12 marks in the annual board exam. This chapter covers the classification of substances as acidic or basic, the pH scale, indicators, reaction of acids and bases with metals, carbonates, and each other (neutralization), and the preparation and uses of important salts. Questions range from 1-mark MCQs to 5-mark comprehensive answers. Understanding the reaction patterns and salt preparation methods is essential for board success.
📑 Table of Contents
🧪 Properties of Acids vs Bases
Acids and bases are two fundamental categories of chemical substances with opposing properties. Understanding their differences is critical for predicting chemical reactions and their applications in daily life.
📊 Comparison Table: Acids vs Bases
🔬 Reaction of Acids with Metals
Acids react with active metals (like Zn, Fe, Mg, Al) to produce hydrogen gas and a salt.
- Zn + 2HCl → ZnCl₂ + H₂↑ (Zinc reacts with hydrochloric acid)
- Mg + H₂SO₄ → MgSO₄ + H₂↑ (Magnesium reacts with sulphuric acid)
- 2Al + 6HCl → 2AlCl₃ + 3H₂↑ (Aluminium reacts with hydrochloric acid)
⚗️ Reaction of Acids with Metal Carbonates & Bicarbonates
Acids react with carbonates and bicarbonates to produce carbon dioxide gas, water, and a salt.
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ (Limestone reacts with HCl)
- NaHCO₃ + HCl → NaCl + H₂O + CO₂↑ (Baking soda reacts with HCl)
- Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ (Washing soda reacts with HCl)
🤝 Neutralization Reaction (Acid + Base)
When an acid and a base react, they neutralize each other to form a salt and water. This is called a neutralization reaction.
- HCl + NaOH → NaCl + H₂O (Hydrochloric acid + Sodium hydroxide)
- H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O (Sulphuric acid + Sodium hydroxide)
- HCl + NH₄OH → NH₄Cl + H₂O (Hydrochloric acid + Ammonium hydroxide)
📘 Everyday Example — Antacids
When you have acidity (excess HCl in the stomach), you take antacids like Milk of Magnesia (Mg(OH)₂) or Eno (NaHCO₃). The base neutralizes excess acid: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
🧂 Reaction of Acids & Bases — Summary Flow
⚗️ Reaction of Base with Metals
Some bases (alkalis) react with certain reactive metals like Zn and Al to produce hydrogen gas.
- 2NaOH + Zn → Na₂ZnO₂ + H₂↑ (Sodium zincate is formed)
- 2NaOH + 2Al + 2H₂O → 2NaAlO₂ + 3H₂↑ (Sodium aluminate is formed)
📊 pH Scale with Examples
The pH scale (potential of Hydrogen) is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14, where 7 represents a neutral solution.
- pH < 7: Acidic solution — lower the pH, stronger the acid
- pH = 7: Neutral solution (pure water)
- pH > 7: Basic (alkaline) solution — higher the pH, stronger the base
🧪 pH Values of Common Substances
🔬 Importance of pH in Daily Life
- Human Body: Blood pH must remain around 7.35–7.45. Stomach pH (~1.5–3.5) helps digestion. pH imbalance can cause health issues.
- Plants: Most plants grow best at soil pH 6.5–7.5. Acidic soil is treated with lime (CaO).
- Digestion: Our stomach produces HCl for digestion. Excess causes acidity, treated with antacids (bases).
- Tooth Decay: Bacteria produce acids (pH < 5.5) that demineralize tooth enamel. Toothpaste (basic) neutralizes the acid.
- Bee Sting: Bee sting injects formic acid (acidic) — treated with baking soda (base).
- Wasp Sting: Wasp sting is alkaline — treated with vinegar (acid).
📘 The pH Formula
The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = –log[H⁺]. For example, if [H⁺] = 10⁻³ M, pH = 3. A ten-fold change in [H⁺] changes pH by 1 unit.
🎨 Indicators (Litmus, Phenolphthalein, Methyl Orange)
Indicators are substances that change colour in acidic or basic media. They help us determine whether a substance is acidic or basic without tasting or touching it.
🔵🔴 Types of Indicators
🧪 Natural Indicators
- Litmus: Extracted from lichens. Blue litmus turns red in acid; red litmus turns blue in base.
- Turmeric: Yellow in acid/neutral, turns red in base.
- Red Cabbage: Contains anthocyanin — red in acid, greenish-yellow in base.
- China Rose (Hibiscus): Dark magenta in acid, bright green in base.
- Methyl Orange: Synthetic indicator — red in acid, yellow in base.
📋 Indicator Colour Chart
🧪 pH Indicators — Universal Indicator
A universal indicator is a mixture of several indicators that gives a distinct colour for each pH value. By comparing the colour with a standard pH colour chart, the approximate pH of a solution can be determined.
- Red → Strongly acidic (pH 1–2)
- Orange/Yellow → Moderately acidic (pH 3–5)
- Green → Neutral (pH 7)
- Blue → Moderately basic (pH 8–10)
- Violet → Strongly basic (pH 11–14)
🧪 Olfactory Indicators
Substances whose smell changes in acidic or basic media are called olfactory indicators.
- Onion: Characteristic smell vanishes when treated with a base.
- Vanilla extract: Smell vanishes when treated with a base.
🧂 Common Salts — Preparation & Uses
A salt is an ionic compound formed by the neutralization reaction between an acid and a base. Salts consist of positive ions (cations) from the base and negative ions (anions) from the acid.
1️⃣ Sodium Chloride (NaCl) — Common Salt
Preparation
- Obtained by evaporation of seawater in salt pans
- Also mined from underground salt deposits (rock salt)
- Impure NaCl is dissolved in water, filtered, and recrystallized to get pure NaCl
Uses
- Essential seasoning in food (table salt)
- Preservation of fish, meat, and pickles
- Raw material for manufacturing Na₂CO₃, NaOH, Cl₂, HCl
- Used in the chlor-alkali process to manufacture NaOH and Cl₂
⚡ Chlor-Alkali Process
When electricity is passed through an aqueous solution of NaCl (brine), it decomposes to form NaOH, Cl₂, and H₂.
This is an important industrial process and is frequently asked in MP Board exams.
2️⃣ Sodium Bicarbonate (NaHCO₃) — Baking Soda
Preparation
Prepared by Solvay process. A cold and concentrated solution of sodium chloride is treated with ammonia and carbon dioxide:
- NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
- NaHCO₃ is sparingly soluble in water, so it precipitates out
Uses
- Used in baking powder (NaHCO₃ + tartaric acid) — produces CO₂ that makes cakes and bread fluffy
- Used as an antacid to neutralize excess stomach acid
- Used in fire extinguishers — produces CO₂ gas when heated
- Used in cleaning and as a mild disinfectant
🔥 Decomposition of Baking Soda
When heated, baking soda decomposes to produce CO₂:
3️⃣ Bleaching Powder (CaOCl₂) — Calcium Oxychloride
Preparation
Prepared by passing chlorine gas over dry slaked lime (Calcium hydroxide):
Conditions: Dry slaked lime, low temperature
Uses
- Used for bleaching cotton, linen, wood pulp in paper industry
- Used as a disinfectant for water purification and sanitation
- Used in the manufacture of chloroform (CHCl₃)
- Used for bleaching in textile and laundry industries
4️⃣ Gypsum (CaSO₄·2H₂O) — Calcium Sulphate Dihydrate
Preparation of Plaster of Paris
Plaster of Paris (CaSO₄·½H₂O) is a white powder that sets into a hard mass when mixed with water.
Uses of Gypsum
- Used in the manufacture of cement (retards setting time)
- Used as a fertilizer (source of calcium and sulphur)
- Used in making chalk and blackboard
- Used in plastering walls and ceilings
Uses of Plaster of Paris
- Making statues, toys, and decorative items
- Used in orthopaedics for setting fractured bones
- Used in construction for false ceilings and decorative mouldings
5️⃣ Sodium Carbonate (Na₂CO₃·10H₂O) — Washing Soda
Preparation
Prepared by heating baking soda (NaHCO₃), followed by recrystallization:
- Step 1: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑ (heating)
- Step 2: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (recrystallization from water)
Uses
- Used as a cleaning agent (washing soda) for laundry
- Used in the manufacture of glass, soap, and paper
- Used for removing permanent hardness of water
- Used in the Solvay process for manufacturing NaHCO₃
📊 Summary Table: Common Salts
📝 Practice Questions (With Answers)
- Q1: What happens when a piece of zinc metal is dropped into dilute hydrochloric acid? Write the balanced chemical equation and name the gas evolved.
- Q2: Two solutions A and B have pH values of 3 and 11 respectively. Which of these is acidic and which is basic? Calculate the hydrogen ion concentration of each.
- Q3: Explain the preparation of bleaching powder with a balanced chemical equation. Write any two uses of bleaching powder.
- Q4: What is the difference between baking soda (NaHCO₃) and washing soda (Na₂CO₃·10H₂O)? How can baking soda be converted into washing soda?
- Q5: Why does blue litmus turn red when dipped in vinegar? Explain using the concept of acids and indicators.
✅ Answer Key
- A1: Zinc reacts with HCl to produce zinc chloride and hydrogen gas.
Zn + 2HCl → ZnCl₂ + H₂↑
Gas: Hydrogen (H₂). It burns with a pop sound near a flame. - A2: Solution A (pH 3) is acidic. Solution B (pH 11) is basic.
[H⁺] for A = 10⁻³ M, [H⁺] for B = 10⁻¹¹ M. - A3: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Uses: (i) Bleaching cotton and linen, (ii) Disinfecting drinking water.
- A4: NaHCO₃ is baking soda (mild base, used in food). Na₂CO₃·10H₂O is washing soda (strong base, used for cleaning).
Conversion: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑ (heating), followed by Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. - A5: Vinegar contains acetic acid (CH₃COOH). Acids release H⁺ ions in solution which react with the colouring agent in blue litmus, turning it red.