Explanation

Egg-shells are primarily composed of calcium carbonate (CaCO₃). When an acid such as dilute hydrochloric acid (HCl) is added, it decomposes the carbonate, releasing carbon dioxide gas (CO₂) with effervescence:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)↑

The evolved carbon dioxide gas reacts with lime water [Ca(OH)₂] to form an insoluble white precipitate of calcium carbonate, making the lime water milky:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ [Milky] + H₂O(l)

Neutral salts like NaCl, LiCl, and KCl do not react with calcium carbonate to liberate any gas.

Explanation

• Acidic solutions: Have a pH value less than 7 (pH < 7) and turn blue litmus red. Values 1, 4, and 5 are acidic.
• Basic/Alkaline solutions: Have a pH value greater than 7 (pH > 7) and turn red litmus blue.

Therefore, a solution that turns red litmus blue must be basic, which corresponds to a pH of 10.

Explanation

During indigestion, the gastric glands in the stomach oversecrete hydrochloric acid (HCl), causing acute hyperacidity, irritation, and pain. Antacids are mild, non-toxic basic substances (such as magnesium hydroxide/milk of magnesia [Mg(OH)₂] or sodium hydrogen carbonate [NaHCO₃]) that neutralize excess stomach acid:

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

• Antibiotics inhibit bacterial growth.
• Analgesics alleviate pain.
• Antiseptics prevent microbial wound infections.

Explanation

The stoichiometric neutralization reaction between sodium hydroxide and hydrochloric acid proceeds in a 1:1 equimolar ratio:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Applying the principle of volumetric proportionality:

• Volume of HCl required to neutralize 10 mL of NaOH = 8 mL
• Volume of HCl required to neutralize 1 mL of NaOH = 8 / 10 = 0.8 mL
• Volume of HCl required to neutralize 20 mL of NaOH = 20 × 0.8 = 16 mL

Explanation

Soda-acid fire extinguishers contain an aqueous solution of sodium hydrogen carbonate / baking soda (NaHCO₃) and a separate glass vial of concentrated sulphuric acid (H₂SO₄). When activated, the two chemicals mix, producing a brisk stream of carbon dioxide gas:

2NaHCO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g)↑

Carbon dioxide blankets the burning surface, cutting off the supply of atmospheric oxygen and smothering the fire.

Explanation

Substances whose characteristic smell or odor changes in acidic or basic media are termed olfactory indicators.

Vanilla essence retains its characteristic sweet pleasant smell in an acidic medium, but its smell is completely destroyed in a basic medium (such as NaOH solution).

Other examples: Onion extract and clove oil. Litmus and turmeric are colour-based natural indicators, while phenolphthalein is a synthetic colour indicator.

Explanation

Most metal oxides are basic in nature because they react with dilute acids to produce salt and water through a classic neutralization pathway:

Metal oxide + Acid → Salt + Water

Example: CuO(s) [Black] + 2HCl(aq) → CuCl₂(aq) [Blue-green] + H₂O(l)

Some metallic oxides like Al₂O₃ and ZnO show both acidic and basic behavior and are classified as amphoteric oxides, whereas non-metal oxides (like SO₂, CO₂) are typically acidic.

Explanation

• NaHCO₃ (Sodium hydrogen carbonate): Commonly known as baking soda, utilized as an antacid, baking ingredient, and in fire extinguishers.
• Na₂CO₃·10H₂O (Sodium carbonate decahydrate): Commonly known as washing soda, containing 10 molecules of water of crystallization, utilized in the glass, soap, paper industries, and for removing permanent hardness of water.
• Note: Caustic soda is sodium hydroxide (NaOH).

Explanation

• Statement (i) is True: Tooth enamel is made up of calcium hydroxyapatite (a crystalline form of calcium phosphate). Tooth decay corrodes enamel when the oral cavity's pH falls below 5.5.
• Statement (ii) is True: Bacteria present in the mouth degrade residual dietary sugars and food particles into organic and acidic by-products (including phosphoric/lactic acid degradation pathways), driving oral pH down below critical levels.

Using toothpaste, which is generally alkaline, neutralizes these excess acids and prevents enamel decay.

Explanation

The chemical compound charged inside the container of a soda-acid fire extinguisher is sodium hydrogen carbonate (NaHCO₃). When brought into contact with dilute sulphuric acid, it reacts instantly to generate a heavy stream of carbon dioxide gas that extinguishes flames:

2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂↑

Explanation

On the universal pH scale measured at 25°C (298 K):

• Neutral solutions: Have an equal concentration of hydronium and hydroxide ions [H₃O⁺] = [OH⁻] = 10⁻⁷ mol/L, giving an exact pH of 7 (e.g., pure water).
• Acidic solutions: Have a pH < 7.
• Basic solutions: Have a pH > 7.

Explanation

In standard chemistry laboratory exercises, when dilute sulphuric acid reacts with a solid metal carbonate/hydrogen carbonate (or when solid pellets are tested), gas evolution takes place:

Board Answer Key Reference: Option (d) is designated as the official key for this question. In chemical systems where solid carbonate or basic salts release gas upon acidification, carbon dioxide gas (Y) is liberated with effervescence.

Explanation

When dry baking soda [sodium hydrogen carbonate, NaHCO₃] is subjected to thermal decomposition (heating), it releases carbon dioxide and steam, yielding anhydrous sodium carbonate (soda ash):

2NaHCO₃(s) —Δ→ Na₂CO₃(s) + H₂O(g) + CO₂(g)↑

Recrystallization of this sodium carbonate in water subsequently yields washing soda crystals:

Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O

Explanation

The microscopic, needle-like stinging hairs on nettle plant leaves inject methanoic acid (formic acid, HCOOH) into the skin upon contact, triggering a sharp burning pain and irritation.

Natural Remedy: Rubbing the stung area with the leaves of a dock plant (which typically grows nearby in the wild and contains mildly basic sap) neutralizes the acid and brings quick relief.

Explanation

Indicators whose characteristic odour changes in acidic or basic media are known as olfactory indicators.

Cloth strips treated with onion extract retain their characteristic pungent smell in an acidic solution, but the smell is completely destroyed and disappears when treated with a strong base like sodium hydroxide (NaOH).

Other examples: Vanilla essence and clove oil. In contrast, phenolphthalein, methyl orange, and litmus are visual indicators that signal pH changes through color transitions.

Explanation

Zinc lies above hydrogen in the electrochemical reactivity series. It displaces hydrogen ions from dilute acids, producing zinc sulphate and evolving hydrogen gas:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑

The gas is insoluble in soap water and forms gas-filled bubbles that burn with a distinctive 'pop' sound when ignited with a flame.

Explanation

All metal carbonates and hydrogen carbonates react with dilute acids to produce corresponding metal salts, water, and brisk effervescence of carbon dioxide gas:

Metal carbonate/bicarbonate + Acid → Salt + CO₂↑ + H₂O

Example:
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)↑

Explanation

Phenolphthalein is an organic acid-base indicator:

• In acidic or neutral solutions (pH ≤ 7), phenolphthalein remains colourless.
• In basic or alkaline solutions (pH > 8.3), phenolphthalein turns distinctly pink.

Because sodium hydroxide (NaOH) is a strong base that produces free hydroxide ions (OH⁻), it turns phenolphthalein bright pink.

Explanation

• Assertion Analysis: Dry HCl gas directed onto dry blue litmus paper produces zero color change (True).
• Reasoning Analysis: The acidic behavior of substances is due to hydronium ions. In the absence of water, covalent HCl molecules cannot ionize:
  
  HCl + H₂O → H₃O⁺ + Cl⁻
  
  Because water is mandatory for the separation of H⁺ ions, dry gas cannot show acidic properties on dry paper (True, and explains A).

Explanation

An alkali is defined as a base that is soluble in water. They are soapy to the touch, bitter to taste, and corrosive in concentrated forms.

Key Rule: "All alkalis are bases, but all bases are not alkalis."

Examples: Sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide [Ca(OH)₂] dissolve in water and are alkalis. Insoluble bases like copper(II) oxide [CuO] or ferric hydroxide [Fe(OH)₃] are bases, but not alkalis.

Explanation

• (a) Gypsum: Calcium sulphate dihydrate → CaSO₄·2H₂O.
• (b) Plaster of Paris: Calcium sulphate hemihydrate → CaSO₄·½H₂O.
• (c) Washing soda: Sodium carbonate decahydrate → Na₂CO₃·10H₂O.
• (d) Baking soda: Sodium hydrogen carbonate → NaHCO₃.

Explanation

Lime water is an aqueous solution of calcium hydroxide. When CO₂ gas is bubbled through it, it reacts to form an insoluble white precipitate of calcium carbonate:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ [White ppt] + H₂O(l)

This white precipitate remains suspended in the liquid, creating the milky appearance.

Explanation

• Assertion Analysis: Connecting a battery, bulb, and electrodes through an aqueous acid (HCl) or alkali (NaOH) causes the bulb to glow brightly, proving electrical conduction (True).
• Reasoning Analysis: Electrical conductivity in liquids requires mobile ions. Acids produce H⁺/H₃O⁺ cations and acid radical anions, whereas bases yield metal cations and OH⁻ anions. These free-moving ions carry electric current through the solution (True, and explains A).

Explanation

On the universal pH scale at 25°C:

• Lemon juice (citric acid): pH ≈ 2.2 (Acidic, < 7)
• Dilute HCl: pH ≈ 1.0 (Strongly acidic, < 7)
• Vinegar (acetic acid): pH ≈ 2.8 (Acidic, < 7)
• Sodium hydroxide (NaOH): pH ≈ 13–14 (Strongly basic/alkaline, pH > 7)

Explanation

When baking soda reacts with dilute hydrochloric acid, sodium chloride, water, and carbon dioxide gas are produced:

NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)↑

The brisk effervescence observed is due to the rapid release of CO₂ gas, which extinguishes a burning splinter.

Explanation

Passing an electric current through saturated aqueous NaCl (brine) is termed the chlor-alkali process because the products are chlorine (chlor) and sodium hydroxide (an alkali):

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g)↑ [At Anode] + H₂(g)↑ [At Cathode]

• Chlorine gas (Cl₂): Evolved at the positive anode.
• Hydrogen gas (H₂): Evolved at the negative cathode.
• Sodium hydroxide (NaOH): Formed in solution near the cathode.

Explanation

• Assertion Analysis: Metal oxides typically act as bases (True).
• Reasoning Analysis: The chemical signature of a base is its ability to neutralize an acid to produce salt and water:
  
  Metal oxide + Acid → Salt + Water
  
  Example: CuO(s) [Black] + 2HCl(aq) → CuCl₂(aq) [Blue-green] + H₂O(l)
  
  Because metallic oxides mimic metallic hydroxides in acid-neutralizing capability, they are classified as basic oxides (True, and explains A).

Explanation

The pH of an aqueous salt solution depends on the relative strengths of its parent acid and parent base:

• Na₂CO₃: Formed from a strong base (NaOH) and a weak acid (H₂CO₃). In water, carbonate ions hydrolyze to generate excess OH⁻ ions, making the solution basic (pH > 7).

• NaCl: Strong acid (HCl) + Strong base (NaOH) → Neutral (pH ≈ 7).
• NH₄Cl: Strong acid (HCl) + Weak base (NH₄OH) → Acidic (pH < 7).
• CuSO₄: Strong acid (H₂SO₄) + Weak base [Cu(OH)₂] → Acidic (pH < 7).

Explanation

At 25°C, the ionic product of water is K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol²/L².

In a neutral solution, the concentration of hydrogen ions equals the concentration of hydroxide ions:

[H⁺] = [OH⁻] = √K_w = 1.0 × 10⁻⁷ mol/L

Taking the negative logarithm:

pH = -&log₁₀[H⁺] = -&log₁₀(10⁻⁷) = 7

Explanation

Hydrated copper sulphate contains five molecules of water of crystallization per formula unit, giving it a bright blue colour: CuSO₄·5H₂O.

Upon heating, these five water molecules evaporate, forming liquid droplets on the cooler upper walls of the test tube, while the remaining solid turns into white anhydrous copper sulphate:

CuSO₄·5H₂O(s) [Blue] —Δ→ CuSO₄(s) [White] + 5H₂O(g)

Adding a few drops of water to this white powder restores the crystalline blue colour.

Explanation

• (a) Strong acids (e.g., HCl, HNO₃, H₂SO₄) dissociate almost 100% in water to yield a high concentration of H⁺ / H₃O⁺ ions.
• (b) Weak acids (e.g., CH₃COOH, H₂CO₃) ionize partially, establishing an equilibrium with low H⁺ concentration.
• (c) Strong bases (e.g., NaOH, KOH) dissociate completely into OH⁻ ions.
• (d) Weak bases (e.g., NH₄OH, Mg(OH)₂) ionize partially to yield a low concentration of OH⁻ ions.

Explanation

• Assertion Analysis: Plaster of Paris quickly ruins if left exposed to open air, requiring airtight packaging (True).
• Reasoning Analysis: The white powder of Plaster of Paris readily combines with water molecules from ambient humidity, rehydrating to form crystalline gypsum:
  
  CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s) [Hard Gypsum]
  
  This hydration reaction sets the material into an unworkable rock-hard mass, destroying its casting utility unless stored away from moisture (True, and explains A).

Explanation

Vinegar is an aqueous solution containing approximately 5% to 8% acetic acid (CH₃COOH / ethanoic acid) in water, produced through the bacterial fermentation of ethanol.

Natural Occurrences:
• Citric acid: Citrus fruits (lemons, oranges).
• Tartaric acid: Tamarind, unripe grapes.
• Oxalic acid: Tomatoes.
• Lactic acid: Curd and sour milk.

Explanation

When an acid reacts with a base, the H⁺ ions from the acid combine with the OH⁻ ions from the base to form water molecules, while the remaining ions constitute an ionic salt:

Acid + Base → Salt + Water

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). This fundamental reaction type is termed a neutralisation reaction.

Explanation

Carbon dioxide (CO₂) is the standard gas tested using lime water. It forms an insoluble white precipitate of calcium carbonate (CaCO₃), giving the solution a milky appearance:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ [White ppt] + H₂O(l)

Gases like H₂, O₂, and N₂ do not react with lime water and produce no milkiness.

Explanation

When excess CO₂ is passed through the suspension containing insoluble calcium carbonate, it converts the insoluble carbonate into calcium hydrogen carbonate [Ca(HCO₃)₂]:

CaCO₃(s) + H₂O(l) + CO₂(g) [Excess] → Ca(HCO₃)₂(aq) [Soluble/Colourless]

Because calcium bicarbonate is fully water-soluble, the suspended particles dissolve and the milkiness completely disappears.

Explanation

• Assertion Analysis: All Arrhenius and Brønsted-Lowry acids (e.g., HCl, H₂SO₄, HNO₃, CH₃COOH) contain hydrogen that can be released as protons (True).
• Reasoning Analysis: Many hydrogen-containing organic molecules—such as glucose (C₆H₁₂O₆) and ethanol (C₂H₅OH)—do not ionize in water to liberate H⁺ ions. Their aqueous solutions do not turn blue litmus red nor conduct electricity, proving that not all hydrogen-containing compounds are acids (False).

Explanation

Conduction of electricity in an aqueous solution requires mobile charged ions:

• HCl, H₂SO₄, and NaOH are strong electrolytes that dissociate completely into mobile ions (H⁺, Cl⁻, Na⁺, OH⁻, SO₄²⁻), conducting electricity effectively.

• Glucose (C₆H₁₂O₆) is a covalent organic compound. Although it dissolves in water through hydrogen bonding, it does not ionize into free ions. In the absence of mobile ions, its aqueous solution cannot conduct electricity.

Explanation

According to the Arrhenius theory, an acid is a substance that releases hydrogen ions in aqueous solution. Because bare hydrogen ions (protons, H⁺) cannot exist alone in water, they combine with polar water molecules to form hydronium ions (H₃O⁺):

H⁺ + H₂O → H₃O⁺

The presence of H⁺(aq) or H₃O⁺(aq) ions accounts for sour taste, electrical conductivity, and the turning of blue litmus paper red.

Explanation

• (a) Antacid: A mild base (e.g., Mg(OH)₂) taken to neutralize excess stomach acid.
• (b) Acid rain: Rainwater with a pH lower than 5.6 due to dissolved SO₂ and NO₂.
• (c) Tooth decay: Bacterial acid production corrodes enamel when the oral pH drops below 5.5.
• (d) Bee sting: Injects acidic methanoic acid, which can be soothed by applying a mild base like baking soda.

Explanation

Active metals positioned above hydrogen in the activity series displace hydrogen from non-oxidizing dilute acids:

Metal + Dilute acid → Metal Salt + Hydrogen gas↑

Example: 2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g)↑

Explanation

• Assertion Analysis: Baking powder is a dry mixture of baking soda (NaHCO₃) and a mild edible acid such as tartaric acid (True).
• Reasoning Analysis: When water is added or the mixture is baked, baking soda reacts with H⁺ ions from tartaric acid to generate CO₂ gas:
  
  NaHCO₃ + H⁺ → CO₂↑ + H₂O + Sodium salt of acid
  
  The expanding bubbles of carbon dioxide get trapped within the batter, making cakes and bread rise to become soft, airy, and spongy (True, and explains A).

Explanation

Analyzing parent acids and bases for each salt:

• Ammonium chloride (NH₄Cl): Synthesized from hydrochloric acid (HCl, a strong acid) and ammonium hydroxide (NH₄OH, a weak base). It forms an acidic salt solution with a pH < 7.

• NaCl: Strong acid (HCl) + Strong base (NaOH) → Neutral salt.
• Na₂CO₃: Weak acid (H₂CO₃) + Strong base (NaOH) → Basic salt.
• KNO₃: Strong acid (HNO₃) + Strong base (KOH) → Neutral salt.

Explanation

Hydration of ions releases significant energy, making the dilution of concentrated acids and bases highly exothermic.

Important Safety Rule: Acid must always be added slowly to water with continuous stirring, never water to concentrated acid. If water is poured into concentrated acid, the sudden localized heat generated can cause the mixture to boil violently and splash out, risking severe acid burns and shattering the glass container.

Explanation

Citrus fruits like lemons, limes, and oranges are rich natural sources of citric acid, which gives their juice a sharp, sour taste and an acidic pH between 2.0 and 2.6.

Natural Sources of Other Acids:

• Acetic acid: Found in vinegar.
• Tartaric acid: Found in tamarind and unripe grapes.
• Methanoic acid (formic acid): Present in ant and nettle leaf stings.

Explanation

Washing soda is chemically known as sodium carbonate decahydrate with the formula Na₂CO₃·10H₂O. It contains ten molecules of water of crystallization bonded within its crystal lattice.

Distinctions:

• Na₂CO₃: Anhydrous sodium carbonate (soda ash).
• NaHCO₃: Sodium hydrogen carbonate (baking soda).
• NaOH: Sodium hydroxide (caustic soda).

Explanation

• Assertion Analysis: Sodium chloride solution neither turns blue litmus red nor red litmus blue; its measured pH at 25°C is 7 (True).
• Reasoning Analysis: The salt is derived from hydrochloric acid (a strong acid) and sodium hydroxide (a strong base):
  
  HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
  
  Neither Na⁺ nor Cl⁻ ions undergo hydrolysis in water. Consequently, the balance of [H⁺] and [OH⁻] remains 10⁻⁷ mol/L, producing a neutral solution (True, and explains A).

Explanation

Baking soda has the chemical formula NaHCO₃ and is named sodium hydrogencarbonate (or sodium bicarbonate). It is an intermediate compound produced during the industrial Solvay process:

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃↓

Explanation

Only metals positioned above hydrogen in the electrochemical reactivity series can displace hydrogen from dilute non-oxidizing acids.

Zinc (Zn) lies above hydrogen and reacts vigorously:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)↑

Copper (Cu), Silver (Ag), and Gold (Au) lie below hydrogen in the reactivity series and do not react with dilute HCl to produce hydrogen gas.

Explanation

• (a) Vinegar: Contains acetic acid (ethanoic acid).
• (b) Curd: Contains lactic acid generated by Lactobacillus bacteria.
• (c) Tamarind: Contains tartaric acid.
• (d) Tomato: Contains oxalic acid.

Explanation

On the Sorenson pH scale (0 to 14):

• pH < 7: Indicates an acidic solution where [H₃O⁺] > 10⁻⁷ mol/L.
• pH = 7: Indicates a neutral solution where [H₃O⁺] = [OH⁻] = 10⁻⁷ mol/L.
• pH > 7: Indicates a basic solution where [OH⁻] > [H₃O⁺].

Explanation

• Assertion Analysis: Testing distilled water in an electric circuit fails to light the bulb, whereas rainwater completes the circuit and illuminates the bulb (True).
• Reasoning Analysis: Electrical conductivity in liquids requires mobile charge-carrying ions. Distilled water consists almost entirely of neutral covalent H₂O molecules with negligible self-ionization. Rainwater absorbs atmospheric carbon dioxide and nitrogen/sulphur oxides to form weak acids (like carbonic acid, H₂CO₃) that dissociate into mobile H⁺ and HCO₃⁻ ions, facilitating electrical conduction (True, and explains A).

Explanation

The chemical formula of Plaster of Paris is CaSO₄·½H₂O.

The half molecule of water indicates that two formula units of CaSO₄ share a single molecule of water of crystallization: (CaSO₄)₂·H₂O. It is prepared by controlled heating of gypsum at 373 K (100°C).

Explanation

When gypsum (CaSO₄·2H₂O) is heated to 373 K, it loses three-fourths of its water of crystallization to form Plaster of Paris:

CaSO₄·2H₂O(s) —373 K→ CaSO₄·½H₂O(s) + 1½H₂O(g)

Caution: If heated above 373 K, all water of crystallization is lost, yielding anhydrous calcium sulphate (CaSO₄), known as "dead burnt plaster", which loses the property of setting with water.

Explanation

Bleaching powder (calcium oxychloride, CaOCl₂) is widely utilized as a disinfectant and germicide for water supplies. When dissolved in water, it releases chlorine gas (available chlorine), which acts as a powerful oxidizing agent to destroy pathogenic bacteria and microorganisms.

Explanation

Bleaching powder is synthesized by passing dry chlorine gas over dry slaked lime [calcium hydroxide, Ca(OH)₂] at approximately 40°C:

Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)

The actual composition is a complex mixture of calcium oxychloride [CaOCl₂], calcium chloride [CaCl₂], and slaked lime.

Explanation

• Assertion Analysis: When non-metallic oxides dissolve in water, they form acids (e.g., CO₂ + H₂O → H₂CO₃) that turn blue litmus red (True).
• Reasoning Analysis: Non-metallic oxides react with alkalis to form salt and water:
  
  Ca(OH)₂(aq) [Base] + CO₂(g) [Non-metal oxide] → CaCO₃(s) [Salt] + H₂O(l)
  
  Because this mirrors the classic Acid + Base → Salt + Water neutralization pathway, it confirms that non-metallic oxides are acidic in character (True, and explains A).

Explanation

The strength of an acid depends entirely on its degree of ionization, which determines the concentration of hydrogen/hydronium ions [H₃O⁺]:

• Strong acids (HCl, HNO₃, H₂SO₄) ionize almost completely in water, giving a high concentration of H⁺ ions.
• Weak acids (CH₃COOH, H₂CO₃) ionize only partially, yielding a low concentration of H⁺ ions at equivalent molar concentrations.

Explanation

Hydrated copper sulphate crystals have the formula CuSO₄·5H₂O, where five water molecules are chemically coordinated to each formula unit, giving the crystals their blue color and geometric shape.

NaCl, KNO₃, and NH₄Cl are anhydrous crystalline salts that do not contain water of crystallization.

Explanation

During the electrolysis of brine (saturated NaCl solution):

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g)↑ + H₂(g)↑

• (a) NaOH: Forms as an alkali in solution near the cathode.
• (b) Cl₂: Oxidized and liberated at the positive anode.
• (c) H₂: Reduced and liberated at the negative cathode.
• (d) Brine: The concentrated aqueous NaCl solution used as the raw material.

Explanation

Normal rainwater has a slightly acidic pH of about 5.6 due to dissolved atmospheric CO₂ forming weak carbonic acid (H₂CO₃).

When air pollutants like sulphur dioxide (SO₂) and nitrogen oxides (NO₂) dissolve in rain clouds, they form sulphuric acid (H₂SO₄) and nitric acid (HNO₃). When the pH of rainwater drops below 5.6, it is classified as acid rain, which lowers the pH of aquatic ecosystems and corrodes marble structures.

Explanation

• Assertion Analysis: Tooth enamel is the hardest substance in the human body; it does not dissolve in neutral water, but begins corroding when oral pH drops below 5.5 (True).
• Reasoning Analysis: Tooth enamel is made of calcium phosphate (calcium hydroxyapatite) (True). However, the cause of the drop in pH is bacterial fermentation of food sugars into organic acids. The acidic environment dissolves calcium phosphate:
  
  Because R simply states the chemical composition of enamel rather than explaining how bacteria generate the acid that causes decay, R does not directly explain A (True, but R is not the correct explanation of A).

Explanation

Heating baking soda triggers a thermal decomposition reaction that yields sodium carbonate, steam, and carbon dioxide gas:

2NaHCO₃(s) —Δ→ Na₂CO₃(s) + H₂O(g) + CO₂(g)↑

The production of carbon dioxide gas causes baking dough to expand, producing light and spongy baked goods.

Explanation

The reaction between a strong acid (HCl) and a strong alkali (NaOH) is a standard neutralization reaction:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Hydrogen ions (H⁺) combine with hydroxide ions (OH⁻) to form neutral water molecules, while sodium and chloride ions form dissolved sodium chloride.

Explanation

Phenolphthalein (and methyl orange) are synthetic indicators synthesized from organic chemical compounds.

Natural Indicators: Litmus (extracted from lichens), turmeric powder, red cabbage extract, and flower petals of hydrangea, petunia, and geranium are naturally occurring plant-based indicators.

Explanation

Certain amphoteric metals like zinc and aluminium react with strong alkalis upon heating to form complex salts and liberate hydrogen gas:

2NaOH(aq) + Zn(s) —Δ→ Na₂ZnO₂(aq) [Sodium zincate] + H₂(g)↑

Note: Such reactions do not occur with all metals; only amphoteric metals react with strong bases to liberate H₂ gas.

Explanation

Hydrochloric acid (HCl) is a strong mineral acid that dissociates nearly 100% into aqueous ions:

HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)

Acetic acid, citric acid, and carbonic acid are weak organic or mineral acids that dissociate only partially in solution.

Explanation

• Assertion Analysis: A solution with [H⁺] = 10⁻¹ M has a pH of 1, whereas [H⁺] = 10⁻⁵ M has a pH of 5; higher acidity corresponds to a lower numerical pH (True).
• Reasoning Analysis: The mathematical relationship is:
  
  pH = -log₁₀[H⁺] = log₁₀(1 / [H⁺])
  
  Because of the negative sign, an increase in [H⁺] concentration produces a proportional decrease on the logarithmic pH scale (True, and explains A).

Explanation

Salts having the same positive or negative radicals are said to belong to a family.

• Chloride family: Salts containing the chloride anion (Cl⁻), such as KCl, NaCl, CaCl₂, and MgCl₂.
• Sulphate family: Salts containing the sulphate anion (SO₄²⁻), such as Na₂SO₄ and CuSO₄.
• Carbonate family: Salts containing the carbonate anion (CO₃²⁻), such as Na₂CO₃.

Explanation

Methyl orange is a synthetic pH indicator:

• In an acidic medium (pH < 3.1): It turns distinctly red.
• In a basic or neutral medium (pH > 4.4): It appears yellow.

Explanation

• (a) Metal + Acid: Yields Salt + Hydrogen gas.
• (b) Acid + Metal Carbonate: Yields Salt + Carbon dioxide + Water.
• (c) Base + Acid: Classic neutralization producing Salt + Water.
• (d) Metal Oxide + Acid: Basic oxide neutralization producing Salt + Water.

Explanation

Hydrogen ion concentration is related to pH by the formula [H⁺] = 10^(-pH) mol/L:

• At pH = 9: [H⁺] = 10⁻⁹ mol/L
• At pH = 7: [H⁺] = 10⁻⁷ mol/L
• At pH = 4: [H⁺] = 10⁻⁴ mol/L
• At pH = 1: [H⁺] = 10⁻¹ mol/L = 0.1 mol/L

The lowest pH value of 1 represents the highest concentration of hydrogen ions.

Explanation

• Assertion Analysis: Baking soda is commonly taken in mild doses to relieve hyperacidity and heartburn (True).
• Reasoning Analysis: NaHCO₃ is a salt derived from a strong base (NaOH) and a weak acid (H₂CO₃), giving it a mildly alkaline nature. It neutralizes excess gastric HCl safely without corroding stomach tissues:
  
  NaHCO₃(aq) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)↑
  
  This neutralizing action brings fast relief, explaining its use as an antacid (True, and explains A).

Explanation

Water of crystallisation is the fixed number of water molecules chemically incorporated in a definite stoichiometric ratio inside one formula unit of a crystalline salt.

Examples:

• Copper sulphate: CuSO₄·5H₂O (5 molecules of water)
• Washing soda: Na₂CO₃·10H₂O (10 molecules of water)
• Gypsum: CaSO₄·2H₂O (2 molecules of water)
• Ferrous sulphate: FeSO₄·7H₂O (7 molecules of water)

These water molecules are integral to the crystal structure, geometry, and color of the salt, and are not simply wet surface moisture.