A solution turns red litmus blue, its pH is likely to be:
(a) 1 (b) 4 (c) 5 (d) 10
Answer: (d) 10
Explanation: Bases turn red litmus blue. Bases have a pH value greater than 7. Among the given options, only 10 is greater than 7.
[Image of pH scale colors]
Q2
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains:
(a) NaCl (b) HCl (c) LiCl (d) KCl
Answer: (b) HCl
Explanation: Egg shells contain Calcium Carbonate ($\text{CaCO}_3$). When an acid (like HCl) reacts with a carbonate, it produces Carbon Dioxide ($\text{CO}_2$) gas, which turns lime water milky.
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution required to neutralise it will be:
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Answer: (d) 16 mL
Calculation:
Since 10 mL NaOH requires 8 mL HCl.
Double the NaOH (20 mL) will require double the HCl.
$$8 \text{ mL} \times 2 = 16 \text{ mL}$$
Q4
Which one of the following types of medicines is used for treating indigestion?
Explanation: Indigestion is caused by excess acid produced in the stomach. An Antacid (like Milk of Magnesia) is a mild base that neutralizes the excess acid and provides relief.
Q5
Write word equations and then balanced equations for the reaction taking place when:
(a) Dilute sulphuric acid reacts with zinc granules.
Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an Activity to prove it.
Activity/Explanation:
1. Take two beakers. In one, put Dilute HCl. In the other, put Alcohol/Glucose solution.
2. Insert two nails connected to a battery and a bulb in both beakers.
3. Observation: The bulb glows in the HCl beaker but does not glow in the alcohol/glucose beaker.
Conclusion: Acids dissociate to produce $\text{H}^+$ ions which conduct electricity. Alcohol and glucose do not produce ions, so they are not categorized as acids.
Q7
Why does distilled water not conduct electricity, whereas rain water does?
Distilled Water: It is a pure form of water and does not contain free ions ($\text{H}^+$ or $\text{OH}^-$) needed to conduct electricity.
Rain Water: While falling, rain absorbs gases like $\text{CO}_2$ and $\text{SO}_2$ from the air, which form acids (like Carbonic acid). These acids dissociate into ions ($\text{H}^+, \text{CO}_3^{2-}$), making rain water conducting.
Q8
Why do acids not show acidic behaviour in the absence of water?
The acidic behavior of acids is due to the presence of Hydrogen ions ($\text{H}^+$). Acids produce $\text{H}^+$ ions only in the presence of water. In dry conditions (absence of water), separation of $\text{H}^+$ ions from acid molecules does not occur, so they do not show acidic properties.
Arrangement in increasing order of H⁺ concentration:
C (11) < E (9) < D (7) < A (4) < B (1)
Q10
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid ($CH_3COOH$) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?
Observation: Fizzing will occur more vigorously in Test Tube A (HCl).
Reason: Hydrochloric acid (HCl) is a strong acid and dissociates completely to produce a high concentration of $\text{H}^+$ ions. Acetic acid is a weak acid. Higher $\text{H}^+$ concentration leads to a faster reaction with Magnesium, producing Hydrogen gas (fizzing) more rapidly.
Q11
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
The pH will decrease (become less than 6).
Reason: When milk turns into curd, bacteria (Lactobacillus) convert the lactose sugar in milk into Lactic Acid. The production of acid increases the concentration of $\text{H}^+$ ions, thus lowering the pH value.
Q12
A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?
(a) He adds baking soda (a base) to prevent the milk from spoiling quickly. By making it slightly alkaline, he neutralizes the lactic acid produced by bacteria, keeping the milk fresh longer.
(b) The lactic acid produced during curd formation must first neutralize the alkaline baking soda present in the milk. Only after that can the acidity rise enough to turn the milk into curd. Hence, it takes longer.
Q13
Plaster of Paris should be stored in a moisture-proof container. Explain why.
Plaster of Paris (POP) absorbs moisture (water) from the air very easily and forms a hard solid mass called Gypsum. Once it hardens, it becomes useless for moulding.