Chemistry

251. Which one of the following is a chemical change ?

Which one of the following is a chemical change ?

[amp_mcq option1=”Cutting of hair” option2=”Graying of hair naturally” option3=”Swelling of resin in water” option4=”Cutting of fruit” correct=”option2″]

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A chemical change results in the formation of new substances with different chemical properties.
A) Cutting of hair is a physical change; the hair material itself (keratin) remains chemically unchanged, only its physical shape and size are altered.
B) Graying of hair naturally involves chemical changes in the hair follicle where the production of melanin (the pigment that gives hair its color) decreases or stops. This is a biological process that alters the chemical composition of the hair shaft over time, leading to a change in color. It is an irreversible change at the level of pigment production.
C) Swelling of resin in water is a physical change; the resin absorbs water but its chemical composition does not fundamentally change. This is often a process of hydration or dissolution/dispersion.
D) Cutting of fruit is primarily a physical change; the fruit’s chemical composition remains the same, although the exposed surface might subsequently undergo chemical reactions like oxidation (browning), but the act of cutting is physical.
Therefore, graying of hair is a chemical change.
– Chemical change involves the formation of new substances.
– Physical change alters form or appearance without changing chemical composition.
– Graying of hair involves a change in pigment (melanin) production, a chemical/biological process.
Distinguishing between physical and chemical changes is a fundamental concept in chemistry. Physical changes are often easily reversible (e.g., melting ice), while chemical changes are typically irreversible under normal conditions (e.g., burning wood). Clues for chemical change include color change (not due to mixing), gas production, heat absorption or release, and precipitate formation.

252. Zinc is used to protect iron from corrosion because zinc is

Zinc is used to protect iron from corrosion because zinc is

[amp_mcq option1=”more electropositive than iron” option2=”cheaper than iron” option3=”a bluish white metal” option4=”a good conductor of heat and electricity” correct=”option1″]

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Zinc is used to protect iron from corrosion through a process called galvanization. Zinc protects iron because it is more electropositive (or more reactive) than iron. In the electrochemical series, zinc is placed above iron, meaning it loses electrons more easily and has a higher tendency to be oxidized. When both metals are present in the presence of an electrolyte (like moisture), zinc acts as the anode and corrodes sacrificially, while iron acts as the cathode and is protected from oxidation (rusting). Even if the zinc coating is scratched and iron is exposed, the zinc in contact with the electrolyte will still corrode preferentially, providing galvanic protection to the iron.
– Zinc is more electropositive/reactive than iron.
– Zinc acts as a sacrificial anode, corroding instead of iron.
– Provides protection even if the coating is scratched (galvanic protection).
Other methods of protecting iron from rusting include painting, greasing, plating with less reactive metals (like tin, although this doesn’t provide sacrificial protection if scratched), and alloying (like stainless steel). Galvanization is a very effective and widely used method due to zinc’s sacrificial nature.

253. How much CO₂ is produced on heating of 1 kg of carbon ?

How much CO₂ is produced on heating of 1 kg of carbon ?

[amp_mcq option1=”11/3 kg” option2=”3/11 kg” option3=”4/3 kg” option4=”3/4 kg” correct=”option1″]

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The balanced chemical equation for the complete combustion of carbon is:
C (s) + O₂ (g) → CO₂ (g)
The molar mass of Carbon (C) is approximately 12 g/mol.
The molar mass of Carbon Dioxide (CO₂) is approximately 12 g/mol (for C) + 2 * 16 g/mol (for O) = 44 g/mol.
According to the stoichiometry of the balanced equation, 1 mole of carbon reacts to produce 1 mole of carbon dioxide.
Therefore, 12 grams of carbon produce 44 grams of carbon dioxide.
To find the amount of CO₂ produced from 1 kg (1000 g) of carbon, we can use the ratio:
(Mass of CO₂ produced / Mass of C reacted) = (Molar mass of CO₂ / Molar mass of C)
Mass of CO₂ produced = (44 g / 12 g) * 1000 g
Mass of CO₂ produced = (11/3) * 1000 g = 11000/3 g
Converting grams to kilograms: 11000/3 g = (11000/3) / 1000 kg = 11/3 kg.
– Chemical reaction: C + O₂ → CO₂
– Molar mass ratio of CO₂ to C is 44:12, which simplifies to 11:3.
– The mass of CO₂ produced is 11/3 times the mass of carbon reacted.
This calculation assumes complete combustion of pure carbon. In real-world scenarios, combustion might be incomplete (producing CO) or the fuel might contain impurities, affecting the actual CO₂ yield. The calculation uses approximate standard atomic weights.

254. The proposition ‘equal volumes of different gases contain equal number

The proposition ‘equal volumes of different gases contain equal numbers of molecules at the same temperature and pressure’ is known as

[amp_mcq option1=”Avogadro’s hypothesis” option2=”Gay-Lussac’s hypothesis” option3=”Planck’s hypothesis” option4=”Kirchhoff’s theory” correct=”option1″]

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The proposition that ‘equal volumes of different gases contain equal numbers of molecules at the same temperature and pressure’ is a fundamental principle in chemistry and physics of gases.
This statement is known as Avogadro’s hypothesis (or Avogadro’s Law). It was first proposed by Amedeo Avogadro in 1811. It directly relates the volume of a gas to the number of particles (molecules or atoms) it contains, under constant temperature and pressure conditions.
Gay-Lussac’s Law of Combining Volumes states that when gases react, the volumes of the reactants and products, measured at the same temperature and pressure, bear a simple whole-number ratio to one another. Planck’s hypothesis (specifically, Planck’s quantum hypothesis) states that energy can be emitted or absorbed only in discrete packets called quanta. Kirchhoff’s laws are important in electrical circuit analysis (Kirchhoff’s circuit laws) and thermal radiation (Kirchhoff’s law of thermal radiation).

255. The compound C 6 H 12 O 4 contains

The compound C6H12O4 contains

[amp_mcq option1=”22 atoms per mole” option2=”twice the mass percent of H as compared to the mass percent of C” option3=”six times the mass percent of C as compared to the mass percent of H” option4=”thrice the mass percent of H as compared to the mass percent of O” correct=”option3″]

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We need to analyze the composition of the compound C₆H₁₂O₄ in terms of the relative masses or mass percentages of its constituent elements.
The compound is C₆H₁₂O₄. Using approximate atomic masses (C=12, H=1, O=16):
Mass of C in one molecule = 6 * 12 = 72
Mass of H in one molecule = 12 * 1 = 12
Mass of O in one molecule = 4 * 16 = 64
Total mass of one molecule (or molar mass) = 72 + 12 + 64 = 148.

Let’s evaluate the options:
A) 22 atoms per mole: One molecule contains 6 + 12 + 4 = 22 atoms. One mole contains Avogadro’s number of molecules, so one mole contains 22 * Avogadro’s number of atoms, not just 22 atoms. Incorrect.
B) twice the mass percent of H as compared to the mass percent of C: Mass of C is 72, Mass of H is 12. Is %H = 2 * %C? (12/148)*100% vs 2 * (72/148)*100%. This simplifies to 12 vs 2*72=144. Clearly incorrect. Is %C = 2 * %H? 72 vs 2*12=24. Incorrect.
C) six times the mass percent of C as compared to the mass percent of H: This is poorly phrased but implies a ratio. Let’s check if %C = 6 * %H. Mass of C is 72, Mass of H is 12. 72 = 6 * 12. Yes, the mass of Carbon in the compound is six times the mass of Hydrogen. Since the mass percentages are (mass of element / total mass) * 100%, if Mass of C = 6 * Mass of H, then %C = 6 * %H. This holds true.
D) thrice the mass percent of H as compared to the mass percent of O: Mass of H is 12, Mass of O is 64. Is %O = 3 * %H? 64 vs 3*12=36. Incorrect. Is %H = 3 * %O? 12 vs 3*64. Incorrect.

Option C is the only statement that accurately reflects the mass composition of the compound, interpreting “six times the mass percent of C as compared to the mass percent of H” as the mass percent of C being six times the mass percent of H.

Calculating mass percentages:
%C = (72/148) * 100% ≈ 48.65%
%H = (12/148) * 100% ≈ 8.11%
%O = (64/148) * 100% ≈ 43.24%
Check C: 6 * %H = 6 * 8.11% ≈ 48.66%, which is very close to %C (48.65%).

256. The species that has the same number of electrons as 35 17 Cl is

The species that has the same number of electrons as 3517Cl is

[amp_mcq option1=”3216S” option2=”3416S+” option3=”4018Ar+” option4=”3516S2-” correct=”option3″]

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The number of electrons in a neutral atom is equal to its atomic number (the subscript). For an ion, the number of electrons is adjusted based on the charge.
The species is 3517Cl. The atomic number is 17, so a neutral Chlorine atom has 17 protons and 17 electrons. We need to find the species with 17 electrons.
A) 3216S: Atomic number 16. Neutral Sulfur has 16 electrons.
B) 3416S+: Atomic number 16. Neutral Sulfur has 16 electrons. S⁺ means it lost 1 electron (16 – 1 = 15 electrons).
C) 4018Ar+: Atomic number 18. Neutral Argon has 18 electrons. Ar⁺ means it lost 1 electron (18 – 1 = 17 electrons).
D) 3516S2-: Atomic number 16. Neutral Sulfur has 16 electrons. S²⁻ means it gained 2 electrons (16 + 2 = 18 electrons).
Therefore, 4018Ar+ has 17 electrons, the same number as neutral 3517Cl. These species are isoelectronic.
The superscript number (mass number) is the sum of protons and neutrons and is irrelevant to the number of electrons in this calculation, unless isotopes are being considered, which is not the case here for electron count. The subscript number is the atomic number, which is crucial as it determines the number of protons and thus the number of electrons in a neutral atom.

257. The principal use of hydrofluoric acid is

The principal use of hydrofluoric acid is

[amp_mcq option1=”in etching glass” option2=”as a bleaching agent” option3=”as an extremely strong oxidizing agent” option4=”in the preparation of strong organic fluorine compounds” correct=”option1″]

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The principal use of hydrofluoric acid is related to its unique property of reacting with silicon dioxide (silica).
Hydrofluoric acid (HF) is highly corrosive and capable of dissolving many materials, particularly oxides and silicates. Its ability to react with SiO₂, the main component of glass, allows it to etch glass surfaces. This reaction is: SiO₂ + 4HF → SiF₄ + 2H₂O. This property is exploited extensively in industries for etching glass, cleaning silicon wafers in electronics, and polishing metals.
While HF is used in the production of organic fluorine compounds, as a catalyst, and in other chemical processes, etching glass and processing silicon are widely cited as principal or distinctive uses due to its unique reactivity with silicates. It is not primarily a bleaching agent or an extremely strong oxidizing agent.

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

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

[amp_mcq option1=”HCl” option2=”CH₃COOH” option3=”CH₃OH” option4=”NaOH” correct=”option4″]

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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.

259. When pure water boils vigorously, the bubbles that rise to the surface

When pure water boils vigorously, the bubbles that rise to the surface are composed primarily of

[amp_mcq option1=”air” option2=”hydrogen” option3=”hydrogen and oxygen” option4=”water vapour” correct=”option4″]

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When pure water boils vigorously, the process involves the phase transition of liquid water into gaseous water.
Boiling occurs when the vapour pressure of the liquid equals the surrounding atmospheric pressure, and the liquid turns into gas within the bulk of the liquid as well as at the surface. The bubbles formed during boiling are filled with the gaseous form of the liquid, which in this case is water vapour.
Air might be present as dissolved gas in water, and tiny air bubbles might be released upon heating before boiling, but the large, vigorous bubbles during boiling are predominantly water vapour. Water molecules (H₂O) decompose into hydrogen and oxygen only under extreme conditions (like electrolysis), not during simple boiling.

260. How many moles of hydrogen atom are present in one mole of Aluminium

How many moles of hydrogen atom are present in one mole of Aluminium hydroxide?

[amp_mcq option1=”One mole” option2=”Two moles” option3=”Three moles” option4=”Four moles” correct=”option3″]

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The chemical formula for Aluminium hydroxide is Al(OH)₃. This formula indicates that one molecule of Aluminium hydroxide contains one atom of Aluminium (Al) and three hydroxide groups (OH). Each hydroxide group (OH) consists of one Oxygen atom (O) and one Hydrogen atom (H). Therefore, one molecule of Al(OH)₃ contains a total of 3 hydrogen atoms (one in each of the three OH groups).
A mole is a unit of amount of substance, equal to Avogadro’s number (approximately 6.022 × 10²³) of elementary entities (atoms, molecules, ions, etc.). If one molecule contains a certain number of atoms of a particular element, then one mole of molecules will contain the same number of moles of those atoms.
In one mole of Al(OH)₃ molecules, there are Avogadro’s number of Al(OH)₃ molecules. Since each molecule has 3 hydrogen atoms, there are 3 × (Avogadro’s number) of hydrogen atoms. This quantity of hydrogen atoms represents 3 moles of hydrogen atoms.
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