Acid, Base and Salt

11. The Soda-acid fire extinguisher contains

The Soda-acid fire extinguisher contains

Subscribe on YouTube
sulphuric acid and solution of sodium hydrogencarbonate
nitric acid and solution of sodium hydrogencarbonate
sulphuric acid and solution of sodium carbonate
sulphuric acid and solution of potassium carbonate
This question was previously asked in
UPSC CAPF – 2019
Download PDFAttempt Online
The correct option is A.
A soda-acid fire extinguisher operates based on the chemical reaction between an acid and a carbonate or bicarbonate to produce carbon dioxide gas. The standard composition of a soda-acid fire extinguisher involves sulphuric acid (H₂SO₄) and a solution of sodium hydrogencarbonate (sodium bicarbonate, NaHCO₃). When the extinguisher is activated (typically by inverting it or breaking a capsule), the acid mixes with the bicarbonate solution, leading to the reaction: H₂SO₄ + 2NaHCO₃ → Na₂SO₄ + 2H₂O + 2CO₂. The released CO₂ gas builds pressure, expelling the solution and also acts as a smothering agent to cut off oxygen supply to the fire.
Soda-acid fire extinguishers are effective against Class A fires (combustible materials like wood, paper, textiles) but are not suitable for Class B (flammable liquids), Class C (flammable gases), or Class D (metals) fires, especially not for electrical fires due to the water content. Modern fire extinguishers often use different agents like dry chemical powders, foam, or CO₂ directly.

Join Our Telegram Channel

12. Which one of the following compounds is NOT considered an acid ?

Which one of the following compounds is NOT considered an acid ?

Join Our Telegram Channel
BF₃
AlCl₃
NH₃
C₆H₅OH
This question was previously asked in
UPSC CAPF – 2015
Download PDFAttempt Online
A) BF₃ (Boron Trifluoride) is a Lewis acid because the Boron atom has an incomplete octet and can accept an electron pair.
B) AlCl₃ (Aluminum Chloride) is also a Lewis acid as the Aluminum atom has an incomplete octet and can accept an electron pair.
C) NH₃ (Ammonia) is a Brønsted-Lowry base because it can accept a proton (H⁺) to form ammonium ion (NH₄⁺). It is also a Lewis base because the nitrogen atom has a lone pair of electrons that it can donate. It is not considered an acid in standard definitions.
D) C₆H₅OH (Phenol) is a weak acid because the hydroxyl group (-OH) attached to the benzene ring can donate a proton, forming the phenoxide ion. The acidity is enhanced by the stability of the phenoxide ion through resonance.
Therefore, NH₃ is the compound that is NOT considered an acid; it is a base.
Acids can be defined in various ways (Arrhenius, Brønsted-Lowry, Lewis). Brønsted-Lowry acids are proton donors. Lewis acids are electron pair acceptors. Bases are proton acceptors (Brønsted-Lowry) or electron pair donors (Lewis). Ammonia is a common example of a base.
Lewis acidity is particularly relevant for species like BF₃ and AlCl₃, which lack a hydrogen ion to donate but can accept electron pairs. Phenol is a classic example of an organic acid, albeit a weaker one compared to carboxylic acids, with a pKa of around 10.

Subscribe on YouTube

13. Which one of the following acid is used by goldsmith for cleaning of g

Which one of the following acid is used by goldsmith for cleaning of gold and silver articles ?

Subscribe on YouTube
Sulphuric acid
Nitric acid
Hydrochloric acid
Phosphoric acid
This question was previously asked in
UPSC NDA-2 – 2023
Download PDFAttempt Online
B) Nitric acid is commonly used by goldsmiths for cleaning gold and silver articles.
– Goldsmiths often need to clean gold and silver items to remove tarnish, dirt, and oxidation.
– Tarnish on silver is typically silver sulfide (Ag₂S), and tarnish on gold can involve sulfides or oxides depending on the alloy.
– Nitric acid (HNO₃) is effective in dissolving many metal oxides and sulfides, including silver sulfide tarnish, while not significantly affecting the noble metals like gold and pure silver (though alloys might react).
– Hydrochloric acid (HCl) can also be used for cleaning certain metal surfaces but is less effective against silver sulfide than nitric acid.
– Sulphuric acid (H₂SO₄) is a strong acid but not typically preferred for cleaning jewellery tarnish compared to nitric or hydrochloric acid.
– Phosphoric acid (H₃PO₄) is a weaker acid used in some cleaning agents (like rust removers) but not commonly by goldsmiths for gold and silver tarnish.
– A mixture of concentrated nitric acid and concentrated hydrochloric acid (in a 1:3 ratio), known as Aqua Regia, is capable of dissolving gold and platinum. This is used for refining or testing gold, NOT for cleaning finished jewellery.

Join Our Telegram Channel

14. Which one of the following salts does not possess water of

Which one of the following salts does not possess water of crystallization?

Subscribe on YouTube
Potassium permanganate
Blue vitriol
Washing soda
Mohr's salt
This question was previously asked in
UPSC NDA-2 – 2022
Download PDFAttempt Online
The correct answer is Potassium permanganate.
Salts that do not possess water of crystallization are called anhydrous salts. Among the given options, Potassium permanganate (KMnO₄) is typically an anhydrous salt. Blue vitriol (CuSO₄·5H₂O), Washing soda (Na₂CO₃·10H₂O), and Mohr’s salt ((NH₄)₂Fe(SO₄)₂·6H₂O) are common examples of hydrated salts, meaning they contain a fixed number of water molecules (water of crystallization) associated with their crystal structure.
Water of crystallization is water that is stoichiometrically bound into a crystal. It is necessary for the maintenance of the crystalline properties but can usually be removed by heating, resulting in the anhydrous salt.

Join Our Telegram Channel

15. The pH value of Milk of Magnesia is approximately

The pH value of Milk of Magnesia is approximately

Join Our Telegram Channel
Zero
7
10
14
This question was previously asked in
UPSC NDA-2 – 2021
Download PDFAttempt Online
Milk of Magnesia is a suspension of magnesium hydroxide (Mg(OH)₂). Magnesium hydroxide is a base. Bases have pH values greater than 7. Milk of Magnesia is a mild base commonly used as an antacid and laxative. Its pH value is approximately 10 to 11. Among the given options, 10 is the closest value.
– Milk of Magnesia is magnesium hydroxide, a basic substance.
– Its pH is alkaline, typically in the range of 10-11.
Antacids work by neutralizing excess stomach acid (hydrochloric acid). The reaction between Mg(OH)₂ and HCl is: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l).

Subscribe on YouTube

16. Which one of the following was the first mineral acid discovered ?

Which one of the following was the first mineral acid discovered ?

Join Our Telegram Channel
Sulphuric acid
Hydrochloric acid
Nitric acid
Phosphoric acid
This question was previously asked in
UPSC NDA-2 – 2020
Download PDFAttempt Online
Nitric acid was one of the first mineral acids discovered.
Mineral acids are acids derived from inorganic compounds. Historically, strong mineral acids like nitric acid, sulfuric acid, and hydrochloric acid were developed by alchemists and early chemists during the Middle Ages. Nitric acid (aqua fortis) is often credited to the Arab alchemist Jabir ibn Hayyan (Geber) in the 8th century, produced by distilling saltpetre (potassium nitrate) with vitriol (sulfates).
While the precise historical order and attribution can be complex due to the nature of early chemical writings, nitric acid is widely recognized as one of the earliest strong mineral acids whose preparation from mineral sources was documented. Sulphuric acid and hydrochloric acid were also known and developed around similar periods by various alchemists like Al-Razi. Phosphoric acid was discovered later, notably by Hennig Brand in the 17th century.

Subscribe on YouTube

17. Which one of the following is the correct sequence of change in colour

Which one of the following is the correct sequence of change in colours when a turmeric stain on white clothes is scrubbed by soap and then washed with water ?

Join Our Telegram Channel
Yellow – pink – blue
Yellow – reddish brown – yellow
Yellow – reddish brown – blue
Yellow – blue – pink
This question was previously asked in
UPSC NDA-2 – 2020
Download PDFAttempt Online
The correct answer is (B) Yellow – reddish brown – yellow.
Turmeric is a natural acid-base indicator. Its original colour is yellow. In acidic or neutral solutions, it remains yellow. In basic (alkaline) solutions, it turns reddish-brown. Soap solution is basic. Therefore, when a turmeric stain (yellow) on white clothes is scrubbed with soap, it turns reddish-brown. When the cloth is then washed with plain water, which is usually neutral or slightly acidic due to dissolved CO2, the basic soap is rinsed away, and the turmeric indicator returns to its yellow colour.
Other indicators change colours differently in acidic and basic media. For example, litmus paper is red in acid and blue in base; phenolphthalein is colourless in acid and pink in base. Turmeric’s characteristic change from yellow to reddish-brown is a common demonstration of its indicator property.

Subscribe on YouTube

18. Number of molecules of water of crystallization in copper sulphate, so

Number of molecules of water of crystallization in copper sulphate, sodium carbonate and Gypsum are

Join Our Telegram Channel
5, 10 and 2 respectively
10, 2 and 5 respectively
5, 2 and 10 respectively
2, 5 and 10 respectively
This question was previously asked in
UPSC NDA-2 – 2020
Download PDFAttempt Online
The correct answer is (A) 5, 10 and 2 respectively.
The number of molecules of water of crystallization are specific to the hydrated forms of these compounds:
– Copper sulphate exists commonly as copper(II) sulphate pentahydrate, CuSO4·5H2O, which has 5 molecules of water.
– Sodium carbonate exists commonly as sodium carbonate decahydrate (washing soda), Na2CO3·10H2O, which has 10 molecules of water.
– Gypsum is calcium sulphate dihydrate, CaSO4·2H2O, which has 2 molecules of water.
Water of crystallization refers to water molecules that are chemically bonded to ions in the crystal structure of a salt. These water molecules are part of the crystalline structure but can be removed by heating the salt.

Subscribe on YouTube

19. At nearly 70°C, sodium bicarbonate shows the property of gradually dec

At nearly 70°C, sodium bicarbonate shows the property of gradually decomposing, which makes it usable in bakery products. The product of decomposition responsible for this use of sodium bicarbonate is

Join Our Telegram Channel
Carbon dioxide
Hydrogen
Water vapour
Oxygen
This question was previously asked in
UPSC NDA-2 – 2020
Download PDFAttempt Online
The correct answer is (A) Carbon dioxide.
Sodium bicarbonate (baking soda, NaHCO3) is used in bakery products as a leavening agent. When heated, it decomposes according to the reaction: 2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g). The gaseous products, water vapour and especially carbon dioxide, are trapped within the dough, causing it to expand and become light and porous. Carbon dioxide is the primary gas responsible for the leavening action.
The resulting sodium carbonate (Na2CO3) is a residual solid. While water vapour also contributes to the rise, carbon dioxide is the main leavening gas produced from this decomposition at baking temperatures. Hydrogen and oxygen are not significant products of this specific decomposition reaction.

Subscribe on YouTube

20. Which one of the following does not represent the salt, Calcium carbon

Which one of the following does not represent the salt, Calcium carbonate ?

Join Our Telegram Channel
Lime water
Limestone
Chalk
Marble
This question was previously asked in
UPSC NDA-2 – 2019
Download PDFAttempt Online
Calcium carbonate ($\text{CaCO}_3$) is a common chemical compound found in various natural forms. Limestone, chalk, and marble are all naturally occurring forms of calcium carbonate. Limestone is a sedimentary rock largely composed of calcium carbonate. Chalk is a type of limestone. Marble is a metamorphic rock formed from limestone.
Lime water, however, is a saturated solution of calcium hydroxide ($\text{Ca(OH)}_2$). It is produced by dissolving calcium oxide (quicklime) in water to form calcium hydroxide (slaked lime), and then dissolving the calcium hydroxide in more water. While calcium carbonate can be *formed* by passing carbon dioxide through lime water (causing precipitation), lime water itself is calcium hydroxide solution, not calcium carbonate.
– Calcium carbonate ($\text{CaCO}_3$) exists in forms like limestone, chalk, and marble.
– Lime water is a solution of calcium hydroxide ($\text{Ca(OH)}_2$).
– Passing $\text{CO}_2$ through lime water produces $\text{CaCO}_3$ precipitate.
Calcium carbonate is a widely used substance, for example, as a building material, as an abrasive, in agriculture to reduce soil acidity, and in medicine as an antacid. Calcium hydroxide (lime water) is also used in various industrial processes and as a test for carbon dioxide.

Subscribe on YouTube
Page 1Page 2Page 3Page 4Page 5