Acid, Base and Salt

41. Suppose you have four test tubes labelled as ‘A’, ‘B’, ‘C’ and ‘D’. ‘A

Suppose you have four test tubes labelled as ‘A’, ‘B’, ‘C’ and ‘D’. ‘A’ contains plain water, ‘B’ contains solution of an alkali, ‘C’ contains solution of an acid, and ‘D’ contains solution of sodium chloride. Which one of these solutions will turn phenolphthalein solution pink ?

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Solution 'A'
Solution 'B'
Solution 'C'
Solution 'D'
This question was previously asked in
UPSC NDA-1 – 2016
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The correct option is B.
Phenolphthalein is a common acid-base indicator. Its color changes depending on the pH of the solution:
– In acidic solutions (pH < ~8.2), phenolphthalein is colorless. - In neutral solutions (pH ~7), phenolphthalein is colorless. - In basic (alkaline) solutions (pH > ~8.2), phenolphthalein turns pink or magenta.
Let’s analyze the solutions:
– Solution ‘A’: Plain water is generally neutral (pH ≈ 7). Phenolphthalein will be colorless.
– Solution ‘B’: Solution of an alkali is basic. Phenolphthalein will turn pink.
– Solution ‘C’: Solution of an acid is acidic. Phenolphthalein will be colorless.
– Solution ‘D’: Solution of sodium chloride (NaCl) is the salt of a strong acid (HCl) and a strong base (NaOH). A solution of such a salt is neutral (pH ≈ 7). Phenolphthalein will be colorless.
Therefore, only the solution of an alkali (Solution ‘B’) will turn phenolphthalein solution pink.
Indicators are substances that show a change in color or some other property in response to a chemical change. Phenolphthalein is widely used in titrations to detect the endpoint of a reaction between an acid and a base. Different indicators change color at different pH ranges.

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42. Which one of the following is the chemical name for baking soda ?

Which one of the following is the chemical name for baking soda ?

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Sodium bicarbonate (Sodium hydrogen carbonate)
Sodium carbonate
Potassium bicarbonate (Potassium hydrogen carbonate)
Potassium carbonate
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UPSC NDA-1 – 2016
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Baking soda is a common household chemical used as a leavening agent in baking, as a cleaning agent, and for various other purposes. Its chemical formula is NaHCO₃. The chemical name for NaHCO₃ is Sodium bicarbonate, also known as Sodium hydrogen carbonate.
– Baking soda chemical formula: NaHCO₃.
– Chemical names: Sodium bicarbonate or Sodium hydrogen carbonate.
Sodium carbonate (Na₂CO₃) is known as washing soda. Potassium bicarbonate (KHCO₃) and Potassium carbonate (K₂CO₃) are related compounds but are not what is commonly referred to as baking soda.

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43. Permanent hardness of water is due to the presence of

Permanent hardness of water is due to the presence of

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sulphates of sodium and potassium
sulphates of magnesium and calcium
carbonates of sodium and magnesium
bicarbonates of magnesium and calcium
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UPSC NDA-1 – 2015
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Hardness of water is primarily caused by the presence of dissolved calcium and magnesium ions (Ca$^{2+}$ and Mg$^{2+}$). Temporary hardness is caused by their bicarbonates, which can be removed by boiling. Permanent hardness is caused by the presence of calcium and magnesium sulphates and chlorides, which are not removed by simple boiling.
Permanent hardness of water is due to the dissolved sulphates and chlorides of calcium and magnesium.
Sodium and potassium ions do not cause water hardness. Carbonates and bicarbonates of Ca and Mg cause temporary hardness. Permanent hardness needs chemical methods like ion exchange (using zeolite or synthetic resins) for removal.

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44. Washing Soda is the common name for A) Calcium Carbonate B) Magnesium

Washing Soda is the common name for
A) Calcium Carbonate
B) Magnesium Carbonate
C) Sodium Carbonate
D) Potassium Carbonate

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Calcium Carbonate
Magnesium Carbonate
Sodium Carbonate
Potassium Carbonate
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UPSC NDA-1 – 2015
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Washing Soda is the common name for Sodium Carbonate ($\text{Na}_2\text{CO}_3$). It is typically sold as the decahydrate, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$. It is widely used as a cleaning agent, in laundry, and in the manufacture of glass and paper.
Knowing the common names of important chemical compounds.
Calcium Carbonate ($\text{CaCO}_3$) is found in limestone, marble, and chalk. Magnesium Carbonate ($\text{MgCO}_3$) is a component of minerals like magnesite. Potassium Carbonate ($\text{K}_2\text{CO}_3$) is also known as potash and is used in various industrial applications, including soap and glass making.

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45. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

List I
(Compound)		 List II
(Nature)
A. Sodium hydroxide 1. Strong acid
B. Calcium oxide 2. Alkali
C. Acetic acid 3. Weak acid
D. Hydrochloric acid 4. Base

Code :

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A-2, B-3, C-4, D-1
A-2, B-4, C-3, D-1
A-1, B-4, C-3, D-2
A-1, B-3, C-4, D-2
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UPSC NDA-1 – 2015
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The correct option is B. The correct match is A-2, B-4, C-3, D-1.
Let’s classify each compound:
A. Sodium hydroxide (NaOH): NaOH is a strong base. Soluble bases are called alkalis. NaOH is highly soluble and completely dissociates in water, making it a strong alkali. Matches with 2 (Alkali).
B. Calcium oxide (CaO): CaO is a metal oxide. Metal oxides are generally basic in nature. When dissolved in water, it forms calcium hydroxide (Ca(OH)₂), a base. Thus, CaO is a base (basic oxide). Matches with 4 (Base).
C. Acetic acid (CH₃COOH): This is an organic acid (carboxylic acid). It is a weak acid because it does not completely dissociate in water. Matches with 3 (Weak acid).
D. Hydrochloric acid (HCl): HCl is a mineral acid. It is a strong acid because it completely dissociates in water. Matches with 1 (Strong acid).
Mapping the compounds to their nature:
A. Sodium hydroxide (NaOH) → 2. Alkali
B. Calcium oxide (CaO) → 4. Base
C. Acetic acid (CH₃COOH) → 3. Weak acid
D. Hydrochloric acid (HCl) → 1. Strong acid
Comparing this mapping with the options provided, option B matches correctly.

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46. Which one among the following statements regarding acids and bases is

Which one among the following statements regarding acids and bases is not correct ?

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A conjugate base of a strong acid is a weak base.
In aqueous solution, the hydroxide ion is hydrated to give several ionic species like H₃O₂⁻, H₅O₃⁻ and H₇O₄⁻ etc.
AlCl₃ is an electron deficient compound, and can act as a Lewis base.
Fluoride ion acts as a Lewis base as it can donate any one of its four electron lone pairs.
This question was previously asked in
UPSC Geoscientist – 2024
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The statement “AlCl₃ is an electron deficient compound, and can act as a Lewis base” is not correct.
AlCl₃ is electron deficient because the central aluminum atom has only six valence electrons (an incomplete octet). According to the Lewis definition, substances that can accept an electron pair are Lewis acids, while substances that can donate an electron pair are Lewis bases.
Since AlCl₃ is electron deficient, it readily accepts electron pairs from other molecules (like water, ammonia, or chloride ions) to complete its octet. Therefore, AlCl₃ acts as a Lewis acid, not a Lewis base. Statement A is correct (e.g., Cl- from HCl is a weak conjugate base). Statement B is correct regarding the hydration of hydroxide ions. Statement D is correct; a fluoride ion has four lone pairs and can donate one to act as a Lewis base.

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47. The natural rain water has a pH value of:

The natural rain water has a pH value of:

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5.6
6.5
7.0
Zero
This question was previously asked in
UPSC Geoscientist – 2021
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The pH of natural, unpolluted rainwater is typically around 5.6.
Rainwater is naturally slightly acidic because carbon dioxide (CO2) from the atmosphere dissolves in it, forming weak carbonic acid (H2CO3).
Acid rain, caused by pollutants like sulfur dioxide (SO2) and nitrogen oxides (NOx), has a pH lower than 5.6. Pure water has a neutral pH of 7.0, but rainwater is never pure due to dissolved atmospheric gases.

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48. Which one of the following statements regarding acids is not correct ?

Which one of the following statements regarding acids is not correct ?

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Hydrochloric acid is present in the gastric juice secreted by the stomach.
Acetic acid is the main constituent of vinegar.
Oxalic acid is found in tamarind paste.
Lemon and orange juices contain citric acid and ascorbic acid respectively.
This question was previously asked in
UPSC CDS-2 – 2023
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The question asks for the statement about acids that is NOT correct.
Let’s evaluate each statement:
A) Hydrochloric acid (HCl) is indeed a major component of gastric juice in the stomach, aiding digestion. (Correct)
B) Acetic acid (CH₃COOH) is the main active component of vinegar, typically present as a dilute solution (around 5-8%). (Correct)
C) Tamarind paste is known for its sour taste, which is primarily due to the presence of tartaric acid. While some other acids might be present in smaller quantities, oxalic acid is not the main acid found in tamarind. Oxalic acid is abundant in foods like spinach, rhubarb, and sorrel. (Incorrect)
D) Lemon and orange juices are rich in citric acid, which gives them their characteristic sourness. They also contain ascorbic acid (Vitamin C). The statement implies both acids are present, which is true. (Correct)
Tartaric acid is a common organic acid found in many fruits, most notably in grapes and tamarinds. It is also a component of cream of tartar.

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49. During white-washing of walls, slaked lime reacts slowly with carbon d

During white-washing of walls, slaked lime reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Which of the following reactions represents this correctly ?

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$ ext{CaO (s) + CO}_2 ext{ (g)} ightarrow ext{CaCO}_3 ext{ (s)}$
$ ext{CaO (l) + CO}_2 ext{ (g)} ightarrow ext{CaCO}_3 ext{ (s)}$
$ ext{Ca(OH)}_2 ext{ (l) + CO}_2 ext{ (l)} ightarrow ext{CaCO}_3 ext{ (s)} + ext{H}_2 ext{O (l)}$
$ ext{Ca(OH)}_2 ext{ (aq) + CO}_2 ext{ (g)} ightarrow ext{CaCO}_3 ext{ (s)} + ext{H}_2 ext{O (l)}$
This question was previously asked in
UPSC CDS-2 – 2023
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White-washing uses slaked lime, which is calcium hydroxide (Ca(OH)2), typically applied as an aqueous suspension or solution. This slaked lime reacts slowly with carbon dioxide (CO2) from the air to form calcium carbonate (CaCO3), a hard, white substance that coats the wall. Water is also produced in this reaction. The correct chemical equation representing this process is Ca(OH)2 (aq) + CO2 (g) $\rightarrow$ CaCO3 (s) + H2O (l).
Slaked lime (Ca(OH)2) is produced by adding water to quicklime (CaO). The reaction of slaked lime with atmospheric CO2 is a slow carbonation process that hardens the white-washed surface. The state symbols are important: slaked lime is in aqueous form (aq), CO2 is a gas (g), calcium carbonate formed is a solid (s), and water is a liquid (l).
Option A and B use CaO (quicklime) instead of Ca(OH)2 (slaked lime). Option C uses incorrect state symbols for both Ca(OH)2 (l instead of aq) and CO2 (l instead of g). Option D correctly shows the reactants, products, stoichiometry, and state symbols for the reaction described.

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50. Milk of Magnesia is used when people suffer from indigestion of food.

Milk of Magnesia is used when people suffer from indigestion of food. It is a

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Strong base
Mild base
Strong acid
Mild acid
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UPSC CDS-2 – 2022
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Milk of Magnesia is a common name for a suspension of magnesium hydroxide (Mg(OH)₂). Magnesium hydroxide is a base. It is used as an antacid to neutralize excess hydrochloric acid (HCl) in the stomach, which causes indigestion. While it is a base, it is not highly soluble in water, and its reaction with acid is not as vigorous or complete as that of a strong base like sodium hydroxide (NaOH). Therefore, it is considered a mild base, suitable for internal consumption as an antacid or laxative without causing severe side effects associated with strong bases.
Milk of Magnesia is magnesium hydroxide, which acts as a mild base to neutralize stomach acid.
The neutralization reaction in the stomach is: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l). By consuming a base like magnesium hydroxide, the acidity in the stomach is reduced, providing relief from indigestion and heartburn.

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