Chapter 2: Acids, Bases and Salts
Learn about chemical properties of acids and bases, pH scale, neutralization reactions, and salts used in daily life. Acids and bases के chemical properties, pH scale, neutralisation reactions, और दैनिक organismन में उपयोग होने वाले salts के बारे में सीखें।
1. Introduction & Indicators (सूचक)
Historically, acids were defined as substances that are sour in taste and turn blue litmus paper red. Bases are bitter in taste, soapy to touch, and turn red litmus paper blue [NCERT p. 17]. However, since many laboratory chemicals are dangerous to taste or touch, we use special substances called indicators to test their nature. इतिहास में, acids को उन substanceों के रूप में परिभाषित किया गया था जो स्वाद में sour (खट्टे) होते हैं और blue litmus paper को लाल कर देते हैं। Bases स्वाद में bitter (कड़वे) होते हैं, छूने में soapy होते हैं, और red litmus paper को नीला कर देते हैं [NCERT p. 17]। लेकिन, चूंकि laboratory के कई chemicals को चखना या छूना खतरनाक होता है, हम उनकी प्रकृति का परीक्षण करने के लिए indicators (सूचकों) का उपयोग करते हैं।
Litmus, Turmeric
Phenolphthalein, Methyl Orange
Onion, Vanilla, Clove
Types of Indicators Indicators के प्रकार
- Natural Indicators: Litmus (extracted from Lichen) and turmeric. Red cabbage leaves and petals of Hydrangea, Petunia, and Geranium also act as natural indicators [NCERT p. 17]. Natural Indicators (प्राकृतिक सूचक): Litmus (जो Lichen से निकाला जाता है) और हल्दी। लाल पत्तागोभी की पत्तियाँ और Hydrangea, Petunia, और Geranium की पंखुड़ियाँ भी प्राकृतिक सूचक का काम करती हैं [NCERT p. 17]।
- Synthetic Indicators: Prepared in labs, e.g., Phenolphthalein [NCERT p. 17] (pink in base, colorless in acid) and Methyl Orange [NCERT p. 17] (red in acid, yellow in base). Synthetic Indicators (संश्लेषित सूचक): ये प्रयोगशाला में तैयार किए जाते हैं, जैसे Phenolphthalein [NCERT p. 17] (base में गुलाबी, acid में रंगहीन) और Methyl Orange [NCERT p. 17] (acid में लाल, base में पीला)।
- Olfactory Indicators (गंधीय सूचक): Substances whose odor changes in acidic or basic media [NCERT p. 18] (e.g., onion, vanilla extract, clove oil). Olfactory Indicators (गंधीय सूचक): वे substance जिनकी गंध acidic या basic medium में बदल जाती है [NCERT p. 18] (जैसे प्याज, vanilla extract, और लौंग का तेल)।
Interactive Indicator & pH Lab इंटरैक्टिव इंडिकेटर और pH लैब
Select a substance below to test it with red/blue litmus, phenolphthalein, methyl orange, olfactory strips, and universal pH indicators: नीचे दिए गए किसी substance को चुनें और red/blue litmus, phenolphthalein, methyl orange, olfactory strips, और universal pH indicators के साथ उसका परीक्षण करें:
Checkpoint 1
- Indicators are chemical substances that change their colour or odour depending on the acidic or basic nature of the medium [NCERT p. 17]. Indicators (सूचक) वे chemical substances हैं जो medium की acidic या basic प्रकृति के आधार पर अपना रंग या गंध बदल देते हैं [NCERT p. 17]।
- Litmus (natural, from lichen), turmeric, and red cabbage are natural indicators, while phenolphthalein and methyl orange are synthetic indicators [NCERT p. 17]. Litmus (lichen से प्राप्त), हल्दी और लाल पत्तागोभी natural indicators हैं, जबकि phenolphthalein और methyl orange synthetic indicators हैं [NCERT p. 17]।
- Olfactory indicators like onion, vanilla extract, and clove oil change their smell in acidic or basic media, helping identify substances by scent [NCERT p. 18]. Olfactory indicators (गंधीय सूचक) जैसे प्याज, vanilla extract और लौंग का तेल acidic या basic media में अपनी गंध बदलते हैं, जो सूंघकर पहचान करने में मदद करते हैं [NCERT p. 18]।
2. Chemical Properties of Acids & Bases
1. Reaction with Metals [NCERT §2.1.2]
When an acid reacts with a metal, it forms a salt and releases hydrogen gas: जब कोई acid किसी metal के साथ react करता है, तो वह salt बनाता है और hydrogen gas मुक्त करता है:
2HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)
H2SO4(aq) + Zn(s) → ZnSO4(aq) + H2(g)
Bases also react with certain active metals (like Zinc and Aluminum) to form hydrogen gas, but the salt contains a complex negative ion containing the metal: Bases भी कुछ सक्रिय metals (जैसे Zinc और Aluminum) के साथ react करके hydrogen gas बनाते हैं, लेकिन बनने वाले salt में धातु युक्त एक complex negative ion होता है:
2NaOH(aq) + Zn(s) → Na2ZnO2(aq) (Sodium Zincate) + H2(g) [NCERT p. 20]
2. Reaction with Metal Carbonates & Hydrogen Carbonates [NCERT §2.1.3]
Metal carbonates and metal hydrogencarbonates react with acids to form salt, water, and carbon dioxide gas: Metal carbonates और metal hydrogencarbonates acids के साथ react करके salt, पानी और carbon dioxide gas बनाते हैं:
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
Metal Hydrogen Carbonate + Acid → Salt + Water + Carbon Dioxide
NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)
Lime Water Test: Passing CO2 gas through lime water Ca(OH)2 turns it milky due to the formation of insoluble calcium carbonate: Lime Water Test (चूने के पानी का परीक्षण): चूने के पानी Ca(OH)2 में CO2 gas गुजारने पर वह अविलेय calcium carbonate बनने के कारण दूधिया (milky) हो जाता है:
If excess CO2 is passed, the milkiness disappears because soluble calcium hydrogen carbonate is formed: यदि अत्यधिक (excess) CO2 प्रवाहित की जाए, तो दूधियापन समाप्त हो जाता है क्योंकि पानी में घुलनशील calcium hydrogen carbonate बन जाता है:
3. Reaction of Acids & Bases with Each Other [NCERT §2.1.4]
When an acid reacts with a base, they cancel out each other's properties. This is called a neutralisation reaction (उदासीनीकरण अभिक्रिया), forming salt and water: जब कोई acid किसी base के साथ react करता है, तो वे एक-दूसरे के प्रभाव को नष्ट कर देते हैं। इसे neutralisation reaction (neutralisation reaction) कहते हैं, जिससे salt और पानी बनते हैं:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
H+(aq) + OH−(aq) → H2O(l) (Net Ionic Reaction) [NCERT p. 25]
4. Reaction of Metallic & Non-metallic Oxides [NCERT §2.1.5, §2.1.6] 4. Metallic और Non-metallic Oxides के साथ reactions [NCERT §2.1.5, §2.1.6]
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Metallic Oxides are Basic: Metal oxides react with acids to form salt and water. Thus, they are basic oxides. For example, Copper oxide (CuO) is black, but reacts with HCl to form copper chloride (CuCl2) which turns the solution blue-green [NCERT p. 22]:
Metallic Oxides are Basic (धात्विक ऑक्साइड क्षारीय होते हैं): धातु के ऑक्साइड acids के साथ react करके salt और पानी बनाते हैं। इसलिए वे basic oxides हैं। उदाहरण के लिए, Copper oxide (CuO) काला होता है, लेकिन HCl के साथ react करके copper chloride (CuCl2) बनाता है जिससे solution नीला-हरा हो जाता है [NCERT p. 22]:
CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)
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Non-metallic Oxides are Acidic: Non-metal oxides (like CO2) react with bases (like Ca(OH)2) to form salt and water, showing they are acidic in nature [NCERT p. 22]:
Non-metallic Oxides are Acidic (अधात्विक ऑक्साइड अम्लीय होते हैं): अधातु के ऑक्साइड (जैसे CO2) bases (जैसे Ca(OH)2) के साथ react करके salt और पानी बनाते हैं, जो यह दर्शाता है कि वे प्रकृति में acidic होते हैं [NCERT p. 22]:
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
Gas Reaction virtual lab bench गैस रिएक्शन वर्चुअल लैब बेंच
Toggle between the two setups to see metal-acid gas evolution and carbonate-acid carbon dioxide tests: Metal-acid gas evolution और carbonate-acid carbon dioxide tests को देखने के लिए दोनों setups के बीच स्विच करें:
Checkpoint 2
- Acids react with active metals to produce salt and hydrogen gas (pop test), whereas bases react with active metals to form hydrogen gas and complex salts [NCERT p. 20]. Acids सक्रिय metals के साथ react करके salt और hydrogen gas (pop test) बनाते हैं, जबकि bases सक्रिय metals के साथ react करके hydrogen gas और complex salts बनाते हैं [NCERT p. 20]।
- Metal carbonates and hydrogen carbonates react with acids to form salt, water, and carbon dioxide gas, which turns lime water milky [NCERT p. 21]. Metal carbonates और hydrogen carbonates acids के साथ react करके salt, पानी और carbon dioxide gas बनाते हैं, जो चूने के पानी को दूधिया (milky) कर देती है [NCERT p. 21]।
- Acids and bases react together in a neutralisation reaction to form salt and water. Metallic oxides are basic, while non-metallic oxides are acidic [NCERT p. 22]. Acids and bases आपस में neutralisation reaction करके salt और पानी बनाते हैं। Metallic oxides basic होते हैं, जबकि non-metallic oxides acidic होते हैं [NCERT p. 22]।
3. Dissociation & pH Scale
1. Dissociation in Water Solution [NCERT §2.2.1]
Acids show acidic properties only in the presence of water because they dissociate to form hydrogen ions (H+). These H+ ions cannot exist alone, so they combine with water molecules to form hydronium ions (H3O+): Acids अम्लीय गुण केवल पानी की उपस्थिति में दिखाते हैं क्योंकि वे टूटकर hydrogen ions (H+) बनाते हैं। ये H+ ions अकेले नहीं रह सकते, इसलिए वे पानी के molecules के साथ मिलकर hydronium ions (H3O+) बनाते हैं:
Dry HCl gas does not change the color of dry litmus paper because without water, it cannot release H+ ions [NCERT p. 24]. Similarly, bases dissociate in water to release hydroxide ions (OH−): सूखी HCl gas सूखे litmus paper का रंग नहीं बदलती क्योंकि पानी के बिना यह H+ ions मुक्त नहीं कर सकती [NCERT p. 24]। इसी प्रकार, bases पानी में टूटकर hydroxide ions (OH−) मुक्त करते हैं:
2. The pH Scale [NCERT §2.3]
The strength of an acid or base is measured quantitatively on a scale called the pH scale (ranges from 0 to 14): किसी acid या base की strength को मात्रात्मक रूप से एक पैमाने पर मापा जाता है जिसे pH scale (सीमा 0 से 14) कहते हैं:
- The 'p' in pH stands for potenz in German, meaning power [NCERT p. 25]. pH में 'p' का अर्थ जर्मन शब्द potenz से है, जिसका अर्थ शक्ति (power) होता है [NCERT p. 25]।
- pH < 7: Acidic solution (higher H+ concentration, lower pH value) [NCERT p. 26]. pH < 7: Acidic solution (यहाँ H+ concentration अधिक और pH value कम होती है) [NCERT p. 26]।
- pH = 7: Neutral solution (like pure distilled water) [NCERT p. 26]. pH = 7: Neutral solution (जैसे शुद्ध distilled water) [NCERT p. 26]।
- pH > 7: Basic / Alkaline solution (higher OH− concentration, higher pH) [NCERT p. 26]. pH > 7: Basic / Alkaline solution (यहाँ OH− concentration अधिक और pH value अधिक होती है) [NCERT p. 26]।
3. Importance of pH in Everyday Life [NCERT §2.3.1] 3. दैनिक organismन में pH का महत्व [NCERT §2.3.1]
- Human Body: Works optimally within a narrow pH range of 7.0 to 7.8. Acid rain (pH < 5.6) flowing into rivers lowers water pH, threatening aquatic life [NCERT p. 27]. Human Body (मानव शरीर): 7.0 से 7.8 के संकीर्ण pH range के भीतर सर्वोत्तम रूप से कार्य करता है। जब acid rain (अम्लीय वर्षा, pH < 5.6) नदियों में बहती है, तो पानी का pH कम हो जाता है, जिससे जलीय organismों का organismन खतरे में पड़ जाता है [NCERT p. 27]।
- Stomach Acidity: The stomach secretes hydrochloric acid (HCl) for digestion. Overeating causes excess acid production, leading to pain and irritation. Antacids (अम्लनाशक) like Milk of Magnesia [Mg(OH)2], which is a mild base, are used as a remedy to neutralise stomach acid [NCERT p. 27]. Stomach Acidity (पेट की अम्लता): हमारा पेट भोजन पचाने के लिए hydrochloric acid (HCl) स्रावित करता है। अपच (indigestion) के दौरान पेट बहुत अधिक acid बनाता है, जिससे दर्द और जलन होती है। इसके उपचार के लिए antacids (जैसे Milk of Magnesia [Mg(OH)2], जो एक दुर्बल base है) का उपयोग पेट के excess acid को neutralise करने के लिए किया जाता है [NCERT p. 27]।
- Tooth Decay: Tooth decay starts when the mouth's pH falls below 5.5. The tooth enamel, made of calcium hydroxyapatite (a crystalline form of calcium phosphate), is corroded by acids produced by bacteria from sugar degradation. Using alkaline toothpaste neutralises the acid [NCERT p. 27]. Tooth Decay (दांतों का क्षय): जब मुंह का pH 5.5 से कम हो जाता है, तो दांतों का सड़ना शुरू हो जाता है। दांतों का enamel, जो calcium hydroxyapatite से बना होता है, मुंह में मौजूद बैक्टीरिया द्वारा भोजन के बचे हुए कणों से बनने वाले acid के कारण संक्षारित (corrode) होने लगता है। Alkaline toothpaste का उपयोग करने से यह acid neutralise हो जाता है [NCERT p. 27]।
- Chemical Warfare by Animals & Plants: Honeybee sting injects acid, causing pain. Applying mild base like baking soda gives relief [NCERT p. 27]. Nettle plant leaves have stinging hair that inject methanoic acid, cured by rubbing dock plant leaves (basic) [NCERT p. 28]. Chemical Warfare by Animals & Plants (पशुओं और पौधों द्वारा आत्मरक्षा): मधुमक्खी का डंक एक acid छोड़ता है, जिससे दर्द और जलन होती है। डंक वाले स्थान पर baking soda जैसे mild base लगाने से राहत मिलती है [NCERT p. 27]। Nettle plant के पत्तों में डंक मारने वाले बाल होते हैं जो methanoic acid छोड़ते हैं, इसे डक पौधे (dock plant) की पत्तियों (जो basic होती हैं) को रगड़कर ठीक किया जाता है [NCERT p. 28]।
Checkpoint 3
- Acids release H⁺(aq) ions and bases release OH⁻(aq) in water, which conducts electricity. Diluting acid is highly exothermic, so always add acid slowly to water [NCERT pp. 24-25]. Acids पानी में H⁺(aq) ions मुक्त करते हैं और bases OH⁻(aq) मुक्त करते हैं, जो electricity conduct करते हैं। Acid का dilution अत्यंत exothermic है, इसलिए हमेशा acid को धीरे-धीरे पानी में मिलाना चाहिए [NCERT pp. 24-25]।
- The pH scale (0 to 14) measures hydrogen ion concentration. A pH of 7 is neutral, under 7 is acidic, and over 7 is basic [NCERT p. 26]. pH scale (0 से 14) hydrogen ion concentration को मापता है। pH 7 neutral होता है, 7 से कम acidic और 7 से अधिक basic होता है [NCERT p. 26]।
- pH is vital for living organisms, digestion, preventing tooth decay, and self-defense mechanisms of plants/animals [NCERT pp. 27-28]. organismों के जीने, पाचन (digestion), दांतों को सड़ने से रोकने और पौधों/पशुओं की आत्मरक्षा प्रणालियों के लिए pH अत्यंत महत्वपूर्ण है [NCERT pp. 27-28]।
4. More About Salts & Downstream Chemicals
1. Family and pH of Salts [NCERT §2.4.1, §2.4.2]
Salts are ionic compounds formed by neutralization. The pH of salt solutions depends on the strength of parent acids and bases: Salts ionic compounds हैं जो neutralization से बनते हैं। Salt solutions का pH उनके मूल acids and bases की strength पर निर्भर करता है:
- Strong Acid + Strong Base → Neutral Salt (pH = 7) (e.g. NaCl, K2SO4) [NCERT p. 28]. Strong Acid + Strong Base → Neutral Salt (pH = 7) (जैसे NaCl, K2SO4) [NCERT p. 28]।
- Strong Acid + Weak Base → Acidic Salt (pH < 7) (e.g. NH4Cl) [NCERT p. 28]. Strong Acid + Weak Base → Acidic Salt (pH < 7) (जैसे NH4Cl) [NCERT p. 28]।
- Weak Acid + Strong Base → Basic Salt (pH > 7) (e.g. CH3COONa, Na2CO3) [NCERT p. 28]. Weak Acid + Strong Base → Basic Salt (pH > 7) (जैसे CH3COONa, Na2CO3) [NCERT p. 28]।
2. Chemicals from Common Salt [NCERT §2.4.3]
Sodium chloride (NaCl) is a crucial raw material for various industrial chemicals: Sodium chloride (NaCl) विभिन्न औद्योगिक chemicals के लिए एक महत्वपूर्ण कच्चा माल (raw material) है:
(NaCl)
NaOH (Caustic Soda)
CaOCl₂
NaHCO₃
Na₂CO₃ • 10H₂O
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Sodium Hydroxide (NaOH) & Chlor-Alkali Process: Electrolysis of brine (concentrated NaCl solution) decomposes it to produce chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide solution near the cathode [NCERT p. 30]:
Sodium Hydroxide (NaOH) & Chlor-Alkali Process: Brine (नमक का सांद्र घोल) के electrolysis (electric decomposition) से यह decompose होकर anode पर chlorine gas, cathode पर hydrogen gas, और cathode के पास sodium hydroxide solution बनाता है [NCERT p. 30]:
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
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Bleaching Powder (CaOCl2): Produced by the action of chlorine gas on dry slaked lime Ca(OH)2 [NCERT p. 30]:
Ca(OH)2 + Cl2 → CaOCl2 (Bleaching powder) + H2OUses: Bleaching cotton, wood pulp, disinfecting drinking water. Bleaching Powder (CaOCl2) (विरंजक चूर्ण): यह सूखे बुझे चूने Ca(OH)2 पर chlorine gas की क्रिया से बनता है [NCERT p. 30]:Ca(OH)2 + Cl2 → CaOCl2 (Bleaching powder) + H2OUses: सूती कपड़ों और लकड़ी की लुगदी (wood pulp) को ब्लीच करना, पीने के पानी को कीटाणुशोधित (disinfect) करना।
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Baking Soda (NaHCO3): Prepared using the Solvay Process where brine reacts with ammonia and carbon dioxide [NCERT p. 31]:
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3 (Baking soda)Heating baking soda during cooking decomposes it, releasing CO2 gas which makes dough rise, making cakes and bread fluffy [NCERT p. 31]:2NaHCO3 → Na2CO3 + H2O + CO2(g)Baking Soda (NaHCO3) (खाने का सोडा): इसे Solvay Process द्वारा तैयार किया जाता है जहाँ brine, ammonia और carbon dioxide के साथ react करता है [NCERT p. 31]:NaCl + H2O + CO2 + NH_{3} → NH_{4Cl + NaHCO3 (Baking soda)खाना पकाते समय baking soda को गर्म करने पर यह टूट जाता है और CO2 gas छोड़ता है जो आटे को फुलाती है, जिससे केक और ब्रेड स्पंजी बनते हैं [NCERT p. 31]:2NaHCO3 → Na2CO3 + H2O + CO2(g)
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Washing Soda (Na2CO3 • 10H2O): Obtained by recrystallizing sodium carbonate [NCERT p. 32]:
Na2CO3 + 10H2O → Na2CO3 • 10H2O (Washing soda)Uses: Cleaning agent, soap/glass industry, removing permanent water hardness. Washing Soda (Na2CO3 • 10H2O) (धोने का सोडा): यह sodium carbonate के पुन: क्रिस्टलीकरण (recrystallization) से प्राप्त होता है [NCERT p. 32]:Na2CO3 + 10H2O → Na2CO3 • 10H2O (Washing soda)Uses: क्लीनिंग एजेंट, कांच/साबुन उद्योग, पानी की स्थायी कठोरता (hardness) को दूर करना।
3. Water of Crystallisation [NCERT §2.4.4] 3. क्रिस्टलन का जल (Water of Crystallisation) [NCERT §2.4.4]
Salts contain fixed water molecules in their crystal structure called water of crystallisation (क्रिस्टलन का जल). Salts की crystal structure में निश्चित पानी के molecules होते हैं जिन्हें water of crystallisation (क्रिस्टलन का जल) कहा जाता है।
- Copper sulphate crystals are blue because of 5 water molecules (CuSO4 • 5H2O). Heating turns them white as water is lost, but adding water restores the blue color [NCERT p. 32]. Copper sulphate crystals 5 water molecules (CuSO4 • 5H2O) की वजह से नीले होते हैं। इन्हें गर्म करने पर पानी खत्म हो जाने से ये सफेद हो जाते हैं, लेकिन फिर से पानी मिलाने पर नीला रंग वापस आ जाता है [NCERT p. 32]।
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Plaster of Paris (CaSO4 • ½H2O): Prepared by heating Gypsum (CaSO4 • 2H2O) at 373 K [NCERT p. 33]:
CaSO4 • 2H2O → CaSO4 • ½H2O (POP) + 1½H2OWhen mixed with water, it re-hydrates back to hard solid gypsum, used by doctors to support fractured bones [NCERT p. 33]. Plaster of Paris (CaSO4 • ½H2O) (प्लास्टर ऑफ पेरिस): इसे Gypsum (CaSO4 • 2H2O) को 373 K पर गर्म करके तैयार किया जाता है [NCERT p. 33]:CaSO4 • 2H2O → CaSO4 • ½H2O (POP) + 1½H2Oजब इसे पानी के साथ मिलाया जाता है, तो यह फिर से कठोर solid gypsum में बदल जाता है, जिसका उपयोग डॉक्टर टूटी हड्डियों को सहारा देने के लिए करते हैं [NCERT p. 33]।
Chlor-Alkali & Salts Factory
Animate the electrolytic cell to see ions move, gases release, and downstream salts form:
Checkpoint 4
- Common salt (NaCl) is used to produce sodium hydroxide (chlor-alkali process), bleaching powder (CaOCl₂), baking soda (NaHCO₃), and washing soda (Na₂CO₃·10H₂O) [NCERT pp. 30-32].
- Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a salt (e.g. copper sulphate crystals) [NCERT p. 32].
- Plaster of Paris (CaSO₄·½H₂O) is a white powder that sets into a hard solid mass of Gypsum (CaSO₄·2H₂O) when mixed with water [NCERT p. 33].
5. NCERT Solutions (अभ्यास प्रश्नोत्तर)
In-Text Questions
Curd and sour substances contain organic acids. If stored in brass (an alloy of copper and zinc) or copper vessels, these acids react with the metal to form toxic metallic compounds. These toxic salts can cause food poisoning and make the food unsafe for consumption.
Hydrogen gas (H2) is liberated when an acid reacts with a metal.
Example: Dilute sulfuric acid reacting with Zinc granules:
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
Test for Hydrogen Gas: Pass the gas through soap water. The gas forms soap bubbles that rise in the air. Bring a burning matchstick or candle near these soap bubbles; the gas burns with a characteristic pop sound, confirming the presence of hydrogen.
Since one of the products is calcium chloride (CaCl2) and the gas evolved extinguishes a burning candle (a property of carbon dioxide, CO2), the metal compound A must be Calcium Carbonate (CaCO3).
Balanced Equation:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
HCl and HNO3 dissociate completely in water to release hydrogen ions (H+ or hydronium ions H3O+) which are responsible for acidic properties. On the other hand, compounds like glucose (C6H12O6) and alcohol (C2H5OH) contain hydrogen but do not dissociate in water to produce H+ ions. Hence, they do not show acidic behavior.
Electric current is carried through a solution by free ions. An acid dissociates into hydrogen ions (H+) and negative acid radicals (like Cl− or SO42−) when dissolved in water. These free-moving ions are responsible for conducting electricity through the solution.
Dry HCl gas is a molecular compound and does not contain free H+ ions. Litmus paper changes color only in the presence of H+ (or H3O+) ions. Since water is required for HCl to dissociate into H+, dry HCl gas does not exhibit acidic properties and fails to change the color of dry litmus paper.
The dilution of a concentrated acid is highly exothermic. If water is added to concentrated acid, a large amount of heat is generated instantly at the point of contact. This heat can cause the acid mixture to boil and splash out, leading to severe burns. It can also cause the glass container to break due to local heating. Adding acid slowly to a large volume of water with continuous stirring dissipates the heat safely.
Dilution is the process of adding water to the solution. When an acid is diluted, the total volume of the solution increases, but the number of H3O+ ions remains constant. Therefore, the concentration of hydronium ions (H3O+) decreases per unit volume of the solution.
When excess base is dissolved in a solution of sodium hydroxide, it releases additional OH− ions. As a result, the concentration of hydroxide ions (OH−) increases per unit volume of the solution.
A lower pH value represents a higher hydrogen ion concentration. Therefore, solution A (pH = 6) has a higher hydrogen ion concentration than solution B (pH = 8).
- Solution A (pH = 6) is acidic because its pH is less than 7.
- Solution B (pH = 8) is basic because its pH is greater than 7.
The concentration of H+ ions determines the acidic strength of a solution. Higher H+ ion concentration makes the solution more acidic (lower pH), whereas lower H+ ion concentration makes the solution less acidic or basic (higher pH).
Yes, basic solutions also contain H+(aq) ions. They arise from the self-ionization of water molecules. However, they are basic because the concentration of hydroxide ions (OH−) is much greater than the concentration of hydrogen ions (H+). हाँ, basic solutions में भी H+(aq) ions होते हैं। वे पानी के molecules के स्वतः आयनीकरण (self-ionization) से उत्पन्न होते हैं। हालाँकि, वे basic इसलिए होते हैं क्योंकि उनमें hydroxide ions (OH−) की concentration hydrogen ions (H+) की concentration से बहुत अधिक होती है।
Quick lime (CaO), slaked lime [Ca(OH)2], and chalk (CaCO3) are all basic substances. A farmer will treat the soil with these compounds if the soil is highly acidic (pH falls below 6.0), which hinders healthy plant growth. Treating it with bases neutralises the soil acidity. Quick lime (CaO), slaked lime [Ca(OH)2], और chalk (CaCO3) सभी basic (क्षारीय) substance हैं। एक किसान इन यौगिकों के साथ मिट्टी का उपचार तब करेगा जब मिट्टी highly acidic (pH 6.0 से कम) हो जाती है, जो पौधों के स्वस्थ विकास को रोकती है। क्षारों (bases) से उपचार करने पर मिट्टी की acidity neutralise हो जाती है।
The common name of Ca(OCl)2 (calcium oxychloride) is Bleaching Powder. Ca(OCl)2 (calcium oxychloride) का साधारण नाम Bleaching Powder (विरंजक चूर्ण) है।
Dry slaked lime [Calcium Hydroxide, Ca(OH)2] yields bleaching powder when treated with chlorine gas. सूखा बुझा चूना [Calcium Hydroxide, Ca(OH)2] chlorine gas के साथ react करने पर bleaching powder देता है।
Sodium Carbonate decahydrate (Washing Soda, Na2CO3 • 10H2O) is used for removing permanent hardness of water. Sodium Carbonate decahydrate (Washing Soda, Na2CO3 • 10H2O) का उपयोग पानी की स्थायी कठोरता (hardness) को दूर करने के लिए किया जाता है।
When a solution of sodium hydrogencarbonate (baking soda) is heated, it decomposes to form sodium carbonate, water vapor, and carbon dioxide gas. जब sodium hydrogencarbonate (baking soda) के solution को गर्म किया जाता है, तो यह टूटकर sodium carbonate, water vapor, और carbon dioxide gas बनाता है।
Equation:
2NaHCO3(aq) [NCERT p. 31] →Heat Na2CO3(s) + H2O(g) + CO2(g)
Plaster of Paris (calcium sulphate hemihydrate) reacts with water to form Gypsum, setting into a hard solid mass. Plaster of Paris (calcium sulphate hemihydrate) पानी के साथ react करके Gypsum बनाता है, जो जमकर एक कठोर ठोस mass में बदल जाता है।
Equation:
CaSO4 • ½H2O(s) + 1½H2O(l) → CaSO4 • 2H2O(s) [NCERT p. 33]
Chapter-End Exercises
Correct Option: (d) 10
Litmus changes from red to blue in basic solutions. Basic solutions have pH values greater than 7. Therefore, a pH of 10 is the only basic pH option.
Correct Option: (b) HCl
Crushed eggshells are made of Calcium Carbonate (CaCO3). Carbonates react with acids (like hydrochloric acid, HCl) to produce carbon dioxide (CO2) gas, which turns lime water milky. NaCl, LiCl, and KCl are neutral salts and do not react with eggshells.
Correct Option: (d) 16 mL
Since the reaction is a quantitative neutralization: 10 mL of NaOH requires 8 mL of HCl. Doubling the amount of NaOH (20 mL) will require double the volume of the same HCl solution, which is 8 • 2 = 16 mL.
Correct Option: (c) Antacid
Indigestion is caused by excess acid secretion in the stomach. Antacids are mild basic substances that neutralise this acidity.
(a) Dilute sulphuric acid reacts with zinc granules.
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
(c) Dilute sulphuric acid reacts with aluminium powder.
(d) Dilute hydrochloric acid reacts with iron filings.
(a) Dilute sulphuric acid + Zinc:
- Word: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
- Balanced: Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
(b) Dilute hydrochloric acid + Magnesium:
- Word: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
- Balanced: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
(c) Dilute sulphuric acid + Aluminium:
- Word: Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
- Balanced: 2Al(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2(g)
(d) Dilute hydrochloric acid + Iron:
- Word: Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen
- Balanced: Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
Activity 2.8 - Conduction test:
- Set up an electric circuit by fixing two iron nails on a rubber cork and placing it inside a 100 mL beaker.
- Connect the nails to the terminals of a 6-volt battery through a switch and a small LED bulb.
- Pour dilute hydrochloric acid (HCl) into the beaker and switch on the current. The bulb glows, showing the solution conducts electricity because HCl dissociates into free H+ and Cl− ions.
- Empty and rinse the beaker, then repeat the experiment using a glucose solution. The bulb does not glow, indicating no current flows.
- Repeat again with an alcohol solution. The bulb remains off.
Conclusion: Although glucose and alcohol contain hydrogen atoms, they do not dissociate into H+ ions in water, showing that they are not acidic.
Distilled water is a pure compound that does not contain dissolved mineral salts. Since there are no free ions available to carry charge, it does not conduct electricity. In contrast, rainwater absorbs gases like carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen oxides from the atmosphere to form carbonic acid, sulfurous acid, and nitric acid. These acids dissociate in water to produce free ions, which conduct electricity.
Acidic behavior is caused by hydrogen ions (H+) formed when an acid dissociates. This dissociation can only take place in the presence of water, which acts as a polar solvent to pull H+ away from Cl−, forming hydronium ions. In the absence of water, dry HCl gas remains molecular and cannot release free ions, thus showing no acidic behavior.
Based on pH values:
- (a) Neutral: Solution D (pH = 7)
- (b) Strongly alkaline: Solution C (pH = 11)
- (c) Strongly acidic: Solution B (pH = 1)
- (d) Weakly acidic: Solution A (pH = 4)
- (e) Weakly alkaline: Solution E (pH = 9)
Arrangement in increasing order of H⁺ concentration:
A higher pH represents a lower H+ concentration. Therefore, the order from lowest to highest H+ is:
pH 11 < pH 9 < pH 7 < pH 4 < pH 1 (Solution C < Solution E < Solution D < Solution A < Solution B)
Fizzing is caused by the release of hydrogen (H2) gas during the metal-acid reaction. The rate of hydrogen release depends on the concentration of H+ ions in the solution.
Hydrochloric acid (HCl) is a strong acid and dissociates completely in water, producing a high H+ concentration. Acetic acid (CH3COOH) is a weak acid and dissociates only partially, yielding a low H+ concentration. Therefore, fizzing will occur much more vigorously in test tube A (containing HCl).
When fresh milk turns into curd, lactose sugar present in the milk is fermented by Lactobacillus bacteria into lactic acid. The formation of lactic acid releases hydrogen ions, making the curd acidic. Consequently, the pH will decrease below 6.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd? [NCERT p. 35]
(a) Shifting pH to alkaline: Milk spoils and turns sour because lactic acid forms over time. Adding basic baking soda (NaHCO3) neutralises any initial lactic acid formed, shifting the pH to slightly alkaline (around 7-8). This delays spoilage and keeps the milk fresh longer.
(b) Delay in setting as curd: Curd needs an acidic environment to set (as lactic acid coagulates milk proteins). Since the baking soda has made the milk alkaline, the lactic acid produced by bacteria must first neutralise the baking soda before it can lower the pH enough for the milk to coagulate into curd. Thus, it takes longer to set.
Plaster of Paris (CaSO4 • ½H2O) is a anhydrous hemihydrate powder. If it absorbs moisture from the air, it undergoes a hydration reaction to form Gypsum (CaSO4 • 2H2O). This causes the powder to set into a hard solid mass, rendering it useless for plastering or molding. Thus, it must be stored in moisture-proof containers.
A chemical reaction between an acid and a base to form salt and water is called a neutralization reaction. The hydrogen ions of the acid combine with the hydroxide ions of the base to form neutral water.
Examples:
- HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
- H2SO4(aq) + Ca(OH)2(aq) → CaSO4(s) + 2H2O(l)
Uses of Washing Soda (Na2CO3 • 10H2O):
- Used in glass, soap, and paper manufacturing industries.
- Used as a cleaning agent for domestic purposes and for removing permanent hardness of water.
Uses of Baking Soda (NaHCO3):
- Used as a raising agent in baking powder for making cakes, bread, and pakoras crispy (releases CO2).
- Used as an ingredient in antacids to neutralise excess stomach acid, and in soda-acid fire extinguishers.
6. Solved PYQs (बोर्ड परीक्षा प्रश्नोत्तर)
1-Mark Questions (Objective / Very Short Answer)
Chemical formula: CaOCl2 (calcium oxychloride). The active ingredient is chlorine gas (Cl2) released when it reacts with atmospheric moisture or acids.
Tomato contains oxalic acid. Vinegar contains acetic acid (ethanoic acid) [NCERT p. 28].
2-Mark Questions (Short Answer - Type I)
1. The substance X used as an antacid that decomposes to release baking gas is Baking Soda (Sodium Hydrogencarbonate, NaHCO3).
2. The gas Y released is Carbon Dioxide (CO2).
Equation:
2NaHCO3(s) →Heat Na2CO3(s) + H2O(l) + CO2(g) [NCERT p. 31]
3-Mark Questions (Short Answer - Type II)
Definition: The chlor-alkali process is the industrial electrolysis of aqueous sodium chloride (brine) to produce chlorine, hydrogen, and sodium hydroxide [NCERT p. 30].
Equation:
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Uses:
- Chlorine (Cl2): Water treatment, manufacturing PVC, disinfectants.
- Hydrogen (H2): Rocket fuel, ammonia for fertilizers, margarine.
- Sodium Hydroxide (NaOH): Degreasing metals, soaps and detergents, paper making.
5-Mark Questions (Long Answer)
(a) Why does tooth decay start when mouth pH is below 5.5?
(b) What is Plaster of Paris? Why should it not be mixed with water during storage?
(c) Give the chemical names and formulae of Gypsum and Plaster of Paris.
(a) Tooth Decay: Enamel is made of calcium phosphate (calcium hydroxyapatite), which is highly resistant to water but gets corroded when mouth pH falls below 5.5 [NCERT p. 27]. Bacteria break down food particles to produce acids. Using basic toothpaste neutralises the acid and prevents corrosion.
(b) Plaster of Paris storage: Plaster of Paris is CaSO4 • ½H2O. If it contacts moisture during storage, it hydrates to form Gypsum, setting into a hard solid mass [NCERT p. 33]. This ruins the product, so it must be kept in airtight containers.
(c) Names and Formulae:
- Gypsum: Calcium sulphate dihydrate, CaSO4 • 2H2O [NCERT p. 33].
- Plaster of Paris: Calcium sulphate hemihydrate, CaSO4 • ½H2O [NCERT p. 33].