In a neutral solution, litmus appears purple in colour. Litmus is extracted from lichens, which belong to the plant division Thallophyta.

Source: Chapter 2, Introduction

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The passage explicitly states: "Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta… When the litmus solution is neither acidic nor basic, its colour is purple." Examiners expect both facts — purple colour AND lichen/Thallophyta — for full credit.

The gas produced is hydrogen (H₂).

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

Test for hydrogen gas: Bring a burning candle or a lighted matchstick near the mouth of the test tube collecting the gas. If the gas burns with a pop sound, it confirms the presence of hydrogen gas.

Source: Chapter 1, Section 1.1.2; Chapter 3, Section 3.2.3

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• 1 mark for naming the gas (hydrogen).
• 1 mark for the balanced chemical equation.
• 1 mark for the correct confirmatory test — examiners specifically want the pop sound detail; just saying "it burns" is incomplete and may lose the mark.
• Always write the equation with state symbols if you can, as it shows better understanding.

Gas produced: Carbon dioxide (CO₂)

Reaction: When sodium carbonate (Na₂CO₃) reacts with dilute hydrochloric acid (HCl), carbon dioxide gas is produced along with sodium chloride and water.

$$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$$

Observation with lime water: When CO₂ is passed through lime water, it turns milky (white). This happens because carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate (CaCO₃):

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$$

Source: Chapter 2 (Acids, Bases and Salts); Chapter 1, Section 1.2.1

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• 1 mark for correctly naming the gas (CO₂).
• 1 mark for the correct observation (lime water turns milky/white).
• 1 mark for the chemical equation/reason — CaCO₃ precipitate forms.
• Always write the equation for the lime water test; examiners expect it.
• The key phrase is "turns milky" — not cloudy or white precipitate alone, though both are acceptable. "Milky" is the standard CBSE term.

(B) A salt and water are produced.

When a base reacts with an acid, neutralisation occurs: Acid + Base → Salt + Water.

This is the neutralisation reaction. The textbook (Chapter 2) clearly states: "Acids and bases neutralise each other to form corresponding salts and water." Option A describes acid reacting with a metal; Option C describes acid reacting with metal carbonate/hydrogencarbonate. Students must not confuse these three different types of reactions.

An acid must ionize in water to produce H⁺ (aq) ions (hydronium ions, H₃O⁺). Although glucose and alcohol contain hydrogen, they do not ionize in water and therefore do not release H⁺ ions. As a result, their solutions do not conduct electricity (the bulb does not glow in the conductivity test), whereas acids like HCl dissociate completely to give H⁺ and Cl⁻ ions, which carry the electric current and are responsible for acidic properties.

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

Source: Chapter 2, Section 2.2

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• The examiner wants the key idea: ionization in water to give H⁺(aq)/H₃O⁺ ions — that's the defining property of an acid.
• Mention the conductivity test evidence (Activity 2.8) to show you understand the experimental basis.
• Include the dissociation equation for full marks.
• Avoid writing that glucose/alcohol "have no hydrogen" — they do; they just don't release H⁺ in solution.

Bases produce OH⁻ (hydroxide) ions when dissolved in water. Bases that are soluble in water are called alkalis.

The examiner expects two facts in one line: the ion (OH⁻) and the term "alkali." Both parts are needed for full credit. Source passage directly states "Formation of OH⁻(aq) ions in solution is responsible for the basic nature of a substance" and refers to soluble bases as alkalis.

The pH scale measures the concentration of hydrogen ions (H⁺) in a solution. A pH value of 7 represents a neutral solution.

Source: Chapter 2, Section 2.3

Examiners expect two things: (1) what pH measures — H⁺ ion concentration — and (2) the exact neutral pH value of 7. Both are needed for full credit even in a 1-mark question. Remember: pH < 7 = acidic, pH > 7 = basic, pH = 7 = neutral.

The solution with pH 3 is acidic (pH < 7), and the solution with pH 11 is basic (pH > 7).

The solution with pH 3 contains a higher concentration of H⁺(aq) ions.

Reason: According to the pH scale, higher the hydronium ion (H⁺) concentration, lower is the pH value. Since pH 3 is lower than pH 11, it has a greater concentration of H⁺(aq) ions. The solution of pH 11 has a higher OH⁻ ion concentration, making it basic.

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

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• 1 mark for correctly identifying pH 3 as acidic and pH 11 as basic.
• 1 mark for stating pH 3 has higher H⁺ concentration.
• 1 mark for the reason: inverse relationship between pH value and H⁺ concentration (lower pH = higher H⁺).
• Remember the key rule: pH < 7 → acidic; pH = 7 → neutral; pH > 7 → basic. Examiners expect you to quote this principle explicitly as your reason.

At equal concentrations, HCl has a lower pH than CH₃COOH (acetic acid).

This is because HCl is a strong acid — it ionises completely in water, producing more H⁺ ions. Acetic acid is a weak acid — it ionises only partially, producing fewer H⁺ ions. Since higher H⁺ concentration means lower pH, HCl has a lower pH than acetic acid at the same concentration.

• Examiners look for two things: (1) stating which pH is lower, and (2) linking it to complete vs. partial ionisation (strong vs. weak acid).
• Key phrase from the textbook: "Acids that give rise to more H⁺ ions are said to be strong acids, and acids that give less H⁺ ions are said to be weak acids."
• Don't just say "HCl is stronger" — explain why in terms of H⁺ ions produced.

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

Baking soda has the chemical name sodium hydrogen carbonate (or sodium bicarbonate) and its chemical formula is NaHCO₃.

This is a standard 1-mark factual recall question. Examiners expect both the chemical name and formula. "Sodium bicarbonate" is also accepted, but "sodium hydrogen carbonate" is the IUPAC-preferred name used in the NCERT textbook. Writing just one without the other may cost you the mark.

When electricity is passed through brine (concentrated NaCl solution), three useful products are formed:

1. Chlorine gas – released at the anode (positive electrode)
2. Hydrogen gas – released at the cathode (negative electrode)
3. Sodium hydroxide (NaOH) – formed in the solution near the cathode

This process is called the chlor-alkali process. The overall reaction is:

$$2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$$

Source: Chapter 1 (Chemical Reactions and Equations), electrolysis/chlor-alkali process

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• The examiner expects all three products named (1 mark each): chlorine, hydrogen, and sodium hydroxide.
• For full marks, state which gas is at which electrode — Cl₂ at anode, H₂ at cathode.
• NaOH is not a gas; it remains in solution, so don't say it is released at an electrode.
• The equation is a bonus but good to include for clarity.

Copper sulphate crystals contain water of crystallisation (CuSO₄·5H₂O), which gives them their blue colour. On strong heating, this water is lost (thermal decomposition), forming anhydrous copper sulphate (CuSO₄), which is white.

When a few drops of water are added to the white powder, it reabsorbs water and turns blue again, as the hydrated form is restored.

Source: Chapter 1, Section 1.2.2 Decomposition Reaction

• The key concept is water of crystallisation: its presence = blue; its absence = white.
• Examiners expect you to use the term anhydrous for the white powder and mention that heating causes loss of water of crystallisation (a thermal decomposition).
• The reversibility (white → blue on adding water) shows this is not a permanent chemical change — a point worth stating clearly.
• This is a classic 2-mark question: one mark for explaining the blue-to-white change, one mark for the white-to-blue reversal on adding water.

(i) The substance is Plaster of Paris.
Chemical formula: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$

(ii) It is prepared from Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$). When Plaster of Paris is mixed with water, it rehydrates and sets into a hard solid mass (gypsum is reformed). This property makes it useful for making casts for broken bones.

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

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• Examiners expect the name Plaster of Paris with its correct formula (the ½ water of crystallisation is essential — a common error is writing 1H₂O).
• For part (ii), naming Gypsum as the parent compound earns 1 mark; stating that it sets hard / reverts to gypsum on mixing with water earns the remaining mark.
• The equation is a bonus/good practice but the key points are the name, formula, parent compound, and the setting reaction.

When baking soda solution is added to beetroot extract, the colour of the extract changes from red/pink to yellow (or greenish-yellow).

This colour change indicates that baking soda (NaHCO₃) solution is basic (alkaline) in nature, as beetroot extract shows a different colour in basic solutions compared to acidic ones.

Source: Chapter 2, Group Activity — Prepare your own indicator

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• Beetroot extract acts as a natural acid-base indicator — it gives one colour in acid and a different colour in base.
• Baking soda = sodium hydrogencarbonate (NaHCO₃), which is a base.
• Examiners expect two points: (1) the colour change observed, and (2) what it tells us about the nature of baking soda solution (i.e., it is basic/alkaline). Each point carries 1 mark.
• Avoid writing that it is a strong base — NaHCO₃ is a mild/weak base.