(C) Lemon juice

Acids turn blue litmus red. Lemon juice is acidic in nature, whereas NaOH, KOH, and baking soda solution are basic and would turn red litmus blue.

The key fact from the textbook (Introduction, Chapter 2) is: "acids change the colour of blue litmus to red, whereas bases change the colour of red litmus to blue." NaOH, KOH, and baking soda (NaHCO₃) are all bases/alkaline — they turn red litmus blue. Only lemon juice is acidic (contains citric acid), so it turns blue litmus red. Always remember: Acid → Blue to Red; Base → Red to Blue.

(C) Hydrogen

When zinc granules are added to dilute sulphuric acid, hydrogen gas is evolved along with zinc sulphate: $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$

Source: Chapter 1, Section 1.1.2

The textbook explicitly gives the word equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen. This is a displacement reaction where zinc (a metal above hydrogen in the activity series) displaces hydrogen from the acid. Examiners expect students to recall both the correct gas and ideally the balanced equation.

(B) HCl does not ionise in the absence of water

The textbook (Activity 2.9) states that H⁺ ions are produced by HCl only in the presence of water. Dry HCl gas has no water to enable ionisation, so no H⁺(aq) ions are formed, and litmus (which responds to ions) does not change colour. HCl is actually a strong acid, so option A is factually wrong.

(D) Oxygen

Electrolysis of brine (chlor-alkali process) produces sodium hydroxide (NaOH), chlorine (Cl₂), and hydrogen (H₂). Oxygen is not produced in this process.

The textbook (Chapter 2, Section 2.4.3) clearly states the chlor-alkali process equation:
$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$
Only three products are formed — NaOH, Cl₂, and H₂. Oxygen does not appear. Students often confuse this with electrolysis of water (which gives H₂ and O₂). Remember: brine electrolysis → chlor-alkali process → no oxygen produced.

(C) 5.5

Tooth enamel begins to corrode when the pH of the mouth falls below 5.5.

Source: Chapter 2, Section 2.3.1 (pH change as the cause of tooth decay)

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The textbook explicitly states: "Tooth decay starts when the pH of the mouth is lower than 5.5." This is a direct recall fact — memorise the exact value 5.5, not 7 (neutral) or any other option. Examiners frequently ask this as a 1-mark MCQ or fill-in-the-blank.

The correct answer is (B) CaSO₄.½H₂O

Plaster of Paris is calcium sulphate hemihydrate, with the chemical formula CaSO₄.½H₂O.

• CaSO₄ (option A) is anhydrous calcium sulphate (dead burnt plaster).
• CaSO₄.½H₂O (option B) is Plaster of Paris — the key point is the half water of crystallisation.
• CaSO₄.2H₂O (option D) is gypsum, the raw material from which Plaster of Paris is made by heating.
• Examiners commonly use all four options to test whether students can distinguish gypsum, POP, and anhydrous calcium sulphate — memorise the formula exactly.

(C) Dissolves to form a soluble compound

When excess CO₂ is passed through lime water, CaCO₃ reacts with CO₂ and water to form calcium bicarbonate [Ca(HCO₃)₂], which is soluble, so the white precipitate dissolves.

The key reaction is: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Calcium bicarbonate is soluble in water, so the milky/white precipitate disappears when CO₂ is in excess. This is a standard two-step lime water test concept tested in board exams. Remember: limited CO₂ → milky (CaCO₃ precipitate); excess CO₂ → clear solution (Ca(HCO₃)₂).

(C) Less than 7

Salts of a strong acid and a weak base are acidic in nature, with pH value less than 7.

Source: More About Salts, Section 2.4.2

The textbook directly states: "salts of a strong acid and weak base are acidic with pH value less than 7." Examples include ammonium chloride (strong acid HCl + weak base NH₄OH) and zinc sulphate. Examiners expect students to recall this rule for all three combinations: strong acid + strong base → pH 7; strong acid + weak base → pH < 7; weak acid + strong base → pH > 7.

(B) A mild edible acid such as tartaric acid

Baking powder is a mixture of baking soda (NaHCO₃) and a mild edible acid such as tartaric acid.

The textbook explicitly states under Uses of Baking Soda: "baking powder is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid." The acid reacts with NaHCO₃ on heating/mixing with water to release CO₂, making bread/cake soft and spongy. Do not confuse baking powder with washing soda or baking soda alone.

Source: Chapter 2, Section 2.4.3 (Uses of Baking Soda)

(C) Onion

Onion is an olfactory indicator because its odour changes in acidic or basic media, unlike litmus, phenolphthalein, and methyl orange, which are colour-change indicators.

Olfactory indicators are substances whose smell (not colour) changes in acidic or basic solutions. The textbook (Activity 2.2) uses onion, vanilla, and clove oil as examples. Litmus, methyl orange, and phenolphthalein are visual (colour-change) indicators. Examiners expect you to know the definition and one example.

(C) An alkali

A base that is soluble in water is called an alkali. For example, NaOH and KOH dissolve in water and are alkalis.

Source: Chapter 2, Section 2.2

The textbook explicitly states: "Bases which are soluble in water are called alkalis." Examiners expect students to recall this precise definition. Do not confuse base (broader term) with alkali (only water-soluble bases).

(C) Sodium zincate

Zinc reacts with NaOH solution to form sodium zincate (Na₂ZnO₂) and hydrogen gas.

Zinc is an amphoteric metal — it reacts with both acids and bases. With NaOH, it forms sodium zincate (Na₂ZnO₂), not a chloride or sulphate (those require different acids). This reaction is a standard example of an amphoteric metal reacting with a base. Remember: Zn + 2NaOH → Na₂ZnO₂ + H₂↑.

Option A is correct. Both A and R are true and R is the correct explanation of A.

On dilution, concentration of H₃O⁺ ions per unit volume decreases, so pH increases (higher pH = lower H⁺ concentration).

• The textbook states: "Mixing an acid or base with water results in decrease in the concentration of ions (H₃O⁺/OH⁻) per unit volume" — this confirms R is true.
• The pH scale states: "Higher the hydronium ion concentration, lower is the pH value" — so decreasing H₃O⁺ concentration directly causes pH to rise, confirming A is true AND that R correctly explains A.
• Choose A, not B, because R is the direct/causal explanation of A.

Option A is correct.

Both A and R are true and R is the correct explanation of A. Metallic oxides are called basic oxides because they react with acids to form salt and water, just like a base does, which justifies their basic nature.

The textbook (Chapter 3) states that "metals combine with oxygen to form basic oxides." The reason correctly explains why they are called basic — their acid-neutralising behaviour mirrors that of bases. Since R directly and logically explains A, option A is the right choice. Avoid choosing B here; R is not merely a coincidental true statement — it is the actual basis for the assertion.

Option A — Both A and R are true and R is the correct explanation of A.

Plaster of Paris (CaSO₄·½H₂O) reacts with water to form gypsum (CaSO₄·2H₂O), losing its setting property, so moisture-proof storage is essential.

The Assertion is correct — moisture ruins PoP by triggering premature setting. The Reason correctly explains why: the reaction with water converts PoP back to gypsum, so it can no longer set when needed. Since R directly and correctly explains A, option A is the right choice. Note: the source passages provided do not cover PoP, but this is standard Class 10 Science (Chapter 2 — Acids, Bases and Salts) factual knowledge expected in the exam.

Answer: A

Both A and R are true and R is the correct explanation of A. Non-metallic oxides are acidic; CO₂ reacts with NaOH to form Na₂CO₃ (salt) and water, confirming its acidic nature.

Source: What you have learnt, Chapter 3

• The textbook states: "Non-metals form oxides which are either acidic or neutral" — so Assertion A is true.
• CO₂ (a non-metallic oxide) reacting with NaOH (a base) to give a salt and water is a classic example of acidic behaviour — so Reason R is true.
• R directly explains why non-metallic oxides are called acidic (they react with bases like acids do), making R the correct explanation of A.
• Choose Option A.

Experimental evidence (Activity 2.8): When glucose and alcohol solutions are tested for electrical conductivity using a bulb-and-battery setup, the bulb does not glow, showing these solutions produce no ions. Acids like HCl make the bulb glow, proving they produce ions in solution.

Nature of H⁺ ions: This tells us that mere presence of hydrogen is not enough for acidity. Acids specifically produce H⁺(aq) / H₃O⁺ ions in water, which are responsible for acidic properties. Glucose and alcohol do not ionise, so they are not acids.

Source: Chapter 2, Section 2.2 (Activity 2.8)

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• 1 mark for the experimental evidence (conductivity test — bulb glows with acids, not with glucose/alcohol).
• 1 mark for the conclusion about H⁺ ions (acids release H⁺(aq)/H₃O⁺; not all H-containing compounds do).
• Examiners want you to name the specific activity/observation, not just state it vaguely.
• Key phrase: "acids produce H⁺(aq) ions in solution" — must appear in your answer.

Two substances the farmer can add are quicklime (CaO) or slaked lime [Ca(OH)₂].

Chemical basis: Acidic soil has excess H⁺ ions (low pH). Lime being basic produces OH⁻ ions, which neutralise H⁺ ions:

$$\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O(l)}$$

This raises the soil pH to a suitable level for crop growth.

Source: Chapter 2, Section 2.2

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• The examiner expects you to name two basic substances used to treat acidic soil — quicklime and slaked lime are the standard answers.
• The key concept is neutralisation: acid + base → salt + water. Showing the ionic equation (H⁺ + OH⁻ → H₂O) demonstrates the chemical basis clearly.
• Do not write baking soda here — it is used for acidity in the stomach, not typically for soil in CBSE context. Lime-based answers score full marks.
• 1 mark for naming two substances; 1 mark for the chemical explanation.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.

Example: Copper sulphate — CuSO₄·5H₂O

Observation on strong heating: The blue crystals turn white (as water of crystallisation is removed), and water droplets appear on the cooler parts of the boiling tube.

The examiner expects: (1) a correct definition, (2) a named example with correct formula, and (3) two observations — colour change (blue → white) and water droplets. Copper sulphate is the textbook example used in Activity 2.15. Washing soda (Na₂CO₃·10H₂O) is also acceptable as an example. Don't forget the formula — it earns a mark by itself.

Source: Chapter 2, Section 2.4.4

A base (alkali) should be applied to relieve the pain, because bee sting is acidic in nature. A base neutralises the acid, providing relief. This is a neutralisation reaction.

The substance traditionally found in the leaves of the dock plant is baking soda (sodium hydrogencarbonate).

(Note: The dock plant leaves naturally contain an alkaline/basic substance that neutralises the acid of the bee sting.)

Source: Chapter 2, Introduction / Acids, Bases and Salts

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• The key concept here is neutralisation — an acid is neutralised by a base.
• Examiners expect you to identify: (1) the type of substance = base/alkali, (2) the reason = neutralisation, and (3) the name = baking soda / sodium hydrogencarbonate from dock plant leaves.
• The dock plant connection is a classic NCERT example from this chapter; remember it as a real-life application of neutralisation.
• Do not write "acid" as the remedy — that is a common mistake since bee sting is already acidic.

Acid rain is rain water having pH less than 5.6, caused by the presence of acidic pollutants like SO₂ and NO₂ dissolved in rainwater.

Harmful effect on aquatic ecosystems: When acid rain flows into rivers, it lowers the pH of river water, making survival of aquatic life difficult.

Source: Chapter 2, Section 2.3.1 — Importance of pH in Everyday Life

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• The examiner expects the exact pH value of 5.6 — writing "less than 7" will not fetch marks.
• One clearly stated harmful effect is enough for a 2-mark answer; do not over-explain.
• The phrase "survival of aquatic life becomes difficult" directly mirrors the textbook language — using it shows the examiner you know the source material.

Decomposition reaction on heating:

$$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\uparrow$$

Why bread becomes soft and spongy:
When baking powder is heated or mixed with water, NaHCO₃ reacts with the edible acid (e.g., tartaric acid) present and releases CO₂ gas. This CO₂ gets trapped in the dough, causing it to rise and making the bread soft and spongy.

Source: Chapter 2, Section 2.4.3 — Baking soda

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• The equation must be balanced and include the heat arrow (Δ); missing it loses marks.
• The key point examiners look for: CO₂ is the gas responsible for making bread spongy — state this explicitly.
• Note the two scenarios given in the textbook: NaHCO₃ decomposes on heating alone, but in baking powder it also reacts with an acid to release CO₂. Since the question asks about baking powder use, mentioning the acid reaction is ideal.
• Do not confuse the decomposition equation (2 mol NaHCO₃) with the single-mol acid reaction equation.

HCl (hydrochloric acid) is the stronger acid.

HCl is a mineral acid that is completely ionised in water, producing a large number of H⁺ ions. Acetic acid (CH₃COOH), being a carboxylic acid, is a weak acid — it only partially ionises in water, producing fewer H⁺ ions at the same concentration.

Since acid strength depends on the number of H⁺ ions produced, HCl is stronger than CH₃COOH at equal concentrations.

Comparison of pH: At the same concentration, HCl produces more H⁺ ions, so it has a lower pH than acetic acid. Acetic acid, being partially ionised, produces fewer H⁺ ions and therefore has a higher pH than HCl.

Source: Chapter 2, Section 2.3; Chapter 4, Section 4.4.2

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• The key phrase from the textbook is: "unlike mineral acids like HCl, which are completely ionised, carboxylic acids are weak acids." Quote or paraphrase this directly — examiners look for it.
• Connect acid strength to H⁺ ion concentration (more H⁺ = stronger acid = lower pH). This links Chapter 2 (pH scale) with Chapter 4 (acetic acid properties).
• Don't just state which is stronger — explain why (ionisation) and compare pH as the question asks both. All three points carry marks.

Manufacture of Bleaching Powder:
Bleaching powder is manufactured by passing chlorine gas over dry slaked lime [Ca(OH)₂] at about 40°C.

Balanced Chemical Equation:

$$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$

(Calcium hydroxide + Chlorine → Bleaching powder + Water)

Two Uses of Bleaching Powder:

1. It is used as a disinfectant and for sterilising drinking water.
2. It is used for bleaching cotton and linen in the textile industry.

This question tests knowledge from the Acids, Bases and Salts chapter. Examiners expect: (1) a one-line description of the manufacturing process mentioning both reactants and the temperature condition, (2) the correctly written and balanced equation with formula CaOCl₂ for bleaching powder, and (3) any two distinct uses. Avoid vague answers — be specific (e.g., "drinking water" not just "water").

When sodium carbonate reacts with dilute hydrochloric acid, it produces sodium chloride, water, and carbon dioxide gas. Effervescence (brisk bubbling) is observed.

Balanced equation:

$$\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

Test for the gas produced (CO₂):
Pass the gas through lime water [Ca(OH)₂ solution]. The lime water turns milky, confirming the gas is carbon dioxide.

$$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$$

The white precipitate of CaCO₃ causes the milkiness.

Source: Chapter 2, Section 2.1.3 — How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

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• Mark distribution (3 marks): 1 mark for stating the products/observation, 1 mark for the balanced equation, 1 mark for the lime water test with its equation or description.
• Always include state symbols (s, aq, l, g) in balanced equations — examiners reward this.
• The lime water test is the standard test for CO₂ at this level; mentioning the white precipitate of CaCO₃ completes the answer.
• Do not confuse Na₂CO₃ (sodium carbonate / washing soda) with NaHCO₃ (baking soda) — both react with HCl to give CO₂, but the equations differ in stoichiometry.

(a) During indigestion, the stomach produces excess hydrochloric acid, which causes discomfort, pain, and a burning sensation.

(b) Antacids are mild bases (alkaline substances) that neutralise the excess acid in the stomach. For example, sodium hydrogencarbonate (baking soda) and magnesium hydroxide.

(c) Magnesium hydroxide [Mg(OH)₂] is preferred over sodium hydroxide (NaOH) because NaOH is a strong base and is highly corrosive, which could harm the stomach lining. Mg(OH)₂ is a mild, non-corrosive base and is safe for ingestion.

Source: Chapter 2, Section 2.4.3 (Baking soda)

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• (a) Simply state that excess HCl is produced — this is the root cause of indigestion.
• (b) The keyword is "base/alkaline" — antacids neutralise the acid by a neutralisation reaction (Base + Acid → Salt + Water).
• (c) This is the most important part: the distinction is mild base vs. strong/corrosive base. Examiners expect you to use the word "corrosive" for NaOH. Mg(OH)₂ is mentioned in Activity 2.1 as a base, and the text explains NaHCO₃ is "mild non-corrosive" — apply the same logic to Mg(OH)₂ vs. NaOH.

Balanced chemical equation for the chlor-alkali process:

$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \xrightarrow{\text{electricity}} 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$

(i) Chlorine is produced at the anode (positive electrode).

(ii) Hydrogen is produced at the cathode (negative electrode).

Industrial use of NaOH: Sodium hydroxide is used in the manufacture of soaps and detergents (also used in paper making and de-greasing metals).

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• The equation must be balanced and include state symbols — examiners check this.
• "Chlor-alkali" refers to the electrolysis of brine (NaCl solution) producing chlorine (Cl₂), alkali (NaOH), and hydrogen (H₂) — remember all three products.
• Anode = oxidation → Cl⁻ loses electrons → Cl₂ formed. Cathode = reduction → H⁺ gains electrons → H₂ formed. This anode/cathode distinction is a common 1-mark trap.
• Any one valid industrial use of NaOH earns the mark (soaps, paper, textiles, de-greasing).

pH change as milk turns to curd: The pH decreases (becomes more acidic). During the formation of curd, bacteria act on milk and produce lactic acid. This increases the H⁺ ion concentration, lowering the pH below 6.

Effect of baking soda added by the milkman:

(a) Baking soda is alkaline, so it raises the pH of fresh milk from 6 to slightly alkaline. This makes the milk last longer before turning sour.

(b) Since lactic acid is produced during curd formation, the added baking soda neutralises this acid. The milk therefore takes a longer time to set as curd, because the acid produced must first neutralise the baking soda before the pH can drop enough for curdling to occur.

Source: Chapter 2 — Acids, Bases and Salts; Section 2.3.1 and Exercise Q.11–12

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• Examiners expect students to name lactic acid as the cause of pH decrease — this is a key fact from the chapter.
• For the baking soda part, the core idea is neutralisation: baking soda (basic) neutralises the lactic acid produced, delaying the pH drop needed for curdling. Mentioning this mechanism earns full marks.
• Table 2.3 confirms lactic acid is present in sour milk/curd, supporting the answer.
• Don't just say "pH increases/decreases" — always explain why using ion concentration or chemical reasoning.

(a) Solution X (pH = 2) is strongly acidic in nature. Solution Y (pH = 12) is strongly basic (alkaline) in nature. Values less than 7 indicate acidic solutions and values greater than 7 indicate basic solutions.

(b) Solution Y (pH = 12) has a higher concentration of OH⁻ ions. As pH value increases from 7 to 14, it represents an increase in OH⁻ ion concentration.

(c) The resulting solution will be acidic. Solution X (pH = 2) is a strong acid with very high H⁺ concentration, while Solution Y (pH = 12) is a strong base. Since equal volumes are mixed but the acid's H⁺ concentration (10⁻² M) exceeds the base's OH⁻ concentration (10⁻² M) — actually they are equal — the solution will be neutral (pH ≈ 7), as H⁺ and OH⁻ ions neutralise each other: H⁺(aq) + OH⁻(aq) → H₂O(l).

Source: Chapter 2, Section 2.3

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• pH 2 and pH 12 are equidistant from 7 — pH 2 means [H⁺] = 10⁻² M and pH 12 means [OH⁻] = 10⁻² M. Equal volumes → equal moles of H⁺ and OH⁻ → neutral solution. Examiners expect students to reason through the neutralisation, not just guess "acidic."
• For part (b), remember: higher pH above 7 = more OH⁻ ions.
• Use the phrase "neutralisation reaction" and the ionic equation H⁺ + OH⁻ → H₂O for full marks on part (c).

(a) A neutralisation reaction is when an acid reacts with a base to form salt and water, with the evolution of heat. Example: NaOH + HCl → NaCl + H₂O.

The ionic equation for all strong acid–strong base neutralisation reactions is:

$$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$$

(b) The blue-green solution indicates formation of copper sulphate, so metal oxide X is copper(II) oxide (CuO).

$$\text{CuO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l)$$

CuO is a basic oxide because it reacts with an acid (H₂SO₄) to form salt and water only, showing no acidic character.

(c) Zinc reacts with sodium hydroxide as follows:

$$\text{Zn}(s) + 2\text{NaOH}(aq) \rightarrow \text{Na}_2\text{ZnO}_2(aq) + \text{H}_2(g)$$

Difference: When a metal reacts with an acid, hydrogen gas is displaced and a salt of the acid is formed. Here, zinc reacts with a base (NaOH) and still produces hydrogen gas, forming sodium zincate. This shows zinc is an amphoteric metal — it can react with both acids and bases, unlike most metals which react only with acids.

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• (a) Examiners expect the word "salt and water" and the net ionic equation $H^+ + OH^- \rightarrow H_2O$. Avoid writing full ionic equations with spectator ions.
• (b) "Blue-green" is the key clue for Cu²⁺ ions in solution → CuSO₄ → CuO was the reactant. Basic oxide reacts with acids (not bases) to give salt + water only.
• (c) The reaction of Zn with NaOH is a standard NCERT example showing amphoteric behaviour. The key contrast to emphasise: metals normally react with acids to give H₂, but Zn also reacts with a base to give H₂ — this is the difference examiners look for.

(a) The pH scale is a scale from 0 to 14 used to measure the concentration of H⁺(aq) ions in a solution. Higher the H⁺ ion concentration, lower the pH value. A neutral solution has pH = 7; values below 7 indicate acidic solutions, and above 7 indicate basic solutions. On dilution, the concentration of H⁺ (or OH⁻) ions per unit volume decreases, so the pH of an acid moves closer to 7 (increases), and that of a base also moves closer to 7 (decreases).

(b) Our body works within the pH range of 7.0 to 7.8; living organisms can survive only in a narrow pH range. Even a small change disturbs metabolic activities.

• Digestion: The stomach produces HCl for digestion. Excess acid (indigestion) causes pain; antacids (mild bases) neutralise it.
• Tooth decay: Bacteria degrade food, producing acids. When mouth pH falls below 5.5, tooth enamel (calcium hydroxyapatite) gets corroded, causing decay.

(c) Solution with pH ≈ 4 (orange) is acidic — high H⁺ concentration, low OH⁻ concentration. Solution with pH ≈ 11 (violet) is strongly alkaline — very low H⁺ concentration, high OH⁻ concentration. Thus, the first solution has much greater H⁺ concentration than the second, while the second has much greater OH⁻ concentration than the first.

Source: Chapter 2, Sections 2.3 and 2.3.1

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• (a): Examiners want the definition, the inverse relationship (higher H⁺ → lower pH), the neutral/acid/base ranges, and the effect of dilution (pH shifts toward 7). Don't just memorise the range — explain the direction of shift on dilution.
• (b): Two specific examples with the mechanism (excess acid → antacid neutralises; pH < 5.5 → enamel corrodes) earn full marks. Just naming examples without explanation loses marks.
• (c): This is a comparison question — clearly state which solution has more H⁺ and which has more OH⁻. No calculation is needed; reasoning from pH values (below 7 = acidic, above 7 = basic) is sufficient.

(a) Sodium Hydroxide — Chlor-Alkali Process:
When electricity is passed through brine (aqueous NaCl), it decomposes. This is called the chlor-alkali process.

$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$

Cl₂ is released at the anode; H₂ at the cathode; NaOH forms near the cathode.

(b) Washing Soda:
Sodium carbonate is recrystallised with water to give washing soda (Na₂CO₃·10H₂O):

$$\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3.10\text{H}_2\text{O}$$

Uses: (i) Used in glass, soap, and paper industries. (ii) Used as a domestic cleaning agent and to remove permanent hardness of water.

(c) Plaster of Paris:
$$\text{CaSO}_4.2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4.\tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O}$$

When mixed with water, Plaster of Paris rehydrates and converts back to gypsum (CaSO₄·2H₂O), forming a hard solid mass:

$$\text{CaSO}_4.\tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4.2\text{H}_2\text{O}$$

Practical use: Used by doctors to set fractured bones in the correct position.

Source: Chapter 2, Section 2.4.3 and 2.4.4

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• (a) Examiners expect the process name ("chlor-alkali"), the balanced equation, and mention of products at each electrode — all three earn marks.
• (b) The key point is that Na₂CO₃ is recrystallised (not just mixed) with water. Write the equation and give two distinct uses from the textbook list.
• (c) Two equations are needed: formation of Plaster of Paris from gypsum, and its setting with water. The chemical explanation (rehydration → gypsum) and one use (bone-setting/fractured bones) are expected. Don't forget the temperature (373 K) in the heating equation.

(i) HCl and H₂SO₄ dissolve in water to produce ions (H⁺ and respective anions), which carry electric current, making the bulb glow. Glucose and alcohol do not ionise in water; they produce no ions, so no current flows and the bulb does not glow.

(ii)
$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
The ion formed (H₃O⁺) is called the hydronium ion.

(iii) In solid state, NaOH ions are held in a fixed crystal lattice and cannot move, so it does not conduct electricity. When dissolved in water, it dissociates into free Na⁺ and OH⁻ ions that move freely. The hydroxide ion (OH⁻) is responsible for the basic nature of NaOH solution.

(iv) Glucose and alcohol do contain hydrogen, but they do not release H⁺ (hydrogen) ions when dissolved in water. Only compounds that produce H⁺(aq) ions in solution are classified as acids. Since glucose and alcohol produce no ions at all, they are not acids.

Source: Chapter 2, Section 2.2 and 2.2.1

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• (i) The key concept is ionisation — examiners want the word "ions" linked to conductivity.
• (ii) Write the equation correctly and name H₃O⁺ as "hydronium ion" — both parts carry marks.
• (iii) Two points needed: why solid doesn't conduct (no free ions) + OH⁻ causes basic nature.
• (iv) The distinction is not the presence of hydrogen but whether H⁺(aq) ions are released in water — this is the core idea examiners test here.

(i) Tooth enamel is made of calcium hydroxyapatite, which does not dissolve in water. It is corroded only when the pH in the mouth falls below 5.5. Above this pH, the acid is not strong enough to corrode the enamel, so decay does not begin.

(ii) Toothpastes are generally basic in nature. They neutralise the excess acid produced in the mouth by bacteria, raising the pH above 5.5, thereby preventing the corrosion of tooth enamel and stopping tooth decay.

(iii) Bacteria present in the mouth degrade sugar and food particles left in the mouth after eating and produce acids. These acids lower the pH of the mouth.

(iv) Quicklime reacts with HCl as follows:

$$\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$$

Source: Chapter 2, Section 2.3.1 (pH change as the cause of tooth decay); Chapter 5 (Dental caries box)

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• (i) The key fact is that enamel (calcium hydroxyapatite) is stable above pH 5.5 — state this explicitly.
• (ii) Always mention: toothpastes are basic, they neutralise the acid — both points needed for the mark.
• (iii) The source of acid is bacterial degradation of sugar/food particles — don't just say "bacteria" without mentioning what they break down.
• (iv) Write the fully balanced equation; note it is 2HCl (not 1). CaO + 2HCl → CaCl₂ + H₂O. Examiners check balancing.

(i) The gas produced is carbon dioxide (CO₂).

$$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$$

The white precipitate is calcium carbonate (CaCO₃), which makes lime water milky.

(ii) When excess CO₂ is passed, the insoluble CaCO₃ reacts with water and excess CO₂ to form soluble calcium hydrogencarbonate, causing the milkiness to disappear.

$$\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3)_2\text{(aq)}$$

(iii) The blue-green colour is due to the formation of copper(II) chloride (CuCl₂). Balanced equation:

$$\text{CuO(s)} + 2\text{HCl(aq)} \rightarrow \text{CuCl}_2\text{(aq)} + \text{H}_2\text{O(l)}$$

(iv) Copper oxide reacts with hydrochloric acid (an acid) to produce a salt and water — this is similar to the reaction of a base with an acid. Since metallic oxides react with acids to give salt and water, they are classified as basic oxides.

Source: Chapter 2, Sections 2.1.3 and 2.1.5

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• (i) The reaction of Na₂CO₃ with HCl releases CO₂; always write the lime water equation with state symbols.
• (ii) Examinees often forget the reason — stress that Ca(HCO₃)₂ is soluble, which is why milkiness disappears.
• (iii) "Blue-green" is the key observation cue; always name the compound responsible (CuCl₂) and balance the equation.
• (iv) The definition-based justification is essential: link "metal oxide + acid → salt + water" directly to the definition of a basic oxide. One sentence of reasoning scores the mark.