Acid, Base and Salt

21. The solution of which one of the following will have pH less than 7?

The solution of which one of the following will have pH less than 7?

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NaOH
KCl
FeCl₃
NaCl
This question was previously asked in
UPSC NDA-2 – 2018
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The pH of a solution indicates its acidity or alkalinity. A pH less than 7 is acidic, pH equal to 7 is neutral, and pH greater than 7 is alkaline (basic). Let’s analyze the given options:
A) NaOH (Sodium hydroxide) is a strong base. Its solution is alkaline (pH > 7).
B) KCl (Potassium chloride) is a salt formed from a strong acid (HCl) and a strong base (KOH). Its solution is neutral (pH ≈ 7).
C) FeCl₃ (Iron(III) chloride) is a salt formed from a weak base (Fe(OH)₃) and a strong acid (HCl). When dissolved in water, metal ions with high charge density, like Fe³⁺, undergo hydrolysis, reacting with water to produce hydrated metal ions and release H⁺ ions, making the solution acidic: FeCl₃(aq) + 3H₂O(l) ⇌ Fe(OH)₃(s) + 3H⁺(aq) + 3Cl⁻(aq). Therefore, its solution will have a pH less than 7.
D) NaCl (Sodium chloride) is a salt formed from a strong acid (HCl) and a strong base (NaOH). Its solution is neutral (pH ≈ 7).
The pH of a salt solution depends on the strength of the acid and base from which the salt is formed. Salts of strong acid and strong base are neutral, salts of strong acid and weak base are acidic, and salts of weak acid and strong base are alkaline.
Hydrolysis is the reaction of a substance with water. In the case of salts formed from weak acids or bases, the ions can react with water, affecting the pH of the solution. Cations from weak bases and anions from weak acids are the ones that undergo significant hydrolysis.

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22. Which one of the following is a tribasic acid?

Which one of the following is a tribasic acid?

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Hydrochloric acid
Nitric acid
Sulphuric acid
Phosphoric acid
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UPSC NDA-2 – 2018
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Phosphoric acid (H₃PO₄) is a tribasic acid because it can donate three protons (H⁺ ions) in a stepwise manner.
– The basicity of an acid refers to the number of acidic hydrogen atoms per molecule that can be donated as protons.
– Monobasic acids donate one proton (e.g., HCl, HNO₃).
– Dibasic acids donate two protons (e.g., H₂SO₄, H₂CO₃).
– Tribasic acids donate three protons (e.g., H₃PO₄).
The dissociation of phosphoric acid occurs in three steps:
1. H₃PO₄ ⇌ H⁺ + H₂PO₄⁻
2. H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻
3. HPO₄²⁻ ⇌ H⁺ + PO₄³⁻
Hydrochloric acid (HCl) and nitric acid (HNO₃) are monobasic. Sulphuric acid (H₂SO₄) is dibasic.

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23. The desirable range of pH for drinking water is

The desirable range of pH for drinking water is

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6.5 to 8.5
5.0 to 6.5
6.5 to 7.0
7.0 to 8.5
This question was previously asked in
UPSC NDA-2 – 2017
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The desirable range of pH for drinking water is 6.5 to 8.5.
pH is a measure of the acidity or alkalinity of water. A neutral pH is 7.0. Water with a pH less than 7 is acidic, and water with a pH greater than 7 is alkaline. The standard recommended range for potable water generally falls within the slightly acidic to slightly alkaline range, as extremely low or high pH can affect palatability, corrosion potential, and the effectiveness of disinfection.
Standards like those set by the World Health Organization (WHO) and the Indian Standard (IS 10500:2012) specify the acceptable limit for pH in drinking water as 6.5 to 8.5. While some standards might allow for slightly wider ranges in specific circumstances, 6.5-8.5 is the widely accepted desirable range to ensure safety and palatability.

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24. Which one among the following chemicals is used as washing soda ?

Which one among the following chemicals is used as washing soda ?

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Calcium carbonate
Calcium bicarbonate
Sodium carbonate
Sodium bicarbonate
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UPSC NDA-2 – 2017
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The correct answer is Sodium carbonate.
Washing soda is the common name for sodium carbonate (Na₂CO₃). It is typically found as a hydrated salt, sodium carbonate decahydrate (Na₂CO₃·10H₂O).
Sodium bicarbonate (NaHCO₃) is commonly known as baking soda. Calcium carbonate (CaCO₃) is found in substances like limestone, marble, and chalk. Calcium bicarbonate is a temporary form of calcium carbonate that exists in aqueous solution and is not a stable solid chemical compound used as “washing soda”. Washing soda is used as a cleaning agent and water softener.

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25. Which compound, when dissolved in water, conducts electricity and form

Which compound, when dissolved in water, conducts electricity and forms a basic solution ?

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HCl
CH₃COOH
CH₃OH
NaOH
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UPSC NDA-2 – 2017
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The compound that conducts electricity when dissolved in water indicates the formation of ions. A basic solution is formed by substances that produce hydroxide ions (OH⁻) in water or accept protons.
Let’s examine the options:
A) HCl (Hydrochloric acid): A strong acid. Dissolves in water to form H⁺ and Cl⁻ ions, conducting electricity. Forms an acidic solution (high concentration of H⁺).
B) CH₃COOH (Acetic acid): A weak acid. Partially dissociates in water to form H⁺ and CH₃COO⁻ ions, conducting electricity weakly. Forms an acidic solution.
C) CH₃OH (Methanol): An alcohol. Does not ionize in water. Does not conduct electricity. Forms a neutral solution.
D) NaOH (Sodium hydroxide): A strong base. Dissolves in water to form Na⁺ and OH⁻ ions, conducting electricity well. Forms a basic solution (high concentration of OH⁻).
Only NaOH satisfies both conditions: conducting electricity (due to ion formation) and forming a basic solution (due to OH⁻ production).
Acids produce H⁺ ions in water (making the solution acidic), bases produce OH⁻ ions (making it basic), and salts dissociate into cations and anions (neutral or affecting pH depending on the nature of the parent acid/base). Substances that don’t ionize, like alcohols, do not conduct electricity and are generally neutral.

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26. Which one of the following gases gives acidic solution on dissolving i

Which one of the following gases gives acidic solution on dissolving in water?

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Hydrogen
Carbondioxide
Nitrogen
Oxygen
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UPSC NDA-2 – 2016
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When carbon dioxide (CO₂) dissolves in water (H₂O), it reacts to form carbonic acid (H₂CO₃) through a reversible reaction: CO₂ + H₂O ⇌ H₂CO₃. Carbonic acid is a weak acid that can dissociate into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻), and further into hydrogen ions and carbonate ions (CO₃²⁻). The presence of hydrogen ions increases the acidity of the solution.
Non-metal oxides that react with water form acidic solutions. CO₂ is an oxide of carbon, a non-metal. Other examples include SO₂ and NO₂, which form sulfurous/sulfuric acid and nitrous/nitric acid respectively, contributing to acid rain.
Hydrogen (H₂), Nitrogen (N₂), and Oxygen (O₂) are diatomic gases that do not react with water to form acids under normal conditions. H₂ is neutral. N₂ and O₂ are relatively inert. Oxides of metals, on the other hand, typically form alkaline solutions when dissolved in water (e.g., Na₂O + H₂O → 2NaOH).

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27. Boric acid is an acid because its molecule:

Boric acid is an acid because its molecule:

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accepts OH<sup>-</sup> from water
combines with proton from water molecule
contains replaceable H<sup>+</sup> ion
gives up a proton
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UPSC NDA-2 – 2015
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Boric acid (H₃BO₃) is a weak monoprotic Lewis acid. It does not directly donate a proton (H⁺) from its own molecule in aqueous solution. Instead, it accepts a hydroxide ion (OH⁻) from a water molecule, releasing a proton from the water molecule in the process. The reaction is: H₃BO₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺. Therefore, it acts as an acid by accepting an OH⁻ ion from water.
Boric acid’s acidity is explained by its behavior as a Lewis acid, accepting an electron pair from the hydroxide ion of water.
Although boric acid contains three hydrogen atoms, they are not easily ionizable as protons in the way that typical Brønsted-Lowry acids like HCl or H₂SO₄ are. Its acidic nature arises from its ability to form a coordinate covalent bond with the hydroxide ion.

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28. Which one among the following is known as Milk of Magnesia ?

Which one among the following is known as Milk of Magnesia ?

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Magnesium bicarbonate
Magnesium carbonate
Magnesium sulphate
Magnesium hydroxide
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UPSC NDA-1 – 2024
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Milk of Magnesia is the common name for an aqueous suspension of magnesium hydroxide.
Magnesium hydroxide (Mg(OH)₂) is a slightly soluble base. When suspended in water, it forms a milky white liquid known as Milk of Magnesia. It is commonly used as an antacid to neutralize stomach acid and as a laxative.
Magnesium carbonate is also used as an antacid but is not called Milk of Magnesia. Magnesium sulphate (Epsom salt) is used as a laxative and for baths. Magnesium bicarbonate exists only in aqueous solution.

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29. Human body works in the pH range of:

Human body works in the pH range of:

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6.8 - 7.2
7.0 - 7.8
6.5 - 7.5
7.5 - 8.0
This question was previously asked in
UPSC NDA-1 – 2024
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The human body, particularly blood, maintains a very narrow pH range for proper functioning.
The normal pH range of human blood is slightly alkaline, typically between 7.35 and 7.45. Among the given options, the range of 7.0 – 7.8 is the one that correctly encompasses the typical physiological pH range.
Significant deviations from this narrow pH range can lead to severe health problems. For example, a pH below 7.35 is called acidosis, and a pH above 7.45 is called alkalosis. The body uses buffer systems (like the bicarbonate buffer system) to maintain this delicate pH balance.

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30. Common salt (NaCl) is not used as a raw material for preparation of

Common salt (NaCl) is not used as a raw material for preparation of which one of the following compounds?

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Bleaching powder
Baking soda
Plaster of Paris
Washing soda
This question was previously asked in
UPSC NDA-1 – 2021
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The correct answer is (C) Plaster of Paris.
Plaster of Paris (Calcium sulfate hemihydrate, CaSO4·½H2O) is produced by heating gypsum (Calcium sulfate dihydrate, CaSO4·2H2O) to about 150°C. The main raw material for Plaster of Paris is gypsum.
Common salt (NaCl) is a key raw material for the production of:
– Bleaching powder: Chlorine gas, used to make bleaching powder, is typically produced by electrolysis of brine (aqueous NaCl).
– Baking soda (NaHCO3) and Washing soda (Na2CO3): Both are primarily manufactured using the Solvay process, which utilizes brine (NaCl solution), ammonia, and carbon dioxide.
Thus, NaCl is a raw material (directly or indirectly) for bleaching powder, baking soda, and washing soda, but not for Plaster of Paris.

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