Metals, Minerals, Ores: Properties, Uses

31. Which one of the following could be the melting point of iron ?

Which one of the following could be the melting point of iron ?

25°C
37°C
500°C
1500°C
This question was previously asked in
UPSC NDA-1 – 2019
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The correct answer is (D) 1500°C.
The melting point of pure iron is approximately 1538°C (2800°F). The melting points of common metals are relatively high compared to typical ambient temperatures.
Options (A) 25°C and (B) 37°C are typical room and body temperatures, far too low for the melting point of a metal like iron. Option (C) 500°C is also too low; this might be closer to the melting point of certain alloys or other materials, but not iron. Option (D) 1500°C falls within the correct range for the melting point of iron. Steel, an alloy of iron and carbon, has a melting point that is typically lower than pure iron but still in the range of 1370–1530 °C, depending on the composition.

32. Which one of the following metals does NOT react with cold water ?

Which one of the following metals does NOT react with cold water ?

Calcium (Ca)
Potassium (K)
Magnesium (Mg)
Sodium (Na)
This question was previously asked in
UPSC NDA-1 – 2019
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Magnesium (Mg) does NOT react with cold water.
Alkali metals (like Potassium and Sodium) react vigorously or even explosively with cold water, producing hydrogen gas and their hydroxides. Calcium, an alkaline earth metal, also reacts with cold water, albeit less vigorously than sodium or potassium, forming calcium hydroxide and hydrogen gas. Magnesium, another alkaline earth metal, reacts very slowly with boiling water and reacts readily with steam to form magnesium oxide and hydrogen, but it does not react with cold water.
The reactivity of these metals with water increases down the group for alkali metals (Li < Na < K < Rb < Cs) and generally for alkaline earth metals (Be < Mg < Ca < Sr < Ba), although the trend for alkaline earth metals reacting with cold water starts effectively at Calcium. Beryllium does not react with water or steam.

33. Which one of the following metals is alloyed with sodium to transfer h

Which one of the following metals is alloyed with sodium to transfer heat in a nuclear reactor ?

Potassium
Calcium
Magnesium
Strontium
This question was previously asked in
UPSC NDA-1 – 2018
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Liquid metals, particularly alloys of Sodium and Potassium (NaK), are used as coolants in certain types of nuclear reactors, especially fast breeder reactors, because they have high thermal conductivity and remain liquid over a wide temperature range. The NaK alloy is used to transfer heat from the reactor core. Therefore, Potassium is the metal alloyed with sodium for this purpose among the given options.
Sodium-Potassium (NaK) alloy is a common liquid metal coolant in nuclear reactors due to its excellent heat transfer properties.
NaK alloys are eutectic, meaning they have a lower melting point than either sodium or potassium individually, making them liquid at room temperature (for certain compositions). This is advantageous for starting up and shutting down the reactor.

34. Which one of the following alkali metals has lowest melting point ?

Which one of the following alkali metals has lowest melting point ?

Sodium
Potassium
Rubidium
Caesium
This question was previously asked in
UPSC NDA-1 – 2018
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The alkali metals are elements in Group 1 of the periodic table: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), and Francium (Fr). As we move down the group, the atomic size increases. The metallic bond strength weakens with increasing atomic size because the valence electron is further from the nucleus and shielded by more inner electron shells, leading to lower melting and boiling points. Therefore, among the given options (Sodium, Potassium, Rubidium, Caesium), Caesium (Cs) has the largest atomic size and thus the weakest metallic bonding, resulting in the lowest melting point.
Melting points of alkali metals decrease down the group due to decreasing metallic bond strength as atomic size increases.
The melting points of the given alkali metals are approximately: Sodium (97.8 °C), Potassium (63.5 °C), Rubidium (39.3 °C), and Caesium (28.5 °C). Caesium’s melting point is just above room temperature.

35. Which one of the following elements corrodes rapidly?

Which one of the following elements corrodes rapidly?

Aluminium
Iron
Zinc
Silver
This question was previously asked in
UPSC NDA-1 – 2017
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Iron corrodes rapidly, especially in the presence of oxygen and moisture, through a process known as rusting. While all the listed metals can corrode, iron’s corrosion product (rust) is porous and flakes off, exposing fresh metal to further corrosion, leading to rapid deterioration compared to the others.
Corrosion is an electrochemical process. Iron corrodes to form hydrated iron oxides, which are commonly called rust. Unlike aluminium and zinc, which form protective oxide layers (passivation), rust does not adhere strongly to the surface and is permeable to air and water, allowing corrosion to continue unchecked. Silver tarnishes by reacting with sulfur, forming a thin layer of silver sulfide, but this is a surface phenomenon and not as destructive as the rusting of iron.
Aluminium is highly reactive but forms a protective, adherent oxide layer. Zinc is more reactive than iron but also forms a protective layer (e.g., zinc carbonate or oxide) that inhibits further corrosion, and it is used in galvanization to protect iron sacrificially. Silver is a relatively noble metal and corrodes slowly, primarily through tarnishing. Therefore, iron corrodes most rapidly among the given options under typical atmospheric conditions.

36. Which one of the following elements is least reactive with water ?

Which one of the following elements is least reactive with water ?

Lithium
Sodium
Potassium
Cesium
This question was previously asked in
UPSC NDA-1 – 2017
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Lithium, Sodium, Potassium, and Cesium are alkali metals belonging to Group 1 of the periodic table. The reactivity of alkali metals with water increases down the group due to the decreasing ionization energy and increasing atomic size, making it easier to lose the valence electron. Therefore, Lithium (Li) is the least reactive with water among the given options, while Cesium (Cs) is the most reactive.
Reactivity of alkali metals with water increases from Lithium to Cesium.
Lithium reacts slowly with water, producing hydrogen gas and lithium hydroxide. Sodium reacts vigorously, often melting due to the heat produced. Potassium, Rubidium, and Cesium react increasingly explosively with water.

37. Which one of the following elements will not react with dilute HCl to

Which one of the following elements will not react with dilute HCl to produce H2 ?

Hg
Al
Mg
Fe
This question was previously asked in
UPSC NDA-1 – 2016
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The correct answer is A) Hg.
Dilute HCl is a non-oxidizing acid. Metals that are more reactive than hydrogen will react with such acids to displace hydrogen gas (H₂). The reactivity of metals can be determined from the electrochemical series (or reactivity series). Metals above hydrogen in the series are more reactive and will displace hydrogen from dilute acids. Metals below hydrogen will not. The relevant part of the reactivity series is (from more reactive to less reactive): … Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au …
– Al, Mg, and Fe are all above hydrogen in the series, so they will react with dilute HCl to produce H₂ gas.
– Hg (Mercury) is below hydrogen in the series, so it will not react with dilute HCl to produce H₂ gas.
The reaction of a reactive metal (M) with dilute HCl is typically represented as: M(s) + nHCl(aq) → MClₙ(aq) + n/2 H₂(g), where ‘n’ is the valency of the metal in the chloride. For example, Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Metals below hydrogen can react with oxidizing acids (like concentrated nitric acid or hot concentrated sulfuric acid) but produce different products (e.g., oxides of nitrogen or sulfur dioxide) instead of hydrogen gas.

38. Which one of the following statements regarding gold is not correct?

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

24 karat gold is very soft.
22 karat gold is generally used to make ornaments.
22 karat gold contains 22 parts of pure gold and 2 parts of iron to make it hard.
18 karat gold comprises of 75% gold mixed with 25% of copper or silver to make it hard.
This question was previously asked in
UPSC Geoscientist – 2022
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The correct answer is that statement C is not correct.
Karat is a measure of gold purity. Pure gold (24 karat) is soft. Gold for jewelry is typically alloyed with other metals like copper, silver, or zinc to increase its hardness and durability.
Statement A is correct: 24 karat gold is pure and very soft. Statement B is correct: 22 karat gold (approx 91.67% pure) is commonly used for making ornaments. Statement D is correct: 18 karat gold is 18/24 = 75% gold, mixed with 25% other metals like copper or silver. Statement C is incorrect because while 22 karat gold contains 22 parts gold and 2 parts alloy, the alloy metals are typically copper, silver, or zinc, not predominantly iron. Iron is not a common alloying metal for standard gold jewelry due to issues like rusting and brittleness.

39. Which one of the following is the correct order of relative reactiviti

Which one of the following is the correct order of relative reactivities of metals Ca, Al, Hg and Au?

”Ca
”Au
”Al
”Ca
This question was previously asked in
UPSC Geoscientist – 2021
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The reactivity of metals is typically determined by their tendency to lose electrons and form positive ions. This is represented by the reactivity series. The standard reactivity series is: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au > Pt.
Comparing the given metals:
Calcium (Ca) is above Aluminium (Al) in the series.
Aluminium (Al) is above Mercury (Hg) and Gold (Au).
Mercury (Hg) is above Gold (Au).
Therefore, the correct order of decreasing reactivity is Ca > Al > Hg > Au.
The reactivity series orders metals from most reactive (top) to least reactive (bottom) based on their tendency to lose electrons. Ca is higher than Al, which is higher than Hg, which is higher than Au.
More reactive metals can displace less reactive metals from their compounds. Gold and Platinum are known as noble metals because they are very unreactive.

40. Why silver chloride turns grey in Sunlight?

Why silver chloride turns grey in Sunlight?

It forms silver metal and is an example of endothermic reaction
It forms silver metal and is an example of exothermic reaction
It forms silver metal and is an example of substitution reaction
It forms silver ion (Ag<sup>+</sup>) and is an example of endothermic reaction
This question was previously asked in
UPSC Geoscientist – 2021
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When silver chloride (AgCl) is exposed to sunlight, it undergoes a photochemical decomposition reaction. The light energy causes the decomposition of AgCl into silver metal (Ag) and chlorine gas (Cl₂). The silver metal produced is grey, which causes the white AgCl to turn grey. This reaction requires energy from sunlight, making it an endothermic process.
The reaction is: 2AgCl(s) + Sunlight → 2Ag(s) + Cl₂(g)
Sunlight decomposes silver chloride (AgCl) into grey silver metal (Ag) and chlorine gas. This decomposition is an endothermic reaction.
This photochemical reaction is the basis for early photography techniques (e.g., using silver halide photographic plates or paper). Silver bromide (AgBr) and silver iodide (AgI) also undergo similar photochemical decomposition when exposed to light.
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