1You have been provided with three test tubes... identify contents using only red litmus paper.
Answer & Reference:
Dip the red litmus paper in each test tube. The one that turns blue contains the basic solution. The one with no change is either acidic or neutral. Now, take the basic solution and add a drop to the remaining two. The one that turns red litmus back to purple (neutralization) is acidic; the other is distilled water. (p. 18)
2Why should curd and sour substances not be kept in brass and copper vessels?
Answer & Reference:
Curd and sour substances contain acids which react with metals like copper and brass to form toxic compounds, making the food unfit for consumption. (p. 22)
3Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Answer & Reference:
Hydrogen gas is liberated. Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑. Test: Bring a burning candle near the gas; it burns with a pop sound. (p. 20)
4Metal compound A reacts with dilute HCl to produce effervescence... gas extinguishes candle... one product is CaCl₂. Write balanced equation.
Answer & Reference:
The gas is CO₂, so compound A is CaCO₃. Equation: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) (p. 21)
5Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while alcohol and glucose do not?
Answer & Reference:
HCl and HNO₃ produce H⁺(aq) ions in water, which are responsible for acidic behavior. Alcohol and glucose do not dissociate to give H⁺ ions in water. (p. 23)
6Why does an aqueous solution of an acid conduct electricity?
Answer & Reference:
Because acids dissociate in water to form ions (e.g., H⁺ and Cl⁻), which carry electric current. (p. 23)
7Why does dry HCl gas not change the colour of dry litmus paper?
Answer & Reference:
H⁺ ions are produced only in the presence of water. Dry HCl does not dissociate, so no acidic character is shown. (p. 23–24)
8While diluting an acid, why is it recommended that acid be added to water and not water to acid?
Answer & Reference:
The process is highly exothermic. Adding water to acid causes sudden heat release, which may splash acid and cause burns. Adding acid to water slowly with stirring allows safe heat dissipation. (p. 24)
9How is [H₃O⁺] affected when an acid solution is diluted?
Answer & Reference:
The concentration of hydronium ions decreases per unit volume on dilution. (p. 25)
10How is [OH⁻] affected when excess base is dissolved in NaOH solution?
Answer & Reference:
The concentration of hydroxide ions increases. (p. 25)
11You have two solutions A (pH=6) and B (pH=8). Which has more H⁺ concentration? Which is acidic/basic?
Answer & Reference:
Solution A (pH=6) has higher H⁺ concentration and is acidic. Solution B (pH=8) is basic. (p. 28)
12What effect does [H⁺(aq)] have on the nature of the solution?
Answer & Reference:
Higher [H⁺] makes the solution more acidic; lower [H⁺] (or higher [OH⁻]) makes it basic. (p. 28)
13Do basic solutions also have H⁺(aq) ions? If yes, why are they basic?
Answer & Reference:
Yes, but [OH⁻] > [H⁺], so the solution is basic. (p. 28)
14Under what soil condition would a farmer treat soil with quick lime, slaked lime or chalk?
Answer & Reference:
When the soil is too acidic (low pH), these basic substances are added to neutralize acidity. (p. 28)
15A solution turns red litmus blue. Its pH is likely to be: (a) 1 (b) 4 (c) 5 (d) 10
Answer & Reference:
(d) 10 — because bases turn red litmus blue and have pH > 7. (p. 34)
16A solution reacts with egg-shells to give gas that turns lime water milky. The solution contains: (a) NaCl (b) HCl (c) LiCl (d) KCl
Answer & Reference:
(b) HCl — egg-shells contain CaCO₃; acid produces CO₂ which turns lime water milky. (p. 34)
1710 mL NaOH neutralized by 8 mL HCl. How much HCl needed for 20 mL NaOH?
Answer & Reference:
(d) 16 mL — neutralization is proportional. (p. 34)
20Why are alcohols and glucose not acids despite containing hydrogen? Describe an activity.
Answer & Reference:
They do not produce H⁺ ions in water. Activity: Set up circuit with battery, bulb, and electrodes in solution. Bulb glows with HCl (conducts), but not with glucose/alcohol (no ions). (p. 23, Activity 2.8)
21Why does distilled water not conduct electricity, but rainwater does?
Answer & Reference:
Distilled water has no ions. Rainwater dissolves atmospheric CO₂, forming carbonic acid (H₂CO₃), which ionizes and conducts electricity. (p. 35)
22Why do acids not show acidic behavior without water?
Answer & Reference:
H⁺ ions are formed only in aqueous solution. Dry acids do not dissociate. (p. 23–24)
Answer & Reference:
(a) D (pH=7), (b) C (pH=11), (c) B (pH=1), (d) A (pH=4), (e) E (pH=9).
Increasing [H⁺]: C (11) < E (9) < D (7) < A (4) < B (1) → actually **decreasing pH** means **increasing [H⁺]**, so order: C, E, D, A, B. (p. 35)
24Equal Mg ribbons in HCl and CH₃COOH (same conc.). In which test tube is fizzing more vigorous and why?
Answer & Reference:
Test tube A (HCl) — because HCl is a strong acid and produces H₂ gas faster than weak acetic acid. (p. 35)
25Fresh milk pH=6. How does pH change as it turns to curd?
Answer & Reference:
pH decreases (<6) because lactic acid is formed during curdling. (p. 35)
26Milkman adds baking soda to milk. (a) Why shift pH to slightly alkaline? (b) Why does it take longer to set as curd?
Answer & Reference:
(a) To prevent spoilage by delaying acid formation.
(b) Baking soda neutralizes acid, so lactic acid must first neutralize base before pH drops enough for curd formation. (p. 35)
27Why store Plaster of Paris in moisture-proof container?
Answer & Reference:
It absorbs moisture and hardens into gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. (p. 35)
28What is a neutralisation reaction? Give two examples.
Answer & Reference:
Reaction between acid and base to form salt and water.
Examples: NaOH + HCl → NaCl + H₂O
Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O (p. 35)
29Give two important uses of washing soda and baking soda.
Answer & Reference: Washing soda: (i) Used in glass, soap, paper industries; (ii) Removes permanent hardness of water. Baking soda: (i) Used in baking powder; (ii) In antacids to neutralize stomach acid. (p. 32, 31)
30What is the common name of Ca(ClO)₂?
Answer & Reference:
Bleaching powder. (p. 31)
31Name the substance which on treatment with chlorine yields bleaching powder.