Question 1

When a few drops of phenolphthalein indicator are added to a solution, the solution turns pink. What can be concluded about the nature of the solution?

Accepted Answer

Phenolphthalein is a pH indicator that remains colourless in acidic and neutral solutions (pH < 8.3) but turns pink/magenta in basic solutions (pH ≥ 8.3). Since the solution turned pink upon addition of phenolphthalein, the solution must be basic in nature. This is a standard NCERT Class 10 indicator test used to distinguish between acidic, neutral, and basic solutions.

Question 2

A student adds 10 mL of dilute hydrochloric acid (HCl) to 10 mL of sodium hydroxide solution (NaOH) in a test tube. The pH of the resulting solution is measured as 7. Which statement correctly explains this observation?

Accepted Answer

This question tests understanding of acid-base neutralization and the pH scale. When equal volumes of dilute HCl and NaOH react, they undergo a neutralization reaction: HCl + NaOH → NaCl + H₂O. Since the acid and base are of equal concentration and equal volume, they completely neutralize each other, producing a neutral salt solution (NaCl) and water. A neutral solution has a pH of exactly 7 at 25°C. The resulting solution contains sodium chloride (a salt formed from a strong acid and strong base), which does not hydrolyze and therefore does not affect the pH. The pH remains 7, confirming complete neutralization. Option B is correct because it identifies that a salt formed from a strong acid and strong base produces a neutral solution. Option A is incorrect because the salt NaCl does not hydrolyze; it is formed from a strong acid (HCl) and strong base (NaOH), so it is neutral. Option C is incorrect because phenolphthalein turns colourless in neutral solutions (pH 7), not pink. Option D is incorrect because the solution is neutral (pH = 7), not basic (pH > 7).

Question 3

A student prepares three solutions and measures their pH values using a pH meter. Solution X has pH = 2, Solution Y has pH = 7, and Solution Z has pH = 12. Which statement correctly describes the nature and relative concentration of H⁺ ions in these solutions?

Accepted Answer

The pH scale measures the concentration of hydrogen ions (H⁺) in a solution. pH is defined as –log₁₀[H⁺]. A lower pH indicates higher H⁺ concentration (acidic), pH = 7 indicates neutral (equal H⁺ and OH⁻), and higher pH indicates lower H⁺ concentration (basic). For Solution X (pH = 2): [H⁺] = 10⁻² = 0.01 M (strongly acidic). For Solution Y (pH = 7): [H⁺] = 10⁻⁷ M (neutral). For Solution Z (pH = 12): [H⁺] = 10⁻¹² M (strongly basic). The H⁺ concentration decreases from X to Y to Z by a factor of 10⁵ between X and Y, and another 10⁵ between Y and Z. Solution X is acidic with the highest H⁺ concentration, Solution Y is neutral, and Solution Z is basic with the lowest H⁺ concentration. Therefore, answer is B.

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