Chemistry Archives

81. Which one of the following metals does not react with oxygen, water or

Which one of the following metals does not react with oxygen, water or acids?

[amp_mcq option1=”Na” option2=”Cu” option3=”Fe” option4=”Au” correct=”option4″]

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Metals vary widely in their chemical reactivity. Some are highly reactive, readily reacting with oxygen, water, and acids, while others are much less reactive, sometimes referred to as noble metals.

Sodium (Na) is an alkali metal and is extremely reactive, reacting vigorously with oxygen, water, and acids. Copper (Cu) is moderately reactive; it reacts with oxygen to form oxides and with certain acids (like nitric acid or hot sulfuric acid) but is relatively stable in air and water compared to alkali metals. Iron (Fe) is also reactive; it rusts in moist air (reacts with oxygen and water) and reacts with acids. Gold (Au) is a noble metal, known for its low reactivity. It resists reaction with oxygen at normal temperatures, water, and most acids. It can only be dissolved by highly corrosive mixtures like aqua regia (a mixture of nitric acid and hydrochloric acid).

The reactivity series of metals ranks metals according to their reactivity. Gold is very low in the reactivity series, indicating its low tendency to participate in chemical reactions.

82. When calcium metal reacts with sulphuric acid, the reaction stops afte

When calcium metal reacts with sulphuric acid, the reaction stops after a short while. Why?

[amp_mcq option1=”The reaction of calcium with sulphuric acid is not feasible at room temperature” option2=”Formed calcium sulphate is insoluble in water and covers the metal, preventing it from reacting any further” option3=”Formed calcium sulphate requires two molecules of water to crystallize as gypsum” option4=”Formed calcium sulphate is unstable” correct=”option2″]

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The correct answer is B) Formed calcium sulphate is insoluble in water and covers the metal, preventing it from reacting any further.

Calcium is a relatively reactive metal that reacts with acids like sulfuric acid to produce hydrogen gas and a salt: Ca(s) + H₂SO₄(aq) → CaSO₄(s) + H₂(g). However, calcium sulfate (CaSO₄) is sparingly soluble in water, and its solubility is even lower in concentrated sulfuric acid. As the reaction proceeds, a layer of solid calcium sulfate forms on the surface of the calcium metal. This layer acts as a barrier, preventing the sulfuric acid from coming into contact with the underlying calcium metal, effectively stopping or significantly slowing down the reaction after a short while. This phenomenon is known as passivation.

Other metals that exhibit passivation with sulfuric acid include lead (forming insoluble PbSO₄) and barium (forming insoluble BaSO₄). With other acids like hydrochloric acid, calcium reacts vigorously because calcium chloride (CaCl₂) is soluble.

83. Which one of the following metal oxides does not usually react with so

Which one of the following metal oxides does not usually react with sodium hydroxide?

[amp_mcq option1=”Copper oxide” option2=”Aluminium oxide” option3=”Potassium oxide” option4=”Calcium oxide” correct=”option1″]

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The correct answer is A) Copper oxide.

Sodium hydroxide (NaOH) is a strong base. Basic metal oxides react with acids, and acidic oxides react with bases. Amphoteric oxides react with both acids and strong bases.
Aluminium oxide (Al₂O₃) is an amphoteric oxide and reacts with sodium hydroxide: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (at high temperature/concentration) or Al₂O₃ + 6NaOH + 3H₂O → 2Na₃[Al(OH)₆] (in aqueous solution).
Potassium oxide (K₂O) and Calcium oxide (CaO) are basic oxides. They react vigorously with water to form potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)₂), respectively, which are strong bases. Being bases, KOH and Ca(OH)₂ do not react with another base like NaOH.
Copper oxide (CuO) is also a basic oxide. It does not react with water and does not react with sodium hydroxide solution under usual conditions.
Among the given options, Aluminium oxide reacts with NaOH. Potassium oxide and Calcium oxide react with water to form hydroxides that do not react with NaOH. Copper oxide is a basic oxide that neither reacts significantly with water nor with NaOH solution under usual conditions. Therefore, Copper oxide is the one that does not usually react with sodium hydroxide.

Reactivity of metal oxides with bases depends on their acidic, basic, or amphoteric nature. Oxides of highly electropositive metals (Group 1 & 2) are strongly basic. Oxides of transition metals can be basic, amphoteric, or even acidic depending on the oxidation state. Oxides of metalloids and non-metals are typically amphoteric or acidic.

84. A milkman adds a small amount of baking soda to milk. This makes the p

A milkman adds a small amount of baking soda to milk. This makes the pH of the milk

[amp_mcq option1=”strongly alkaline” option2=”weakly alkaline” option3=”strongly acidic” option4=”weakly acidic” correct=”option2″]

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The correct answer is B) weakly alkaline.

Baking soda is sodium bicarbonate (NaHCO₃), which is a weak base. Milk is slightly acidic or close to neutral, typically having a pH between 6.7 and 6.9. Adding a small amount of a weak base like baking soda to milk will neutralize some of the natural acidity and shift the pH towards the alkaline side. A small amount of a weak base will make the solution weakly alkaline, not strongly alkaline or acidic. Milkmen traditionally added baking soda to milk to prevent it from souring quickly, as it neutralizes the lactic acid produced by bacteria, which causes souring and lowers the pH.

The pH scale measures acidity and alkalinity. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is alkaline (basic). Adding a base increases the pH.

85. Packets of potato chips are usually flushed with which one of the foll

Packets of potato chips are usually flushed with which one of the following gases to prevent rancidity?

[amp_mcq option1=”Carbon dioxide” option2=”Hydrogen” option3=”Nitrogen” option4=”Oxygen” correct=”option3″]

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The correct answer is C) Nitrogen.

Packets of potato chips and other similar snacks contain fats and oils. These fats and oils can undergo oxidation when exposed to oxygen in the air, leading to rancidity, which results in unpleasant smells and flavors. To prevent this, the air inside the packets is replaced with an inert gas, most commonly nitrogen. Nitrogen is chosen because it is readily available, inexpensive, and does not react with the food components. This flushing process removes oxygen, significantly slowing down the oxidation process and preventing rancidity, thus extending the shelf life of the product.

This technique is an example of modified atmosphere packaging (MAP), where the gaseous atmosphere surrounding the food is altered to improve preservation. Other gases like carbon dioxide are also sometimes used in food packaging, often in combination with nitrogen.

86. Which one of the following is an example of endothermic reaction?

Which one of the following is an example of endothermic reaction?

[amp_mcq option1=”CaO(s) + H₂O(l) → Ca(OH)₂(aq)” option2=”CaCO₃(s) → CaO(s) + CO₂(g)” option3=”C(s) + O₂(g) → CO₂(g)” option4=”CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)” correct=”option2″]

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The correct answer is B) CaCO₃(s) → CaO(s) + CO₂(g).

An endothermic reaction is a reaction that absorbs heat from its surroundings.
A) CaO(s) + H₂O(l) → Ca(OH)₂(aq) (Slaking of lime) is a highly exothermic reaction, releasing significant heat.
B) CaCO₃(s) → CaO(s) + CO₂(g) (Decomposition of calcium carbonate) requires heating to occur, meaning it absorbs heat from the surroundings, making it an endothermic reaction.
C) C(s) + O₂(g) → CO₂(g) (Combustion of carbon) is an exothermic reaction, releasing heat and light.
D) CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) (Combustion of methane) is an exothermic reaction, releasing a large amount of heat.

Exothermic reactions have a negative enthalpy change (ΔH < 0), while endothermic reactions have a positive enthalpy change (ΔH > 0). Decomposition reactions often require energy input to break chemical bonds, making them endothermic. Combustion reactions always release energy, making them exothermic.

87. Which one of the following minerals is NOT a naturally occurring silic

Which one of the following minerals is NOT a naturally occurring silicate ?

[amp_mcq option1=”Feldspar” option2=”Zeolite” option3=”Mica” option4=”Bauxite” correct=”option4″]

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UPSC CAPF – 2020

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Bauxite is the principal ore of aluminium and is primarily composed of hydrated aluminium oxides and hydroxides, such as gibbsite, boehmite, and diaspore, often mixed with iron oxides, clay minerals, and quartz. It does not have the silicon-oxygen tetrahedral structure characteristic of silicate minerals. Feldspar, Zeolite, and Mica are all well-known families of naturally occurring silicate minerals.

Silicate minerals are composed of silicon and oxygen, often with other metals, forming structures based on the SiO₄⁴⁻ tetrahedron unit. Non-silicate minerals lack this fundamental structure.

Feldspars are tectosilicates, zeolites are porous tectosilicates, and micas are phyllosilicates (sheet silicates). Bauxite is classified as an oxide mineral.

88. Which one of the following is NOT a heterocyclic aromatic compound ?

Which one of the following is NOT a heterocyclic aromatic compound ?

[amp_mcq option1=”Tropolone” option2=”Furan” option3=”Thiophene” option4=”Pyridine” correct=”option1″]

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Tropolone is a seven-membered carbocyclic compound containing a ketone and a hydroxyl group. Its aromaticity arises from the cyclic conjugated system within the carbon ring, but the ring itself is made entirely of carbon atoms. Furan, Thiophene, and Pyridine are all five- or six-membered rings containing heteroatoms (Oxygen in Furan, Sulfur in Thiophene, Nitrogen in Pyridine) within the ring structure, along with conjugated pi systems that satisfy Hückel’s rule, making them heterocyclic aromatic compounds.

A heterocyclic compound contains at least one atom other than carbon (such as O, N, S) in its ring structure. An aromatic compound is cyclic, planar, conjugated, and obeys Hückel’s rule (4n+2 pi electrons).

Tropolone is a carbocyclic aromatic compound. Common examples of heterocyclic aromatic compounds include pyridine, furan, thiophene, pyrrole, and imidazole.

89. Which one of the following is the average atomic mass of chlorine atom

Which one of the following is the average atomic mass of chlorine atom ?

[amp_mcq option1=”35.9 u” option2=”35.5 u” option3=”35.0 u” option4=”37.0 u” correct=”option2″]

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The average atomic mass of chlorine atom is approximately 35.5 u.

Chlorine occurs naturally as a mixture of two main isotopes: chlorine-35 (³⁵Cl) and chlorine-37 (³⁷Cl). Their approximate natural abundances are about 75% for ³⁵Cl (mass ≈ 34.97 u) and 25% for ³⁷Cl (mass ≈ 36.97 u). The average atomic mass is a weighted average of the masses of its isotopes based on their natural abundance.

Average atomic mass = (0.75 × 34.97 u) + (0.25 × 36.97 u) ≈ 26.23 u + 9.24 u ≈ 35.47 u, which is rounded to 35.5 u.

90. Which one of the following methods can be used to remove permanent har

Which one of the following methods can be used to remove permanent hardness of water ?

[amp_mcq option1=”Boiling” option2=”Ion-exchange” option3=”Lime treatment” option4=”Bubbling sulphur dioxide gas” correct=”option2″]

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UPSC CAPF – 2020

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The Ion-exchange method can be used to remove permanent hardness of water.

Permanent hardness in water is caused by dissolved sulfates and chlorides of calcium and magnesium, which do not precipitate out upon boiling. The ion-exchange method, using materials like zeolite or synthetic resin, replaces the hard ions (Ca2+, Mg2+) with soft ions (usually Na+). This process is effective in removing both temporary and permanent hardness.

A) Boiling removes only temporary hardness (caused by bicarbonates). C) Lime treatment primarily removes temporary hardness by precipitating calcium and magnesium bicarbonates. Adding lime and soda ash (lime-soda process) can remove both. D) Bubbling sulfur dioxide gas through water forms sulfurous acid, which does not effectively remove the ions responsible for permanent hardness and is not a standard water softening method.