Chemistry

71. Calcium oxide reacts with water to produce slaked lime. It is an examp

Calcium oxide reacts with water to produce slaked lime. It is an example of

[amp_mcq option1=”combination reaction” option2=”decomposition reaction” option3=”oxidation reaction” option4=”addition reaction” correct=”option1″]

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The reaction described is between calcium oxide ($\text{CaO}$) and water ($\text{H}_2\text{O}$) to produce slaked lime, which is calcium hydroxide ($\text{Ca(OH)}_2$).
The balanced chemical equation for this reaction is:
$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)}$

Let’s analyze the types of reactions given in the options:
A) Combination reaction: A reaction in which two or more reactants combine to form a single product. In this reaction, calcium oxide and water (two reactants) combine to form calcium hydroxide (a single product). This matches the definition of a combination reaction.
B) Decomposition reaction: A reaction in which a single compound breaks down into two or more simpler substances. This is the opposite of the given reaction.
C) Oxidation reaction: A reaction involving the loss of electrons or increase in oxidation state. While redox aspects might be present in the formation of bonds, the primary classification based on the change in the number of substances is combination. The oxidation states of Ca (+2), O (-2), and H (+1) do not change overall.
D) Addition reaction: A reaction in which atoms are added to a molecule across a multiple bond (like a double or triple bond). This term is mainly used in organic chemistry for reactions involving unsaturated hydrocarbons. While water is added to CaO, the term ‘combination reaction’ is the standard classification for this type of inorganic reaction.

The reaction fits perfectly the definition of a combination reaction. It is also a highly exothermic reaction, releasing a significant amount of heat, which is why it is also called the slaking of lime.

– Identifying the reactants and products of the reaction: Calcium oxide + Water -> Calcium hydroxide.
– Understanding the definition of different types of chemical reactions.
– Classifying the reaction based on the change in the number of reactants and products. Two reactants form one product, characteristic of a combination reaction.
This reaction is a classic example of an inorganic combination reaction and the process of slaking quicklime ($\text{CaO}$) to produce slaked lime ($\text{Ca(OH)}_2$). Slaked lime has many uses, including in mortars, plasters, and water treatment. The reaction is also an example of a hydrolysis reaction because water is a reactant. However, among the given choices, “combination reaction” is the most appropriate primary classification.

72. Hydrogenation of alkenes can be carried out in the presence of

Hydrogenation of alkenes can be carried out in the presence of

[amp_mcq option1=”copper” option2=”zinc” option3=”aluminium” option4=”nickel” correct=”option4″]

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Hydrogenation is a chemical reaction between molecular hydrogen ($\text{H}_2$) and another compound, usually in the presence of a catalyst. Hydrogenation of alkenes involves adding hydrogen across the carbon-carbon double bond, converting the alkene into a saturated alkane. This reaction typically requires a catalyst to lower the activation energy, as the $\text{H-H}$ bond in hydrogen is strong.
Common catalysts used for the hydrogenation of alkenes are transition metals, particularly from Group 8, 9, and 10 of the periodic table. These metals, such as platinum ($\text{Pt}$), palladium ($\text{Pd}$), rhodium ($\text{Rh}$), and nickel ($\text{Ni}$), adsorb both the alkene and hydrogen molecules onto their surface, facilitating the reaction.

Looking at the options provided:
A) Copper: Copper can catalyze some hydrogenation reactions, but it is generally less active than Ni, Pt, or Pd for alkene hydrogenation.
B) Zinc: Zinc is not a typical catalyst for the hydrogenation of alkenes.
C) Aluminium: Aluminium is not used as a catalyst for the hydrogenation of alkenes; it is a reactive metal itself.
D) Nickel: Nickel is a very common and widely used catalyst for the hydrogenation of alkenes, especially in industrial processes (e.g., hydrogenation of vegetable oils to margarine – the Sabatier-Senderens process often uses Ni).

Therefore, nickel is a standard catalyst for the hydrogenation of alkenes.

– Hydrogenation of alkenes is the addition of H₂ to the double bond.
– This reaction requires a catalyst.
– Transition metals like Ni, Pt, Pd, and Rh are common hydrogenation catalysts.
– Nickel is a common and cost-effective catalyst for alkene hydrogenation.
The catalytic hydrogenation of alkenes is a heterogeneous catalytic process where the reactants and catalyst are in different phases (alkenes/hydrogen are gases or liquids, catalyst is a solid metal). The reaction proceeds via a mechanism where both the alkene and hydrogen are adsorbed onto the catalyst surface. Raney nickel is a porous form of nickel alloy commonly used for hydrogenation.

73. White gold is an alloy of

White gold is an alloy of

[amp_mcq option1=”gold, nickel and palladium” option2=”gold, cobalt and palladium” option3=”gold, titanium and platinum” option4=”gold, magnesium and palladium” correct=”option1″]

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White gold is an alloy of gold with at least one white metal. The purpose of alloying gold with white metals is to produce a white appearance and often increase its hardness and durability. Common white metals used in white gold alloys include nickel, palladium, silver, and platinum.
– Nickel is a common and inexpensive white metal used in white gold alloys, especially in the United States. It provides hardness but can cause allergic reactions in some individuals.
– Palladium is a more expensive white metal used in white gold alloys, particularly in Europe. It produces a whiter alloy and is hypoallergenic.
– Platinum is also a white metal and can be used, often as part of a platinum group metal mix including palladium.
– Silver is a white metal sometimes used in smaller quantities to modify color and workability.

Option A lists gold, nickel, and palladium, which are all standard components found in various white gold alloys.
Option B lists cobalt, which is less commonly used than nickel or palladium in standard white gold jewelry alloys.
Option C lists titanium, which is not typically used in gold alloys for jewelry purposes.
Option D lists magnesium, which is also not typically used in gold alloys for jewelry purposes.

Therefore, gold, nickel, and palladium represent a typical composition of white gold alloys.

– White gold is an alloy of gold with white metals.
– Common white metals alloyed with gold include nickel, palladium, silver, and platinum.
– Option A correctly lists common alloying metals for white gold.
The actual composition of white gold varies. Nickel-white gold alloys are typically 10-20% nickel, with possibly some copper or zinc. Palladium-white gold alloys are typically 10-20% palladium, which is more expensive but produces a softer, more easily worked, and hypoallergenic alloy. Higher karat white gold alloys often use palladium. White gold jewelry is usually plated with rhodium to give it a brighter white finish.

74. Cinnabar is an ore of

Cinnabar is an ore of

[amp_mcq option1=”mercury” option2=”zinc” option3=”copper” option4=”lead” correct=”option1″]

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Cinnabar is a mineral with the chemical formula $\text{HgS}$ (mercury(II) sulfide). It is the most common ore from which the element mercury ($\text{Hg}$) is extracted. Historically, it was mined extensively, and its red color made it a valuable pigment (vermilion). Roasting cinnabar in air converts the mercury(II) sulfide to elemental mercury and sulfur dioxide:
$\text{HgS(s)} + \text{O}_2\text{(g)} \rightarrow \text{Hg(g)} + \text{SO}_2\text{(g)}$
– Cinnabar is a mineral composed of mercury sulfide ($\text{HgS}$).
– An ore is a natural occurrence of a rock or sediment which contains sufficient minerals with economically important elements, metals or gems.
– Cinnabar is the primary ore for the extraction of mercury.
Other common ores of the metals listed are:
– Zinc: Sphalerite ($\text{ZnS}$), Calamine ($\text{ZnCO}_3$).
– Copper: Chalcopyrite ($\text{CuFeS}_2$), Malachite ($\text{Cu}_2(\text{CO}_3)(\text{OH})_2$), Azurite ($\text{Cu}_3(\text{CO}_3)_2(\text{OH})_2$).
– Lead: Galena ($\text{PbS}$).

75. Borax is prepared from

Borax is prepared from

[amp_mcq option1=”calcium carbonate” option2=”magnesium carbonate” option3=”potassium carbonate” option4=”sodium carbonate” correct=”option4″]

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Borax is sodium tetraborate decahydrate ($\text{Na}_2\text{B}_4\text{O}_7\cdot10\text{H}_2\text{O}$). It is a compound of sodium and boron. While industrially it is often extracted from naturally occurring borate minerals like kernite or colemanite, it can also be synthesized. One method of synthesizing borax in the lab involves reacting boric acid ($\text{H}_3\text{BO}_3$) with a sodium compound. Among the given options, sodium carbonate ($\text{Na}_2\text{CO}_3$) is a common reactant used in the preparation of borax from boric acid. The reaction is:
$4\text{H}_3\text{BO}_3 + \text{Na}_2\text{CO}_3 \rightarrow \text{Na}_2\text{B}_4\text{O}_7 + 6\text{H}_2\text{O} + \text{CO}_2$
This yields anhydrous sodium tetraborate, which can then be hydrated to form borax. Given the options, sodium carbonate is the most suitable precursor listed for the preparation of borax.
– Borax is a sodium borate compound.
– Its preparation often involves a sodium source and a boron source.
– Sodium carbonate is a common sodium source used in synthesizing borax from boric acid.
Other sodium compounds like sodium hydroxide ($\text{NaOH}$) can also be used to react with boric acid to produce borax, but $\text{Na}_2\text{CO}_3$ is explicitly listed as an option. Borax is an important boron compound used in various applications, including detergents, cosmetics, and as a flux in metallurgy.

76. Which of the following makes bread soft and spongy when baking soda is

Which of the following makes bread soft and spongy when baking soda is added?

[amp_mcq option1=”Sodium salt of acid” option2=”NaHCO₃” option3=”CO₂” option4=”H₂O” correct=”option3″]

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The correct answer is C, CO₂.
Baking soda is sodium bicarbonate (NaHCO₃). When used in baking, it acts as a leavening agent.
In the presence of heat or an acidic ingredient (like those often found in dough or baking powder), baking soda undergoes a chemical reaction that produces carbon dioxide (CO₂) gas.
The main reactions are:
1. Decomposition by heat (slow without acid): 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)
2. Reaction with an acid (e.g., citric acid, tartaric acid, or acids in buttermilk/yogurt): NaHCO₃(s) + H⁺(aq) → Na⁺(aq) + H₂O(l) + CO₂(g)
The carbon dioxide gas gets trapped within the gluten matrix of the dough, causing it to expand and rise. This creates small air pockets, which give the bread or cake a light, fluffy, soft, and spongy texture.
While NaHCO₃ is the source of the leavening action, it is the carbon dioxide gas produced from its reaction or decomposition that physically causes the dough to rise and become soft and spongy. Sodium carbonate (Na₂CO₃), formed from heat decomposition, can leave a slightly soapy taste if not neutralized by an acid.

77. Which one of the following oxides reacts with both acid and base?

Which one of the following oxides reacts with both acid and base?

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

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The correct answer is A, Aluminium oxide.
Oxides are classified as acidic, basic, neutral, or amphoteric based on their reaction with acids and bases.
Amphoteric oxides are oxides that exhibit both acidic and basic properties, meaning they can react with both acids and bases to form salts and water.
Aluminium oxide (Al₂O₃) is a common example of an amphoteric oxide.
It reacts with acids: Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
It reacts with bases: Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq) (Sodium tetrahydroxoaluminate, also written as 2NaAlO₂ + 4H₂O or similar)
Calcium oxide (CaO), Sodium oxide (Na₂O), and Potassium oxide (K₂O) are typically basic oxides, reacting with acids but not bases (except perhaps very strong, concentrated bases in specific conditions not characteristic of amphoterism).
Other examples of amphoteric oxides include Zinc oxide (ZnO) and Lead(II) oxide (PbO). The amphoteric nature of oxides often relates to the position of the metal in the periodic table; elements like Aluminium, Zinc, and Lead are on the border between metals and nonmetals.

78. Which one of the following properties decreases across the periodic ta

Which one of the following properties decreases across the periodic table from left to right and increases from top to bottom?

[amp_mcq option1=”Ionization energy” option2=”Electron affinity” option3=”Electronegativity” option4=”Atomic radius” correct=”option4″]

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Atomic radius decreases across the periodic table from left to right and increases from top to bottom.
Across a period (left to right), the atomic radius decreases because the nuclear charge increases while electrons are added to the same energy shell, pulling the electron cloud closer to the nucleus. Down a group (top to bottom), the atomic radius increases because new electron shells are added, increasing the distance between the nucleus and the valence electrons, despite the increased nuclear charge.
Ionization energy, electron affinity (generally), and electronegativity all tend to increase across a period from left to right and decrease down a group from top to bottom, showing the opposite trend to atomic radius. This is because these properties are related to the attraction between the nucleus and electrons; a smaller radius generally means a stronger attraction.

79. Manganese is extracted by heating manganese dioxide with aluminium pow

Manganese is extracted by heating manganese dioxide with aluminium powder. Which one of the following statements with regard to the reaction is correct?

[amp_mcq option1=”The reaction is exothermic.” option2=”The reaction is endothermic.” option3=”Manganese is produced as a solid.” option4=”Manganese is more reactive than aluminium.” correct=”option1″]

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The reaction between manganese dioxide and aluminium powder is a type of thermite reaction. Thermite reactions are highly exothermic, releasing a significant amount of heat.
Thermite reactions involve the reduction of a metal oxide by a more reactive metal (typically aluminium). The general form is Metal Oxide A + Metal B → Metal A + Oxide of Metal B, where Metal B is more reactive than Metal A. These reactions are characterized by their high exothermicity, often producing molten metal.
In this specific reaction, Al is more reactive than Mn, displacing it from MnO2. The equation is 3MnO₂ + 4Al → 2Al₂O₃ + 3Mn + Heat. While Manganese is indeed produced as a solid at room temperature, during the reaction itself, the high temperature can cause the product metal (depending on its melting point and the reaction’s heat) to be in a molten state. However, the statement that the reaction is exothermic is a fundamental property of this reaction type.

80. Which one of the following metals produces hydrogen with cold water?

Which one of the following metals produces hydrogen with cold water?

[amp_mcq option1=”Silver” option2=”Sodium” option3=”Copper” option4=”Iron” correct=”option2″]

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Sodium is a highly reactive alkali metal. It reacts vigorously with cold water, producing hydrogen gas and sodium hydroxide.
The reactivity of metals with water varies. Very reactive metals (like Group 1 alkali metals) react with cold water. Less reactive metals (like zinc or iron) may react with steam but not cold water. Even less reactive metals (like copper, silver, gold) do not react with water at all.
The reaction of sodium with cold water is exothermic and can even ignite the hydrogen produced due to the heat generated. The general reaction is 2M + 2H₂O → 2MOH + H₂ (where M is an alkali metal). Other metals like Potassium and Calcium also react with cold water, while Iron reacts with steam to produce iron oxides and hydrogen.
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