Acid, Base and Salt – TNPSC Study Notes (Samacheer Kalvi Based)

Introduction

Acids, Bases and Salts form one of the most important chapters in the Chemistry section of TNPSC exams. This topic is drawn primarily from Samacheer Kalvi Class 10 Science (Chapter on Acids, Bases and Salts) with foundational concepts also introduced in Class 8 and 9 Science. TNPSC regularly asks direct factual questions, pH-based conceptual questions, and everyday-life application questions from this topic, making it a high-scoring area if prepared systematically.

Basic Concepts

Acids, bases and salts are classified based on their chemical behaviour when dissolved in water. Acids release hydrogen ions (H+), bases release hydroxide ions (OH−), and salts are formed when an acid reacts with a base in a neutralisation reaction. Understanding their properties, classification and reactions is essential for TNPSC preparation.

Important Definitions

Detailed Concept Explanation

1. Acids

Acids are compounds that produce H+ ions in aqueous solution. They have a sour taste, conduct electricity in solution, react with metals to produce hydrogen gas, and turn blue litmus paper red.

Types of Acids

2. Bases

Bases are compounds that produce OH− ions in aqueous solution. They taste bitter, feel soapy to touch, react with acids to form salt and water, and turn red litmus blue. Water-soluble bases are called alkalis.

Common Bases

• Sodium hydroxide (NaOH) – caustic soda
• Potassium hydroxide (KOH) – caustic potash
• Calcium hydroxide (Ca(OH)2) – slaked lime
• Ammonium hydroxide (NH4OH)
• Magnesium hydroxide (Mg(OH)2) – used in antacids (Milk of Magnesia)

3. The pH Scale

The pH scale ranges from 0 to 14 and measures the hydrogen ion concentration of a solution.

TNPSC Important: pH was introduced by Danish chemist Søren Sørensen in 1909. "p" stands for "potenz" (power) and "H" for hydrogen ion concentration.

4. Indicators

Litmus is a natural indicator obtained from lichen. Other natural indicators include turmeric, red cabbage extract and China rose (Gudhal) extract.

5. Salts

Salts are formed when an acid reacts completely with a base. This is called a neutralisation reaction: Acid + Base → Salt + Water.

Types of Salts

6. Important Chemical Reactions

• Acid + Metal → Salt + Hydrogen gas (e.g., Zn + 2HCl → ZnCl2 + H2)
• Acid + Metal carbonate → Salt + Water + Carbon dioxide
• Acid + Base → Salt + Water (neutralisation)
• Base + Metal (some) → Salt + Hydrogen gas

7. Common Salts and Their Uses (Chemistry in Everyday Life)

Important Facts for TNPSC

• The pH scale was devised by Søren Sørensen (1909).
• Antacids like Milk of Magnesia (Mg(OH)2) neutralise excess stomach acid (HCl).
• Tooth decay begins when the pH of the mouth falls below 5.5 due to acid formed by bacteria.
• Ant sting and nettle sting contain formic acid; a mild base like baking soda is applied to neutralise it.
• Rain with pH below 5.6 is called acid rain, caused mainly by sulphur dioxide and nitrogen oxides.
• Plaster of Paris is obtained by heating gypsum at 373 K.
• Baking soda is made using the Solvay process (sodium chloride, ammonia and carbon dioxide).

Common Mistakes Aspirants Make

• Confusing washing soda (Na2CO3) with baking soda (NaHCO3) — they have different formulas and uses.
• Assuming all salts are neutral; acidic and basic salts also exist (e.g., NH4Cl is acidic, Na2CO3 is basic).
• Mixing up litmus colour change direction — remembering "acid turns blue litmus red" is often reversed by mistake.
• Confusing pH value direction — lower pH means stronger acid, not weaker.
• Mixing up gypsum and Plaster of Paris formulas (difference lies in water of crystallisation).

Important TNPSC Tips

• Memorise the formulas and common names of everyday salts (baking soda, washing soda, bleaching powder, gypsum, Plaster of Paris) — these are frequently tested.
• Focus on the pH scale and indicator colour changes, as TNPSC often asks direct and statement-based questions on these.
• Understand the logic behind acid-base reactions (neutralisation) rather than memorising blindly, since TNPSC sometimes frames application-based questions (e.g., why toothpaste is basic).

TNPSC Important: Toothpaste is basic in nature. It neutralises the excess acid produced by bacteria in the mouth, which is why using toothpaste helps prevent tooth decay.

TNPSC Model Questions

Q1. Which of the following has the lowest pH value?

1. Pure water
2. Lemon juice
3. Sodium hydroxide solution
4. Baking soda solution

Answer: B) Lemon juice
Explanation: Lemon juice contains citric acid, a strong organic acid, giving it a low pH (around 2), whereas pure water is neutral (pH 7) and NaOH/baking soda are basic (pH above 7).

Q2. Consider the following statements about Plaster of Paris:

1. It is obtained by heating gypsum at 373 K.
2. Its chemical formula is CaSO4.2H2O.

Which of the statements given above is/are correct?

1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2

Answer: A) 1 only
Explanation: Plaster of Paris is CaSO4.½H2O, while CaSO4.2H2O is gypsum. Statement 1 is correct; statement 2 describes gypsum, not Plaster of Paris.

Q3. Match List I with List II and select the correct answer:

(TNPSC-style question pattern based on indicator behaviour)

Answer: Both pairs are correctly matched — phenolphthalein is colourless in acid, and methyl orange turns red/pink in acid.

Q4. Baking soda is chemically known as:

1. Sodium carbonate
2. Sodium bicarbonate
3. Calcium carbonate
4. Sodium hydroxide

Answer: B) Sodium bicarbonate (NaHCO3)
Explanation: Sodium carbonate (Na2CO3) is washing soda, a commonly confused option.

Q5. Acid rain is primarily caused by which of the following gases?

1. Carbon monoxide and methane
2. Sulphur dioxide and nitrogen oxides
3. Oxygen and nitrogen
4. Hydrogen and helium

Answer: B) Sulphur dioxide and nitrogen oxides
Explanation: These gases combine with atmospheric moisture to form sulphuric and nitric acids, lowering rainwater pH below 5.6.

Quick Revision

• Acid: releases H+ ions; turns blue litmus red; pH less than 7.
• Base: releases OH− ions; turns red litmus blue; pH greater than 7.
• Neutral: pH = 7 (e.g., pure water).
• Neutralisation: Acid + Base → Salt + Water.
• Baking soda = NaHCO3; Washing soda = Na2CO3.10H2O.
• Plaster of Paris = CaSO4.½H2O; Gypsum = CaSO4.2H2O.
• Bleaching powder = CaOCl2.
• pH scale devised by Søren Sørensen (1909).
• Acid rain: pH below 5.6, caused by SO2 and NOx.

TNPSC Exam Strategy

This topic is largely factual and definition-based, so focus first on memorising the common names, chemical formulas, and uses of everyday salts (baking soda, washing soda, bleaching powder, Plaster of Paris, gypsum), as these are the most frequently tested items. Next, build conceptual clarity on the pH scale and indicator colour changes, since TNPSC often frames statement-based and match-the-following questions here. Finally, revise real-life applications (antacids, tooth decay, acid rain) as these connect chemistry to general awareness and are commonly tested through application-based questions. Avoid rote memorisation without understanding the underlying acid-base logic, as this leads to errors in statement-based questions.