UPSC NDA-2 Archives

31. The raw materials used for the manufacture of Portland cement are

The raw materials used for the manufacture of Portland cement are

Lime, silica and sulphur dioxide

Lime, silica and carbon dioxide

Lime, silica and alumina

Lime, silica and boric acid

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The primary raw materials used for the manufacture of Portland cement clinker are calcium oxide (usually derived from limestone or chalk), silicon dioxide (silica, typically from clay or sand), and aluminium oxide (alumina, from clay or bauxite). These materials are mixed in specific proportions, heated in a kiln to high temperatures (calcination and clinkering), and then ground to produce cement. Option C lists lime, silica, and alumina, which are the essential components derived from these raw materials.

– The main chemical components of Portland cement are calcium silicates, calcium aluminates, and calcium ferroaluminates.
– These compounds are formed by heating a mixture of calcareous material (like limestone, source of CaO) and argillaceous material (like clay, source of SiO2 and Al2O3) in a kiln.
– Minor components like iron oxide and gypsum (added during grinding) are also involved but lime, silica, and alumina are the core raw material oxides.

– Limestone (CaCO3) is heated to produce lime (CaO) and carbon dioxide (CO2).
– Clay provides silica (SiO2), alumina (Al2O3), and iron(III) oxide (Fe2O3).
– Sulphur dioxide and boric acid are not standard raw materials for Portland cement. Carbon dioxide is a byproduct of the calcination process, not a raw material that is added.

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32. Which one of the following statements about diamond and graphite is no

Which one of the following statements about diamond and graphite is not correct ?

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Diamond has a tetrahedral structure, whereas graphite has a hexagonal planar structure.

Both physical and chemical properties of diamond and graphite are different.

Graphite is soft but diamond is hard.

Graphite is a good conductor of electricity while diamond is not.

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Statement B, “Both physical and chemical properties of diamond and graphite are different”, is arguably the least accurate or potentially misleading statement compared to the others, which are clear factual distinctions. While their physical properties are vastly different, their chemical properties, stemming from being elemental carbon, share some fundamental similarities (e.g., both combust in oxygen to form CO2, although under different conditions and rates). The difference in chemical reactivity is significant due to their structures, but claiming *all* chemical properties are different might be considered an overstatement in some contexts. Statements A, C, and D are unequivocally correct and describe key differences between diamond and graphite. In competitive exams, if multiple statements are true, the question might hinge on identifying the statement that is not *completely* true or contains a subtle inaccuracy or overgeneralization.

– Diamond and graphite are allotropes of carbon with distinct crystal structures (tetrahedral for diamond, hexagonal planar layers for graphite).
– These structural differences lead to significant differences in physical properties like hardness, density, electrical conductivity, and thermal conductivity.
– While their reactivity differs significantly (graphite is more reactive than diamond), they are both carbon and participate in similar fundamental chemical reactions like combustion.

– Diamond is an electrical insulator and the hardest known natural material.
– Graphite is a good electrical conductor and is soft, used as a lubricant.
– The difference in properties arises from the difference in bonding and structure: sp3 hybridization in diamond forms a rigid 3D network, while sp2 hybridization in graphite forms layers held together by weak van der Waals forces.
– Statement B is considered “not correct” likely because the term “different” might be interpreted as “completely distinct with no commonalities”, which isn’t strictly true for chemical properties as they are both forms of carbon.

33. Which one of the following statements is correct?

Which one of the following statements is correct?

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Both boiling and evaporation are surface phenomena.

Boiling is a surface phenomenon, but evaporation is a bulk phenomenon.

Both boiling and evaporation are bulk phenomena.

Boiling is a bulk phenomenon, but evaporation is a surface phenomenon.

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The correct option is D) Boiling is a bulk phenomenon, but evaporation is a surface phenomenon.

Evaporation is the process where molecules at the surface of a liquid gain enough kinetic energy to overcome the intermolecular forces and escape into the gaseous phase. This occurs at any temperature below the boiling point and only happens at the liquid’s surface.
Boiling is the process where a liquid turns into a vapor when its vapor pressure equals the surrounding atmospheric pressure. This occurs at a specific temperature (the boiling point) and involves the formation of vapor bubbles *throughout* the volume (bulk) of the liquid, which then rise to the surface. It is a phenomenon that occurs throughout the bulk of the liquid, not just at the surface.

Think of boiling water: bubbles form at the bottom and rise. Think of a puddle drying up: the surface reduces as water evaporates. Sublimation is another phase transition, from solid directly to gas, and it is typically a surface phenomenon. Melting is a bulk phenomenon.

34. Which one of the following statements about glass is not correct?

Which one of the following statements about glass is not correct?

Glass is often said to be a supercooled liquid.

Glass has no definite melting point.

Soda glass is harder than pyrex glass.

Boron is present in pyrex glass.

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The correct option is C) Soda glass is harder than pyrex glass. This statement is incorrect. Pyrex glass (borosilicate glass) is generally harder and more durable than soda-lime glass (soda glass).

A) Glass is an amorphous solid, lacking a long-range ordered crystalline structure. Its structure is similar to a liquid’s, and it becomes increasingly viscous as it cools, without a sharp transition like freezing. Hence, it is often described as a supercooled liquid or an amorphous solid. This statement is generally considered correct.
B) Due to its amorphous nature, glass does not have a specific melting point. It softens gradually over a range of temperatures. This statement is correct.
C) Soda-lime glass (soda glass) is the most common type of glass (window panes, bottles). Pyrex is a brand of borosilicate glass, which contains boron oxide (B₂O₃) in addition to silica. The addition of boron oxide significantly changes the properties, making it more resistant to thermal shock and chemical corrosion, and generally harder and more resistant to scratches compared to soda-lime glass. Therefore, soda glass is *not* harder than pyrex glass; it is softer.
D) Pyrex glass is a type of borosilicate glass, meaning it contains boron (specifically in the form of boron oxide). This statement is correct.

The composition differences between soda-lime glass and borosilicate glass lead to their different properties. Soda-lime glass typically contains silicon dioxide (silica, ~70%), sodium oxide (soda), and calcium oxide (lime). Borosilicate glass typically contains silica (~80%), boron oxide (~13%), and smaller amounts of sodium oxide and aluminium oxide. The higher silica and boron content contribute to the superior properties of borosilicate glass.

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35. Which one of the following compounds does not exhibit a different oxid

Which one of the following compounds does not exhibit a different oxidation number of the same element?

Pb₃O₄

Fe₃O₄

Fe₂O₃

Mn₃O₄

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The correct option is C) Fe₂O₃. In Fe₂O₃, iron exists in only one oxidation state.

We need to determine the oxidation state of the metal element in each compound:
A) Pb₃O₄ is a mixed oxide, often written as 2PbO·PbO₂. In PbO, O is -2, so Pb is +2. In PbO₂, O is -2 (total -4 for 2 O atoms), so Pb is +4. Lead has oxidation states +2 and +4 in Pb₃O₄.
B) Fe₃O₄ is a mixed oxide, FeO·Fe₂O₃. In FeO, O is -2, so Fe is +2. In Fe₂O₃, O is -2 (total -6 for 3 O atoms), so 2 Fe atoms are +6, meaning each Fe is +3. Iron has oxidation states +2 and +3 in Fe₃O₄.
C) Fe₂O₃: Assuming oxygen is -2, the total charge from 3 oxygen atoms is 3 * (-2) = -6. For the compound to be neutral, the total charge from 2 iron atoms must be +6. Thus, the oxidation state of each Fe atom is +6 / 2 = +3. All iron atoms in Fe₂O₃ are in the +3 oxidation state.
D) Mn₃O₄ is a mixed oxide, MnO·Mn₂O₃. In MnO, O is -2, so Mn is +2. In Mn₂O₃, O is -2 (total -6 for 3 O atoms), so 2 Mn atoms are +6, meaning each Mn is +3. Manganese has oxidation states +2 and +3 in Mn₃O₄.

Mixed oxides like Pb₃O₄, Fe₃O₄, and Mn₃O₄ contain the metal element in more than one oxidation state combined within the same compound structure. Simple binary oxides like Fe₂O₃ (or FeO, FeO₂, etc.) typically have the metal in a single oxidation state throughout the compound.

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36. When a ball bounces off the ground, which of the following changes sud

When a ball bounces off the ground, which of the following changes suddenly ?
(Assume no loss of energy to the floor)

Its speed

Its momentum

Its kinetic energy

Its potential energy

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The correct option is B) Its momentum. When a ball bounces off the ground, its velocity changes direction suddenly, leading to a sudden change in its momentum.

Velocity is a vector quantity, having both magnitude (speed) and direction. Momentum is mass multiplied by velocity (p = mv), so it is also a vector quantity. When a ball bounces, its downward velocity just before impact changes to an upward velocity just after impact. Even if the speed remains the same (in a perfectly elastic collision with no energy loss), the change in direction causes a significant change in the velocity vector, and thus a sudden change in the momentum vector.

Speed is the magnitude of velocity. In an idealized elastic collision with no energy loss to the floor, the speed of the ball remains the same before and after the bounce. Kinetic energy (KE = 1/2 mv²) depends on the square of the speed, so it also remains the same if there’s no energy loss. Potential energy (PE = mgh) changes gradually with height and is at a minimum (or zero) at the point of bounce; it does not change suddenly during the bounce event itself.

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37. Which one of the following statements regarding Ohm’s law is not cor

Which one of the following statements regarding Ohm’s law is not correct ?

Ohm's law is an assumption that current through a conductor is always directly proportional to the potential difference applied to it.

A conducting device obeys Ohm's law when the resistance of a device is independent of magnitude and polarity of applied potential difference.

A conducting material obeys Ohm's law when the resistance of material is independent of the magnitude and direction of applied electric field.

All homogeneous materials obey Ohm's law irrespective of whether the field is within range or strong.

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The correct option is D) All homogeneous materials obey Ohm’s law irrespective of whether the field is within range or strong. This statement is incorrect.

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain unchanged (V = IR). A material or device is said to be “ohmic” if its resistance R is constant and independent of the applied voltage/current/electric field over a significant range.
Statements A, B, and C describe aspects of Ohm’s law or materials obeying it. Statement A describes Ohm’s law as the proportionality between V and I (though the “always” can be debated depending on interpretation, compared to D it’s less definitively wrong). Statements B and C correctly define an ohmic device/material as one whose resistance is independent of the magnitude and direction of the applied potential difference/electric field. Statement D is a universal claim that is false. Many homogeneous materials (e.g., semiconductors, diodes, electrolytes) do *not* obey Ohm’s law. Even ohmic materials like metals can deviate from Ohm’s law at very high electric fields or extreme temperatures.

Materials that do not obey Ohm’s law are called non-ohmic materials. Their resistance can vary with voltage, current, field strength, temperature, etc. Examples include diodes, transistors, and gas discharge lamps.

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38. In which of the following phenomena do heat waves travel along a strai

In which of the following phenomena do heat waves travel along a straight line with the speed of light ?

Thermal conduction

Thermal convection

Thermal radiation

Both, thermal conduction and radiation

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The correct option is C) Thermal radiation. Heat transfer by radiation involves electromagnetic waves, which travel at the speed of light.

Heat can be transferred by conduction, convection, and radiation. Conduction is the transfer of heat through direct contact via vibrations of particles. Convection is the transfer of heat through the movement of fluids (liquids or gases). Both conduction and convection require a medium and the speed of heat transfer is much slower than the speed of light. Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation. These waves can travel through a vacuum and propagate at the speed of light (approximately 3 x 10⁸ m/s in vacuum).

Heat from the Sun reaches the Earth primarily through radiation, traveling across the vacuum of space at the speed of light. Conduction is how a metal spoon gets hot when placed in hot soup. Convection is how water heats up in a pot or how air circulates in a room.

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39. Let there be an object having some chemicals in it. It starts moving w

Let there be an object having some chemicals in it. It starts moving with a uniform velocity v and a chemical reaction starts happening. In this case, which of the following statement/s is/are correct ?

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1. Chemical reactions happening in the system cannot change the velocity v of the center of mass of the object.
2. Chemical reactions happening in the system cannot change the kinetic energy of the particles inside with respect to the center of mass of object.

Select the correct answer using the code given below :

1 only

2 only

Both 1 and 2

Neither 1 nor 2

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The correct option is A) 1 only. Statement 1 is correct, while statement 2 is incorrect.

Statement 1: The velocity of the center of mass of a system can only be changed by external forces acting on the system. Chemical reactions happening *within* the object involve internal forces between constituent particles. Internal forces cannot change the motion of the center of mass of the system. Therefore, if the object is moving with uniform velocity (implying zero net external force or balanced external forces), a chemical reaction inside will not change the velocity of the center of mass.
Statement 2: Chemical reactions often involve changes in the internal energy of the system, which can include changes in the kinetic energy of the particles relative to the center of mass (manifesting as temperature changes). For example, an exothermic reaction releases energy, which increases the kinetic energy of the constituent particles. Therefore, chemical reactions *can* change the kinetic energy of the particles inside with respect to the center of mass of the object.

The total momentum of the system remains constant in the absence of external forces, which means the velocity of the center of mass remains constant if mass is also constant. The total energy of the isolated system is conserved, but this energy can be converted between different forms, including internal potential energy (chemical energy) and internal kinetic energy (thermal energy, kinetic energy of particles relative to CM).

40. A thin disc and a thin ring, both have mass M and radius R. Both rotat

A thin disc and a thin ring, both have mass M and radius R. Both rotate about axes through their center of mass and are perpendicular to their surfaces at the same angular velocity. Which of the following is true ?

The ring has higher kinetic energy

The disc has higher kinetic energy

The ring and the disc have the same kinetic energy

Kinetic energies of both the bodies are zero since they are not in linear motion

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The correct option is A) The ring has higher kinetic energy. The rotational kinetic energy is given by KE = (1/2) * I * ω², where I is the moment of inertia and ω is the angular velocity. Since both the ring and the disc have the same mass (M) and radius (R) and rotate at the same angular velocity (ω), the kinetic energy depends on their moments of inertia.

The moment of inertia of a thin ring about an axis through its center and perpendicular to its plane is I_ring = M * R². The moment of inertia of a thin disc about an axis through its center and perpendicular to its plane is I_disc = (1/2) * M * R². Since I_ring > I_disc (M*R² is greater than (1/2)*M*R²), and ω is the same for both, the rotational kinetic energy of the ring is higher than that of the disc (KE_ring = (1/2)*I_ring*ω² > KE_disc = (1/2)*I_disc*ω²).

Moment of inertia represents the resistance of an object to rotational motion; it depends on the mass distribution relative to the axis of rotation. For the same total mass and radius, the ring has mass distributed further from the axis (all at radius R) compared to the disc (mass distributed from center to R), resulting in a higher moment of inertia for the ring. Kinetic energy of a body rotating is non-zero unless its angular velocity is zero, so option D is incorrect.

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