The H⁺(aq) ion (hydronium ion, H₃O⁺) is responsible for the acidic nature of a solution. Acids produce H⁺ ions in aqueous solution, which give them their acidic properties.

Source: Chapter 2, Section 2.2

The examiner expects you to name H⁺(aq) or H₃O⁺ as the ion responsible. Either notation is acceptable, but mentioning both shows thorough understanding. The parallel fact — that OH⁻(aq) is responsible for basic nature — is often asked alongside, so keep it in mind.

The pH of a neutral solution is 7.

Source: Chapter 2, Section 2.3 (How Strong Are Acid or Base Solutions?)

This is a direct fact-based question. The examiner expects the single value "7" with the word "neutral." You may also add that values below 7 are acidic and above 7 are basic, but for 1 mark, just stating pH = 7 for a neutral solution is sufficient. Avoid writing lengthy explanations.

The process is called the chlor-alkali process. Electrolysis of brine (aqueous NaCl solution) produces sodium hydroxide, chlorine gas, and hydrogen gas.

Source: Chapter 2, Section 2.4.3

The examiner expects the specific name "chlor-alkali process" for 1 mark. The name comes from the products: chlor for chlorine and alkali for sodium hydroxide. Simply writing "electrolysis" alone is insufficient — the named process is what the question asks for.

The chemical name of baking soda is sodium hydrogencarbonate (NaHCO₃).

Baking soda is mentioned in Chapter 2 as NaHCO₃ (sodium hydrogencarbonate). Examiners expect the full IUPAC/common chemical name. Writing just the formula without the name may not fetch full marks.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt (e.g., CuSO₄·5H₂O contains 5 water molecules).

Source: Chapter 2, What you have learnt

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The examiner expects the exact definition from the textbook. The key phrase is "fixed number of water molecules" in "one formula unit of a salt" — both parts are needed for full marks. Adding a familiar example (like copper sulphate) strengthens the answer but is not compulsory for 1 mark.

Onion (or turmeric) is an olfactory indicator. Its smell changes in the presence of acids and bases, helping to detect them.

Source: Acids, Bases and Salts, Chapter 2

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The chapter on Acids, Bases and Salts mentions olfactory indicators — substances whose smell changes in acidic or basic conditions (e.g., onion, vanilla). Examiners expect you to name one such indicator. "Onion" is the most commonly cited example in NCERT/CBSE context. Do not confuse olfactory indicators with visual ones like litmus or turmeric.

Tooth decay begins when the pH of the mouth falls below 5.5. At this pH, the tooth enamel (calcium hydroxyapatite) gets corroded by acids produced by bacteria.

Source: Chapter 2, Section 2.3.1

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The examiner expects the specific pH value (below 5.5) and ideally a brief reason (acid corrodes enamel). Since it's 1 mark, just the pH value with one supporting detail is sufficient. Do not write more than one or two lines.

The chemical formula of Plaster of Paris is CaSO₄·½H₂O (calcium sulphate hemihydrate).

This question is from Chapter 2 (Acids, Bases and Salts), not Chapter 4. The source passages provided are from Chapter 4 (Carbon and its Compounds) and do not contain information about Plaster of Paris. However, the standard CBSE answer is CaSO₄·½H₂O. Examiners expect the correct formula with the half water of crystallisation clearly written. Some textbooks also write it as (CaSO₄)₂·H₂O — either form is accepted.

(c) Acidic

A solution that turns blue litmus red is acidic in nature. Acids change blue litmus to red.

Source: Introduction, Chapter 2

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The key fact from the textbook introduction: acids change blue litmus to red; bases change red litmus to blue. This is a direct, one-line recall question. Always remember the direction of colour change — blue→red = acid, red→blue = base. "Amphoteric" is not a concept tested at this level in this context.

(c) Hydrogen

When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved:
$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$

Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations

---

The textbook explicitly gives this reaction as an example while explaining balanced chemical equations. Zinc displaces hydrogen from dilute sulphuric acid, producing zinc sulphate and hydrogen gas. This is a displacement reaction. Remember: active metals (like Zn, Fe, Mg) react with dilute acids to release H₂ gas — never O₂, CO₂, or SO₂.

(c) Sodium hydroxide solution

Sodium hydroxide (NaOH) is a base that produces OH⁻ ions in solution. Bases have pH greater than 7, while lemon juice, vinegar, and hydrochloric acid are all acids with pH less than 7.

Source: Chapter 2, Section 2.3

The pH scale runs from 0–14: acids have pH < 7, neutral substances = 7, and bases (alkalis) have pH > 7. NaOH is a strong base, so its pH is well above 7. Lemon juice, vinegar (acetic acid), and HCl are all acidic (pH < 7). Examiners expect you to recall that bases/alkalis have pH > 7.

(b) Na₂CO₃.10H₂O

Washing soda is sodium carbonate decahydrate, with the chemical formula Na₂CO₃.10H₂O.

Source: Chapter 2, Section 2.4.3

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The key distinction here is between Na₂CO₃ (anhydrous sodium carbonate) and Na₂CO₃.10H₂O (washing soda). Washing soda is the hydrated form — it contains 10 water molecules of crystallisation. The textbook explicitly states: "Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O (washing soda)." Options (c) NaHCO₃ is baking soda, and (d) NaOH is caustic soda — common distractors in this type of MCQ.

When CO₂ is passed in excess through lime water, the white milky precipitate of calcium carbonate dissolves to form soluble calcium bicarbonate, and the solution turns colourless again.

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

Source: Chapter 1, Section 1.2.1 (and general chemistry knowledge consistent with the passage)

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• The first reaction (lime water turning milky) is: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O — this is the standard test for CO₂.
• The excess CO₂ reaction converts insoluble CaCO₃ into soluble Ca(HCO₃)₂, clearing the solution. Examiners expect both the observation and the balanced equation.
• Note that the source passage mentions CaCO₃ formation from Ca(OH)₂ + CO₂ (equation 1.14); the excess CO₂ step is the standard follow-up reaction tested in board exams.

The reaction between sodium carbonate and dilute hydrochloric acid is:

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

Sodium carbonate reacts with dilute HCl to produce sodium chloride, water, and carbon dioxide gas. The CO₂ gas turns lime water milky.

Source: Chapter 2, Section 2.1.3

• The examiner expects the correctly balanced equation (1 mark) with correct formulae and coefficients — note the coefficient 2 before HCl and 2NaCl.
• The second mark is for state symbols (s, aq, l, g) and/or a brief statement of products. Many students lose marks by omitting CO₂ or forgetting to balance the equation. Remember: metal carbonates + acid → salt + water + CO₂.

Two uses of bleaching powder are:

1. Disinfecting drinking water — It is used to make drinking water free from germs and safe for consumption.
2. Bleaching cotton and linen in the textile industry — It removes colour/stains from fabrics.

(It is also used as an oxidising agent in the chemical industry.)

Bleaching powder (Ca(OCl)Cl) is a product of Chapter 2 (Acids, Bases and Salts). The source passages provided do not directly discuss bleaching powder, so this answer draws on the standard NCERT Chapter 2 content. Examiners expect any two distinct uses — disinfection of water and bleaching of fabrics are the most commonly credited. Write each use as a clear, separate point to ensure both marks are awarded.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime [Ca(OH)₂]:

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

$$\text{(Slaked lime)} \hspace{1cm} \hspace{2cm} \text{(Bleaching powder)}$$

Bleaching powder is the common name for calcium oxychloride (CaOCl₂).

Source: Chapter 2, Acids, Bases and Salts (Bleaching Powder section)

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• The examiner expects the balanced chemical equation with state symbols or at least the names of reactants and products — this fetches 1 mark.
• Naming the product as calcium oxychloride / CaOCl₂ and mentioning the condition (dry slaked lime + Cl₂) fetches the second mark.
• Although this specific equation isn't in the provided passages, it is the standard NCERT Class 10 Chapter 2 content. Write the equation clearly and state the chemical name of bleaching powder.

A reaction in which an acid and a base react with each other to form salt and water is called a neutralisation reaction. Heat is also released in this process.

General equation:

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

Example: NaOH + HCl → NaCl + H₂O

Source: Acids, Bases and Salts, Chapter 2

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• The examiner expects the definition (acid + base → salt + water) and the general equation — both are needed for full 2 marks.
• Mentioning heat release is a good addition but not compulsory for this mark allocation.
• A concrete example (like NaOH + HCl) strengthens the answer but keep it brief since this is only 2 marks.
• Do not write a long paragraph — one definition sentence + the equation is sufficient.

HCl and H₂SO₄ show acidic character because they dissociate in water to produce H⁺(aq) / H₃O⁺ ions. Glucose, though it contains hydrogen atoms, does not dissociate to produce H⁺ ions in solution.

Activity 2.8:
Two nails fixed on a cork in a 100 mL beaker are connected to a 6V battery through a bulb. When dilute HCl or H₂SO₄ is poured in, the bulb glows, showing that ions (H⁺ and Cl⁻/SO₄²⁻) are present and conduct electricity. When glucose solution is used, the bulb does not glow, showing no ions are present.

Conclusion: Acidic character depends on the production of H⁺(aq) ions in solution, not merely on the presence of hydrogen atoms. Since glucose does not ionise, it shows no acidic character.

Source: Chapter 2, Section 2.2, Activity 2.8

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• The examiner expects two parts: (1) the conceptual reason — ionisation producing H⁺ ions — and (2) a description of Activity 2.8 with observations and conclusion.
• Key contrast: HCl/H₂SO₄ → ions → bulb glows; glucose → no ions → bulb does not glow.
• Always state the conclusion explicitly — that acidic character is due to H⁺(aq) ions, not just hydrogen atoms.
• Avoid writing the full circuit diagram description; a brief procedural mention is enough for 3 marks.

Dry HCl gas does not ionise in the absence of water, so it cannot produce H⁺ ions. Since no H⁺(aq) ions are formed, it shows no acidic property and does not change the colour of dry litmus paper.

When HCl is dissolved in water, it ionises completely:

$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$

The H⁺ ions (hydronium ions) produced are responsible for its acidic nature, which turns blue litmus red.

Thus, water is essential for an acid to show its acidic behaviour.

Source: Chapter 2, Section 2.2.1

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• The key concept tested is: water is necessary for ionisation of HCl.
• Examiners expect you to state that dry HCl has no H⁺ ions → no colour change, and that in solution H₃O⁺ ions form → acidic properties shown.
• Writing the equation $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$ earns a dedicated mark.
• This is directly from Activity 2.9 and the textbook statement: "The separation of H⁺ ion from HCl molecules cannot occur in the absence of water."

Problem faced by crops:
Soil with pH 4 is highly acidic. Most crops cannot grow well in such acidic soil because nutrients become unavailable and minerals like aluminium and manganese may reach toxic levels, hampering healthy plant growth.

What should be added and why:
The farmer should add quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) to the soil. These are basic substances that neutralise the excess acid in the soil, raising the pH to a range suitable for healthy crop growth.

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

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• The question tests knowledge of soil pH and its agricultural importance from Section 2.3.1.
• Key point: pH 4 = highly acidic → unfavourable for crops.
• The textbook specifically states farmers treat acidic soil with quick lime / slaked lime / chalk — name all three for full marks.
• Mention neutralisation as the reason; that's the chemical concept being tested.
• 3-mark split: 1 mark for identifying the problem, 1 mark for naming the substance(s), 1 mark for explaining why (neutralisation).

Chlor-alkali process: When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes. This process is called the chlor-alkali process (chlor = chlorine; alkali = sodium hydroxide).

Chemical equation:

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

Products at each electrode:

• Anode (+): Chlorine gas (Cl₂)
• Cathode (−): Hydrogen gas (H₂); sodium hydroxide (NaOH) solution is also formed near the cathode.

Source: Chapter 2, Section 2.4.3

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• The name "chlor-alkali" must be explained (1 mark).
• The balanced equation is essential (1 mark).
• Products at each electrode must be stated clearly — examiners specifically check anode → Cl₂ and cathode → H₂ + NaOH (1 mark).
• Do not confuse: NaOH forms near the cathode, not directly at it. All three points are directly from the textbook passage.

Copper sulphate crystals (CuSO₄·5H₂O) are blue because they contain water of crystallisation. On heating, this water is lost and the crystals become anhydrous copper sulphate (CuSO₄), which is white. This is a decomposition reaction.

$$\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \xrightarrow{\text{Heat}} \text{CuSO}_4 + 5\text{H}_2\text{O}$$

When water is added to the white anhydrous copper sulphate powder, it reabsorbs the water and turns blue again, as the hydrated form is restored. This reaction is also exothermic.

Source: Chapter 1, Chemical Reactions and Equations

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• The key concept here is water of crystallisation — water molecules that are part of the crystal structure and give CuSO₄ its blue colour.
• Examiners expect you to name both forms: hydrated (blue) and anhydrous (white).
• Mentioning that the blue colour is restored on adding water scores the third mark.
• The equation is a bonus but strengthens the answer; write it if you remember it.

When a concentrated acid is added to water, the large volume of water absorbs the heat generated, keeping the solution safe. However, if water is added to a concentrated acid, the small amount of acid cannot absorb the heat produced and the mixture may splash or boil, causing severe burns. The glass container may also crack due to excessive local heating.

Property involved: The dissolving of an acid in water is a highly exothermic process. Adding acid to water ensures the heat is distributed over a large volume of water, making the process safe.

Source: Chapter 2, Section 2.2.1

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• The examiner expects: (1) what happens when water is added to acid (splashing, burns, breakage), (2) what happens when acid is added to water (safe), and (3) naming the property — exothermic process.
• The phrase "acid must always be added slowly to water with constant stirring" is from the textbook — use it.
• Do not confuse "exothermic" with "endothermic." Mixing acid/base with water releases heat — that's exothermic.
• 3 marks = roughly 3 distinct points: safety reason (1) + consequence of wrong method (1) + property name (1).

When baking soda (sodium hydrogen carbonate) is heated, it decomposes as follows:

$$2\text{NaHCO}_3(s) \xrightarrow{\Delta} \text{Na}_2\text{CO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$$

The carbon dioxide (CO₂) gas released during this reaction gets trapped in the dough, causing it to rise and become soft and spongy during baking.

• The balanced equation must show 2 moles of NaHCO₃ on the left; examiners deduct marks for unbalanced equations.
• Clearly name CO₂ as the product responsible for the spongy texture — this is the direct answer to the second part and earns a separate mark.
• The three marks split roughly as: 1 mark for correct reactant/products, 1 mark for balancing, 1 mark for identifying CO₂ as the cause of soft/spongy bread.

Fizzing will occur more vigorously in the test tube containing HCl (hydrochloric acid).

HCl is a strong acid that completely dissociates in water, producing a high concentration of H⁺ ions:
$$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\uparrow$$

CH₃COOH (acetic acid) is a weak acid that only partially dissociates, producing fewer H⁺ ions at the same concentration.

Since the rate of reaction between magnesium and the acid depends on the concentration of H⁺ ions, HCl provides more H⁺ ions, causing faster and more vigorous evolution of hydrogen gas (fizzing).

---

The key concept here is strong acid vs. weak acid. Examiners expect students to:

1. Correctly identify HCl as the test tube with more vigorous fizzing.
2. Link the reason to degree of dissociation → H⁺ ion concentration → rate of reaction.
3. Write the balanced equation for at least one reaction (HCl with Mg is sufficient).

Do not just say "HCl is stronger" without explaining why that leads to more fizzing — the H⁺ ion concentration argument is what earns the marks.

The pH of milk decreases as it turns into curd.

Fresh milk has a pH of 6. When milk turns into curd, bacteria present in the milk convert lactose (milk sugar) into lactic acid. As lactic acid is produced, the concentration of H⁺ ions in the solution increases. Since a higher H⁺ ion concentration means a lower pH value, the pH of milk decreases below 6, making it more acidic.

Source: Chapter 2, Section 2.3.1 (Table 2.3 — Sour milk/curd contains lactic acid)

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• The key facts examiners look for: (1) pH decreases, (2) lactic acid is formed by bacteria, (3) lactic acid increases H⁺ concentration, (4) higher H⁺ = lower pH.
• Table 2.3 in the textbook explicitly lists "Sour milk (Curd) → Lactic acid," so mentioning lactic acid is essential.
• Don't just state the answer — you must explain the reason (acid formation → H⁺ ions → lower pH) to earn all 3 marks.

Strong acids are acids that produce a large number of H⁺ ions when dissolved in water. Weak acids produce fewer H⁺ ions in solution even at the same concentration.

pH scale (0–14) measures hydrogen ion concentration. A strong acid has a very low pH (closer to 0), while a weak acid has a pH slightly below 7. Thus, pH helps us compare the strength of acids quantitatively.

• Strong acid example: Hydrochloric acid (HCl) — completely ionised, very low pH.
• Weak acid example: Acetic acid (CH₃COOH) — partially ionised, pH closer to 7.

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

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• Examiners expect the definition of both terms linked to the number of H⁺ ions produced — this is the key phrase from the textbook.
• Mention the pH scale's role in quantitative measurement of H⁺ ion concentration, not just identification of acid/base.
• HCl (strong) and acetic/ethanoic acid (weak) are the textbook's own examples — always use these.
• 3 marks = definition of strong acid (1) + definition of weak acid and pH role (1) + one example each (1). Keep it concise.

Plaster of Paris (CaSO₄·½H₂O) must be stored in a moisture-proof container because it readily reacts with atmospheric moisture (water) and gets converted back into gypsum (CaSO₄·2H₂O). Once this happens, the Plaster of Paris loses its ability to set hard when water is added later, making it useless.

Chemical Equation:

$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$

$$\text{(Plaster of Paris)} \hspace{1.5cm} \text{(Gypsum)}$$

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• This question tests knowledge of Plaster of Paris from the Acids, Bases and Salts chapter (Chapter 2).
• The key point examiners look for: why moisture is harmful (it converts POP → Gypsum, causing it to set prematurely and lose utility).
• Always write the balanced chemical equation with correct formula — the ½H₂O (hemihydrate) notation is essential for full marks.
• Note: The source passages provided did not cover this topic; the answer is drawn from the standard CBSE Class 10 Chemistry syllabus (Chapter 2 — Acids, Bases and Salts).

(a) Reaction of Zinc Granules with Dilute Sulphuric Acid

Labelled Diagram:

```
Zn granules
|
______[Test tube]______
| Dilute H₂SO₄ |
|_____________________|
|
Gas evolved (H₂) → collected / tested with burning splint
```

(Diagram should show a test tube containing zinc granules and dilute H₂SO₄, with a delivery tube/mouth for gas collection.)

Balanced Chemical Equation:

$$\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$$

Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. Bubbles are observed, and the zinc granules dissolve.

Test for the Gas:
Bring a burning splint near the mouth of the test tube. The gas burns with a pop sound, confirming it is hydrogen gas.

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(b) General Summary Equation — Metal + Acid:

$$\text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen gas}$$

Source: Chapter 1 – Chemical Reactions and Equations, Section 1.1.2

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• The diagram is worth marks — label clearly: test tube, zinc granules, dilute H₂SO₄, and the gas outlet.
• Write the balanced equation with state symbols for full marks; this equation is already balanced (1:1:1:1 ratio).
• The "pop sound" with a burning splint is the standard CBSE accepted test for hydrogen — don't just say "it burns."
• For part (b), the general equation (Metal + Acid → Salt + Hydrogen) is a direct textbook statement; write it exactly.

(a) The pH scale is a scale from 0 to 14 used to measure the hydrogen ion concentration in a solution, indicating its acidic or basic nature.

• pH less than 7 → acidic solution (higher H⁺ concentration)
• pH equal to 7 → neutral solution
• pH greater than 7 → basic (alkaline) solution (higher OH⁻ concentration)

(b)

• pH less than 7 (acidic): Lemon juice, tomato juice
• pH greater than 7 (basic): 1M NaOH solution, baking soda solution

(c) Living organisms can survive only within a narrow range of pH. When acid rain (pH less than 5.6) flows into rivers, it lowers the pH of the river water. This makes survival of aquatic life difficult, as most aquatic organisms require a near-neutral pH to carry out their life processes.

Source: Chapter 2, Section 2.3 and 2.3.1

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• (a) Examiners expect the definition of pH scale and all three cases (acidic/neutral/basic) — missing any one loses a mark.
• (b) Give only two examples per category; lemon juice, tomato juice, vinegar, HCl are all valid for acidic; NaOH, milk of magnesia, baking soda for basic.
• (c) The key chain is: acid rain → lowers river pH → aquatic life cannot survive. Mention the acid rain threshold (pH < 5.6) for full marks. Keep it brief — this sub-part is worth ~1 mark.

(a) A universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions, allowing us to measure the pH of a solution (scale 0–14). A single indicator like litmus only tells us whether a substance is acidic or basic (turns red or blue) — it cannot tell how strong the acid or base is. The universal indicator gives a range of colours, enabling quantitative measurement of H⁺ ion concentration.

(b) Procedure: Take about 2 g of backyard soil in a test tube, add 5 mL water, shake well, filter, and collect the filtrate. Test the filtrate with universal indicator paper and note the colour to find the approximate pH.

Conclusion: We can determine whether the soil is acidic, neutral, or alkaline, and find out the ideal pH range required for healthy growth of plants in that region. If soil pH is very low (acidic), a farmer may treat it with lime to neutralise excess acid.

(c) Our body performs all metabolic activities within a pH range of 7.0 to 7.8. Living organisms can survive only within a narrow pH range. If the pH goes beyond this range — due to disease, diet, or acid rain entering water bodies — enzyme activity is disrupted and cells cannot function normally, making survival difficult.

Source: Chapter 2, Section 2.3 and 2.3.1

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• (a): Examiners want the key phrase "mixture of several indicators" and the contrast that litmus is qualitative (acid/base only) while universal indicator is quantitative (gives pH value).
• (b): Follow Activity 2.12 steps precisely. The conclusion must mention "ideal soil pH for plant growth" — that's the exact line the textbook uses. Bonus: linking low pH to lime treatment (from the Questions section) shows application.
• (c): Stick to the textbook line about "narrow range of pH change" and metabolic activity. Don't over-explain — two to three crisp sentences are enough for the marks available.

(a) When sodium hydroxide reacts with zinc metal, it produces sodium zincate and hydrogen gas. The balanced chemical equation is:

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

(b) When zinc reacts with dilute sulphuric acid, it also produces hydrogen gas along with zinc sulphate (a salt):

$$\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$$

The key difference is that the reaction with H₂SO₄ is a reaction of zinc with an acid, forming a simple salt (ZnSO₄). The reaction with NaOH is a reaction of zinc with a base, forming a complex salt called sodium zincate (Na₂ZnO₂). Not all metals react with bases; zinc is amphoteric (reacts with both acids and bases).

(c) General summary equation for the reaction of a base with a metal:

$$\text{Base} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas}$$

Source: Chapter 2, Section 2.1.2

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• Part (a) tests recall of the specific equation from Activity 2.4 — write it with state symbols for full marks.
• Part (b) requires a comparison: both produce H₂, but the nature of the other product differs — acid gives a normal salt; base gives a complex salt (zincate). Mentioning "not all metals react with bases" or "zinc is amphoteric" adds value.
• Part (c) is a one-line summary modelled on the acid + metal pattern given in the textbook. Keep it in the exact word-equation format the textbook uses.

(a) Preparation of Sodium Hydrogencarbonate (Baking Soda):

Baking soda is prepared from sodium chloride (common salt) by passing carbon dioxide and ammonia gas through a concentrated solution of sodium chloride (brine). The chemical equation is:

$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$

Ammonium chloride remains dissolved while sodium hydrogencarbonate, being less soluble, precipitates out and is collected.

(b) Three Uses of Baking Soda:

1. Used in making baking powder (mixture with tartaric acid); releases CO₂ to make cakes/bread soft and spongy.
2. Used as an antacid to neutralise excess acid in the stomach.
3. Used in soda-acid fire extinguishers.

(c) Baking Soda as Antacid:

The stomach produces hydrochloric acid during digestion. When excess acid is secreted, it causes indigestion/acidity. Baking soda (NaHCO₃) is a mild, non-corrosive basic salt. Being alkaline, it neutralises the excess acid in the stomach, providing relief from acidity.

Source: Chapter 2, Section 2.4.3

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• Part (a) is typically 2 marks — include the equation and one line on the process.
• Part (b) is 2 marks — list exactly three uses; the textbook gives three, so write all three.
• Part (c) is 1 mark — key phrase is "alkaline/basic nature neutralises excess acid". Keep it to 2–3 lines.
• Do not confuse "antacid" (neutralises acid) with "acid" — baking soda is basic, not acidic.
• The equation in part (a) must be written exactly as given in the textbook.

(a) A bee sting injects an acidic substance (formic acid/methanoic acid). The acid involved in nettle sting is methanoic acid (formic acid).

(b) Baking soda is sodium hydrogen carbonate (NaHCO₃), which is a base (mild alkali). When applied to the bee sting, it neutralises the acid injected, thereby relieving the pain and irritation.

(c) The dock leaf must be basic (alkaline) in nature. Since rubbing it relieves the acidic nettle sting, it must contain a basic substance that neutralises the methanoic acid present in the sting.

(d) This illustrates neutralisation: when an acid and a base react together, they cancel each other's effect to form salt and water.
$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$
Both cases show that the opposite substance (base for acid sting) removes the effect of the sting by neutralising it.

Source: Chapter 2 (Acids, Bases and Salts), Sections 2.2 and Introduction

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• (a) Bee sting = acidic; nettle sting acid = methanoic (formic) acid — both are standard textbook facts from this chapter.
• (b) Baking soda is a base; the key word examiners want is neutralisation.
• (c) This is a deduction question — since an acidic sting is relieved, the remedy must be basic. Say so clearly.
• (d) Always write the word equation for neutralisation; it fetches marks. The principle is: acid + base → salt + water, which nullifies the effect of either substance.
• For a 5-mark question, each sub-part carries roughly 1–1.5 marks, so keep each answer to 1–2 lines.

(a) Bulb glows with dilute HCl and dilute NaOH only.

Dilute HCl produces H⁺(aq) and Cl⁻ ions in solution, while dilute NaOH produces Na⁺(aq) and OH⁻(aq) ions. These free ions carry electric current through the solution, completing the circuit and making the bulb glow.

Glucose and alcohol solutions do not ionise in water — they contain no free ions — so they do not conduct electricity and the bulb does not glow.

(b) This experiment proves that acids produce H⁺(aq) ions and bases produce OH⁻(aq) ions in aqueous solution. These ions are responsible for electrical conductivity and hence for the acidic and basic properties respectively. Compounds like glucose and alcohol, though they contain hydrogen, do not ionise and are therefore not acidic.

(c) Distilled water is pure and contains no dissolved ions, so it cannot conduct electricity. Rainwater dissolves CO₂ from the atmosphere to form carbonic acid (H₂CO₃), which ionises to give H⁺ and HCO₃⁻ ions. These ions make rainwater a conductor.

Source: Chapter 2, Section 2.2

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• The key examiner expectation is ions = conductivity: always link glowing bulb → ions present → current flows.
• For part (b), name both ions explicitly: H⁺(aq) for acids, OH⁻(aq) for bases.
• For part (c), the contrast is no ions (distilled) vs. ions from dissolved CO₂ (rainwater) — both points needed for full credit.
• Glucose/alcohol: stress they contain hydrogen but do not ionise — this directly answers the textbook's investigative question in Activity 2.8.

(a) This phenomenon is called acid rain. When the pH of rain water falls below 5.6, it is termed acid rain. It is caused by the dissolving of pollutant gases (like SO₂ and NO₂ released from industries) in rainwater, forming sulphuric and nitric acids, which lower the pH.

(b) When acid rain flows into rivers, it lowers the pH of river water. Living organisms can survive only within a narrow pH range. The reduced pH makes survival of aquatic life in such rivers difficult, as their metabolic processes are disrupted.

(c) Our body functions within a pH range of 7.0 to 7.8. All living organisms are highly pH-sensitive and can tolerate only a narrow range of pH change. A significant drop in environmental pH — as caused by acid rain — threatens biological processes essential for survival.

(d) Adding lime (calcium hydroxide / slaked lime) to the river water can neutralise the excess acid, raising the pH back to a range suitable for aquatic life.

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

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• The question has four sub-parts worth a combined 5 marks (~1–1.5 marks each), so keep each part to 2–3 lines.
• Examiners expect the term "acid rain" explicitly in (a) with a cause.
• For (b) and (c), directly quote/paraphrase the textbook line: "Living organisms can survive only in a narrow range of pH change" and "survival of aquatic life becomes difficult" — these are scoring phrases.
• For (d), any base that neutralises acid is acceptable; lime/slaked lime is the standard textbook-friendly answer. Don't over-explain the chemistry here.

Metallic oxides are basic in nature because they react with acids to form salt and water.

Example: Copper oxide (a metallic oxide) reacts with dilute hydrochloric acid:

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

This shows CuO is basic as it neutralises an acid.

Non-metallic oxides are acidic in nature because they react with bases to form salt and water.

Example: Carbon dioxide (a non-metallic oxide) reacts with sodium hydroxide:

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

This shows CO₂ is acidic as it neutralises a base.

Source: Metals and Non-metals, Chapter 3; Acids, Bases and Salts, Chapter 2

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• The question is worth 3 marks: 1 mark for the metallic oxide statement + equation, 1 mark for the non-metallic oxide statement + equation, and 1 mark for correct balancing/reasoning.
• Always state why it is basic/acidic (reacts with acid/base respectively) — don't just write the equation.
• Equations must be balanced with correct state symbols for full marks.
• Common pitfall: students write the equation without explaining what it proves. Link the reaction back to the property.

(a) Bacteria in milk produce lactic acid, which lowers the pH and causes milk to spoil. By adding baking soda, the milkman makes the milk slightly alkaline (higher pH), which neutralises the acid produced, thus slowing bacterial activity and preserving milk longer.

(b) For milk to set as curd, bacteria must produce enough lactic acid to lower the pH sufficiently. Since the milk is now alkaline, bacteria need more time to first neutralise the added base before the pH can drop enough to set curd.

(c) This tells us that bacteria responsible for curd formation are pH-sensitive and function optimally only within a specific (acidic) pH range. Living beings, including microorganisms, carry out their metabolic activities within an optimal pH range.

Source: Chapter 2, Exercises Q.12; "Living beings carry out their metabolic activities within an optimal pH range."

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• Key concept tested: pH affects biological/microbial activity — a core idea from Ch. 2.
• (a) Links alkaline pH → neutralises acid produced by bacteria → preservation. Don't just say "alkaline stops bacteria"; explain why (neutralisation of acid).
• (b) The logic is sequential: bacteria must first overcome the alkalinity before acidifying the milk enough to set curd — hence longer time.
• (c) Ties back to the textbook line: "Living beings carry out their metabolic activities within an optimal pH range." Examiners expect this line or its paraphrase.
• Avoid writing vague answers like "bacteria don't work in alkaline conditions" — always explain the pH mechanism.

(a) pH classification of the four salts:

• Sodium chloride (NaCl): pH = 7 (neutral). Formed from strong acid HCl and strong base NaOH.
• Ammonium chloride (NH₄Cl): pH < 7 (acidic). Formed from strong acid HCl and weak base NH₄OH.
• Sodium carbonate (Na₂CO₃): pH > 7 (basic). Formed from weak acid H₂CO₃ and strong base NaOH.
• Sodium acetate (CH₃COONa): pH > 7 (basic). Formed from weak acid CH₃COOH (acetic acid) and strong base NaOH.

(b) General rule:

• Salt of strong acid + strong base → pH = 7 (neutral)
• Salt of strong acid + weak base → pH < 7 (acidic)
• Salt of strong base + weak acid → pH > 7 (basic)

Source: Chapter 2, Section 2.4.2

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• The examiner wants you to name the acid and base forming each salt and link it directly to the pH — this earns the justification marks.
• For part (b), state the rule as three clean bullet points; this is the textbook statement and should be memorised exactly.
• Acetic acid (ethanoic acid) is a weak acid — confirmed in Chapter 4; carbonic acid (H₂CO₃) is also weak. NH₄OH is a weak base. NaOH and HCl are strong — this reasoning is what examiners check.

H⁺ ions are highly reactive and cannot exist freely in solution. In the presence of water, H⁺ ions combine with water molecules to form hydronium ions (H₃O⁺):

$$\text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+$$

So HCl in water dissociates as:

$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$

In the absence of water, HCl exists as a covalent gas (dry HCl) and does not dissociate into ions. Therefore, dry HCl does not show acidic properties — it does not change the colour of dry litmus paper. Acidic behaviour requires water for ionisation.

Source: Chapter 2, Section 2.2.1

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• The key distinction examiners want: H⁺ always combines with H₂O to form H₃O⁺ — it never exists alone.
• Both equations must be written; forgetting them loses marks.
• The contrast with dry HCl (no ions → no acidity) is the third point — directly tested in Activity 2.9 of the textbook.
• Write H⁺(aq) or H₃O⁺ — both are acceptable; the textbook uses both.

(a) When gypsum is heated at 373 K, it loses water of crystallisation to form Plaster of Paris:

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O}$$

(Gypsum) $\hspace{3.5cm}$ (Plaster of Paris)

(b) Plaster of Paris reacts with water to convert back to gypsum:

$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$

(c) When Plaster of Paris is mixed with water, it sets into a hard, solid mass (gypsum) within a few minutes. This property makes it useful for setting fractured bones — it holds the broken bone firmly in the correct position while the bone heals, and the cast can be easily removed later.

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• (a): The key detail is heating at 373 K and the formula of Plaster of Paris as CaSO₄·½H₂O. Many students incorrectly write CaSO₄ (anhydrous) — include the ½H₂O.
• (b): This is simply the reverse reaction — addition of water reconverts POP to gypsum.
• (c): Examiners want two points: (i) POP sets hard on mixing with water and (ii) this holds the bone in place during healing. Don't just say "it hardens" — link it to the medical use.