Acids, Bases and Salts (Class 10): Notes और MCQ Practice

आसान भाषा में:

Acids वो substances होते हैं जो स्वाद में खट्टे (sour) होते हैं और water में H⁺ ions रिलीज़ करते हैं। Bases स्वाद में कड़वे (bitter), छूने में soapy होते हैं और OH⁻ ions देते हैं। जब Acid और Base आपस में react करते हैं, तो Neutralisation reaction होती है और Salt + Water बनता है।

Important Points & Chemical Formulas:

• Indicators: Blue Litmus Acid में Red हो जाता है; Red Litmus Base में Blue हो जाता है। Phenolphthalein Acid में colourless और Base में pink रहता है।
• Acid + Metal: Acid + Metal → Salt + H₂ gas (Hydrogen gas 'pop' sound के साथ जलती है)।
• Neutralisation: Acid + Base → Salt + H₂O (e.g., HCl + NaOH → NaCl + H₂O).
• pH Scale (0 to 14): pH < 7 (Acidic), pH = 7 (Neutral), pH > 7 (Basic). pH कम मतलब stronger acid।
• Common Salts & Formulas:
• Bleaching Powder: CaOCl₂ (Dry slaked lime Ca(OH)₂ + Cl₂)
• Baking Soda: NaHCO₃ (Sodium hydrogen carbonate)
• Washing Soda: Na₂CO₃.10H₂O
• Plaster of Paris (POP): CaSO₄.½H₂O (Gypsum: CaSO₄.2H₂O को 373 K पर heat करने से बनता है)

2 Memory Tricks:

1. BAR Trick (Litmus): Blue to Red = Acid (Red means danger/acid!). याद रखें: Base turns Blue.
2. POP vs Gypsum: Plaster of Paris में 'हाफ' (½) water molecule होता है (चोट लगने पर half-bandage लगती है), Gypsum में पूरे 2 water molecules होते हैं।

2 Common Board-Exam Mistakes:

1. Dilution of Acid: Acid में कभी भी पानी नहीं डालते (violently splash हो सकता है)। हमेशा Water में drop-by-drop Acid डालते हैं with constant stirring.
2. Dry HCl Gas Test: Dry HCl gas dry litmus paper का colour change नहीं करती, क्योंकि H⁺ ions सिर्फ water/moisture की presence में ही produce होते हैं।

Acids, Bases and Salts - Class 10 NCERT Study Notes

1. Chapter in Short

This chapter introduces the chemical nature of substances around us by classifying them into acids, bases, and salts. You will learn:

• How to identify acids and bases using natural and synthetic indicators.
• How acids and bases react with metals, carbonates, and each other.
• Why acids and bases conduct electricity only in water (presence of ions).
• What the pH scale is and why maintaining the right pH is crucial for our body, plants, and teeth.
• How everyday chemical compounds—like caustic soda, bleaching powder, baking soda, washing soda, and Plaster of Paris—are prepared from common salt and used in daily life.

2. Key Concepts Explained Simply

A. What are Acids and Bases?

• Acids: Sour in taste, turn blue litmus red, and release Hydrogen ions (H+ or H3O+) in water.

Everyday examples: Lemon juice (citric acid), vinegar (acetic acid), curd (lactic acid), tamarind (tartaric acid).

• Bases: Bitter in taste, soapy to touch, turn red litmus blue, and release Hydroxide ions (OH-) in water. Water-soluble bases are called alkalis.

Everyday examples: Soap solution, baking soda solution, whitewash (slaked lime).

B. Indicators

Substances that change colour or smell to show whether a solution is acidic or basic.

1. Natural Indicators: Litmus (purple dye from lichen), turmeric (turns reddish-brown in bases).
2. Synthetic Indicators: Phenolphthalein (colourless in acid, pink in base), Methyl orange (red in acid, yellow in base).
3. Olfactory Indicators: Substances whose smell changes in acidic or basic media, e.g., onion juice, vanilla extract, clove oil (lose their smell in bases).

C. Key Chemical Reactions

1. Acid + Metal -> Salt + Hydrogen gas

• Example: Zinc + Dilute Sulphuric acid -> Zinc sulphate + Hydrogen gas
• Test for H2 gas: Bring a burning candle near the mouth of the test tube; the gas burns with a 'pop' sound.

1. Base + Metal -> Salt + Hydrogen gas

• Example: 2NaOH + Zn -> Na2ZnO2 (Sodium zincate) + H2
• Note: Such reactions do not occur with all metals.

1. Acid + Metal Carbonate / Hydrogen Carbonate -> Salt + Carbon dioxide + Water

• Test for CO2 gas: Pass it through lime water; it turns milky due to insoluble calcium carbonate (CaCO3). On passing excess CO2, milkiness disappears because soluble calcium hydrogen carbonate Ca(HCO3)2 is formed.

1. Acid + Base -> Salt + Water (Neutralisation Reaction)

• Acid and base cancel each other's effects.
• Example: NaOH + HCl -> NaCl + H2O

1. Metallic Oxides and Non-Metallic Oxides:

• Metal oxides are basic (e.g., CuO + 2HCl -> CuCl2 + H2O; solution turns blue-green).
• Non-metal oxides are acidic (e.g., Ca(OH)2 + CO2 -> CaCO3 + H2O).

D. What Happens in Water Solution?

• Acids produce H+ ions only in the presence of water. Since a bare H+ cannot exist alone, it joins water to form hydronium ion (H3O+).
• Dry HCl gas does not change the colour of dry litmus paper because no H+ ions are produced without water.
• Dilution: Mixing an acid with water is highly exothermic (generates intense heat). Always add acid slowly to water with constant stirring, never water to concentrated acid!

E. The pH Scale and Everyday Life

• The pH scale measures hydrogen ion concentration from 0 (very acidic) to 14 (very basic). Neutral solutions have a pH of 7.
• Our Body: Works within a narrow pH range of 7.0 to 7.8.
• Acid Rain: When rain water has a pH less than 5.6.
• Digestion: Our stomach produces hydrochloric acid (HCl). Excess acid causes acidity; antacids like magnesium hydroxide (Milk of Magnesia) neutralise it.
• Tooth Decay: Starts when the pH in the mouth falls below 5.5. Bacteria produce acids from leftover food sugars. Toothpaste (which is basic) neutralises this acid.
• Stings: Honeybee and nettle plant leaves inject methanoic acid (causes burning pain). Rubbing a mild base like baking soda or dock plant leaf gives relief.

F. Important Salts from Common Salt (NaCl)

1. Sodium Hydroxide (NaOH): Made by the Chlor-alkali process (electrolysis of brine/aqueous NaCl). Gives NaOH at cathode, Cl2 gas at anode, and H2 gas at cathode.
2. Bleaching Powder (CaOCl2): Produced by the action of chlorine on dry slaked lime [Ca(OH)2]. Used for bleaching cotton, wood pulp, and disinfecting drinking water.
3. Baking Soda (NaHCO3 - Sodium hydrogen carbonate): Mild, non-corrosive basic salt. On heating, it releases CO2 bubbles that make bread and cakes spongy and soft.
4. Washing Soda (Na2CO3.10H2O): Formed by recrystallisation of sodium carbonate. Contains 10 molecules of water of crystallisation. Used in glass, soap, and paper industries, and to remove permanent hardness of water.
5. Plaster of Paris (CaSO4.1/2H2O): Formed by heating Gypsum (CaSO4.2H2O) at 373 K. It is a white powder that sets into a hard solid mass upon mixing with water. Used for supporting fractured bones and making toys/statues.

3. Important Definitions, Formulas, and Facts

Key Definitions

• Olfactory Indicator: An indicator whose odour changes depending on whether it is placed in an acidic or basic medium.
• Neutralisation Reaction: The reaction between an acid and a base to form salt and water.
• Alkali: A base that dissolves in water (e.g., NaOH, KOH, Ca(OH)2).
• pH Scale: A scale for measuring hydrogen ion concentration in a solution (p stands for potenz, German for power).
• Water of Crystallisation: The fixed number of water molecules chemically attached to each formula unit of a salt in its crystal form.

Chemical Formulas and Equations to Memorise

• Chlor-alkali process:

2NaCl + 2H2O -> 2NaOH + Cl2 + H2

• Bleaching powder formation:

Ca(OH)2 + Cl2 -> CaOCl2 + H2O

• Baking soda formation:

NaCl + H2O + CO2 + NH3 -> NH4Cl + NaHCO3

• Heating of baking soda:

2NaHCO3 (heat) -> Na2CO3 + H2O + CO2

• Plaster of Paris (POP) formation:

CaSO4.2H2O (at 373 K) -> CaSO4.1/2H2O + 1 1/2 H2O

• Setting of POP back to Gypsum:

CaSO4.1/2H2O + 1 1/2 H2O -> CaSO4.2H2O

Important Facts

• Copper sulphate crystals are blue due to 5 water molecules (CuSO4.5H2O). When heated, they lose water and turn white.
• Tooth enamel is made of calcium hydroxyapatite (a crystalline form of calcium phosphate), the hardest substance in the human body.

4. Solved Board-Style Questions

Q1: Why do HCl, HNO3 show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not?

Ans: Acidic character is shown only when a substance dissociates to release free Hydrogen ions [H+ (aq) or H3O+] in water. HCl and HNO3 completely ionise in water to produce H+ ions, so they conduct electricity and show acidic properties. Although glucose (C6H12O6) and alcohol (C2H5OH) contain hydrogen, they do not ionise in water to produce H+ ions. Hence, they do not show acidic character.

Q2: While diluting an acid, why is it recommended that acid should be added to water and not water to acid?

Ans: The process of dissolving an acid in water is highly exothermic. If water is poured into concentrated acid, a huge amount of heat is generated suddenly in a small volume. This can cause the acidic mixture to splash out and cause chemical burns. The glass container may also break due to excessive local heating. Therefore, acid must always be added slowly to water with continuous stirring.

Q3: A white chemical compound 'X' is used by doctors to support fractured bones.

(a) Write the chemical name and formula of 'X'.

(b) How is 'X' prepared? Write the balanced chemical equation.

(c) What precautions should be taken regarding its storage?

Ans:

(a) Chemical name: Calcium sulphate hemihydrate. Chemical formula: CaSO4.1/2H2O (Plaster of Paris).

(b) It is prepared by heating gypsum (CaSO4.2H2O) carefully at 373 K (100°C):

CaSO4.2H2O (heat at 373 K) -> CaSO4.1/2H2O + 1 1/2 H2O

(c) Precaution: It must be stored in moisture-proof containers because it absorbs moisture from air and turns into hard solid gypsum, becoming useless.

Q4: What is baking powder? How does it make cakes soft and spongy, and why is tartaric acid added to it?

Ans: Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid like tartaric acid.

When mixed with water and heated in cake batter, baking soda reacts with the acid to produce carbon dioxide gas:

NaHCO3 + H+ -> CO2 + H2O + Sodium salt of acid.

The trapped bubbles of CO2 gas cause the dough to rise, making the cake soft and spongy.

Tartaric acid is added to neutralise the sodium carbonate formed upon heating, which would otherwise make the cake taste bitter.

5. Common Mistakes and Quick Revision Points

Common Mistakes to Avoid

• Formula of Plaster of Paris: Do not write CaSO4.2H2O (that is Gypsum!). POP has half a water molecule: CaSO4.1/2H2O (which means two formula units of CaSO4 share one molecule of H2O).
• Baking Soda vs. Baking Powder: Baking soda is purely NaHCO3. Baking powder is NaHCO3 PLUS a mild edible acid (tartaric acid).
• pH values and acidity: Remember, lower pH means higher H+ concentration and stronger acid. (pH 2 is stronger than pH 5, not weaker!).
• Testing gases: CO2 turns lime water milky; H2 burns with a pop sound. Do not mix up the tests.

Quick Revision Summary

• Acid + Base -> Salt + Water (Neutralisation).
• pH < 7: Acidic | pH = 7: Neutral | pH > 7: Basic.
• Litmus: Acid turns Blue to Red; Base turns Red to Blue.
• Phenolphthalein: Colourless in Acid, Pink in Base.
• Chlor-alkali process products: NaOH (base), Cl2 (anode), H2 (cathode).
• Permanent hardness of water: Removed using Washing Soda (Na2CO3.10H2O).