Physical Properties of Materials Archives

31. Which one of the following schematic graphs correctly represents quali

Which one of the following schematic graphs correctly represents qualitatively the variation of resistivity ρ with respect to temperature T for a semiconductor?

Graph (a)

Graph (b)

Graph (c)

Graph (d)

This question was previously asked in

UPSC Geoscientist – 2023

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The correct answer is (B) Graph (b).

For a semiconductor, resistivity generally decreases as temperature increases. This is because increasing thermal energy excites more electrons into the conduction band and creates more holes in the valence band, increasing the number of free charge carriers available for conduction. More charge carriers lead to higher conductivity and thus lower resistivity. Graph (b) qualitatively shows this inverse relationship where resistivity decreases with increasing temperature.

In contrast, for a typical metal, resistivity increases with increasing temperature due to increased scattering of electrons by lattice vibrations. Insulators have very high resistivity that changes significantly with temperature, usually decreasing as temperature increases, but starting from a much higher value than semiconductors. Graph (a) represents the behavior of a metal, while graphs (c) and (d) do not represent the typical qualitative behavior of a semiconductor over a relevant temperature range.

32. What happens to a bar magnet when it is heated?

What happens to a bar magnet when it is heated?

Its polarity reverses

Its magnetism is increased

Its magnetism remains unchanged

Its magnetism is either reduced or lost

This question was previously asked in

UPSC Geoscientist – 2022

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When a bar magnet is heated, the increased thermal energy causes the atoms within the magnetic material to vibrate more vigorously. This increased vibration disrupts the alignment of magnetic domains, leading to a reduction or complete loss of magnetism.

– There is a specific temperature, called the Curie temperature, above which a ferromagnetic material loses its ferromagnetism and becomes paramagnetic.
– Heating below the Curie temperature can still weaken the magnet’s field, especially if the magnet is not uniformly heated or has structural imperfections.

Heating is one way to demagnetize a magnet. Other methods include hammering or exposing it to a strong alternating magnetic field (as in a degausser). The loss of magnetism is due to the disorganization of the microscopic magnetic moments within the material.

33. The magnetization in ferromagnetic materials

The magnetization in ferromagnetic materials

can never vanish

is always opposite to the direction of applied magnetic field

is always in the direction perpendicular to the applied magnetic field

may not vanish even when the applied magnetic field is reduced to zero

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UPSC Geoscientist – 2022

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Ferromagnetic materials exhibit hysteresis. When a magnetic field is applied, they become strongly magnetized. When the applied magnetic field is removed, a significant amount of magnetization often remains, even if the external field is reduced to zero. This retained magnetization is called remanent magnetization or remanence. This property allows ferromagnetic materials to be used to create permanent magnets.

– Ferromagnetic materials show hysteresis in their magnetization curve.
– Remanence is the residual magnetization left after the applied field is removed.
– This property is essential for creating permanent magnets.

Magnetization in ferromagnetic materials is caused by the alignment of magnetic domains. Applied fields cause domains oriented with the field to grow and those opposed to shrink. Even after the external field is removed, some of this alignment persists due to the strong internal interactions between domains.

34. Which one of the following has the least density at room temperature?

Which one of the following has the least density at room temperature?

Milk

Acetone

Red soil

Iron

This question was previously asked in

UPSC Geoscientist – 2022

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Among the given options, Acetone has the least density at room temperature.

– Approximate densities at room temperature:
– Milk: Approx. 1.03 g/cm³ (slightly denser than water).
– Acetone: Approx. 0.79 g/cm³ (a lightweight organic solvent).
– Red soil: Bulk density varies greatly but is typically in the range of 1.2 to 1.6 g/cm³ or higher, depending on composition and compaction. Even the solid mineral components have densities around 2.6-2.7 g/cm³.
– Iron: Approx. 7.87 g/cm³ (a dense metal).
– Comparing these values, Acetone (0.79 g/cm³) has the lowest density.

– Density is defined as mass per unit volume.
– Different substances have different densities due to the mass of their constituent particles and how closely they are packed.
– Solids are generally denser than liquids, which are generally denser than gases, though there are exceptions (e.g., ice is less dense than water).

35. Which one of the following is the correct sequence of substances havin

Which one of the following is the correct sequence of substances having densities in decreasing order?

Iron, Water, Honey, Air

Iron, Honey, Water, Air

Iron, Honey, Air, Water

Water, Iron, Air, Honey

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UPSC Geoscientist – 2020

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The correct sequence of substances in decreasing order of density is Iron, Honey, Water, Air.

Density is defined as mass per unit volume. Generally, solids are denser than liquids, and liquids are denser than gases. Among liquids, denser substances have more mass packed into the same volume.

Typical densities are: Iron (~7.87 g/cm³), Honey (~1.42 g/cm³), Water (~1.00 g/cm³), Air (~0.0012 g/cm³ at sea level and 15°C). This confirms the order: Iron > Honey > Water > Air.

36. When water is heated from 0°C to 4°C, its density

When water is heated from 0°C to 4°C, its density

remains constant.

increases.

decreases.

first increases then decreases to its original value.

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UPSC CDS-2 – 2022

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The correct answer is B) increases.

Water exhibits anomalous expansion between 0°C and 4°C. Unlike most substances whose density decreases upon heating (as volume increases), water’s volume *decreases* when heated from 0°C to 4°C. Density is mass per unit volume (ρ = m/V). Since the mass remains constant and the volume decreases in this temperature range, the density of water increases.

The density of water is maximum at 4°C (approximately 1000 kg/m³). Above 4°C, water behaves normally; its volume increases with temperature, and thus its density decreases. This peculiar property of water is crucial for aquatic life in cold climates, as lakes and rivers freeze from the top down, allowing aquatic organisms to survive in the denser, warmer water at the bottom.

37. Which one of the following is used for storing biological tissues?

Which one of the following is used for storing biological tissues?

Liquid nitrogen

Liquid helium

Liquid argon

Liquid bromine

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UPSC CDS-2 – 2020

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Liquid nitrogen is commonly used for cryopreservation, which is the process of preserving biological tissues, cells, and organs at very low temperatures.

Liquid nitrogen boils at -196°C (-320°F). This extremely low temperature effectively halts biological activity, including enzymatic reactions and cellular degradation, allowing long-term storage of samples such as sperm, eggs, embryos, blood cells, tissue biopsies, and even whole organs for transplantation in some cases.

Liquid helium boils at an even lower temperature (-269°C) but is more expensive and typically used for specialized scientific applications like cooling superconducting magnets. Liquid argon (-186°C) has some industrial uses but is not standard for biological cryopreservation. Bromine is a liquid at room temperature and is highly toxic and corrosive, not suitable for storage.

38. Consider the following statements: Particles of matter intermix on t

Consider the following statements:

Particles of matter intermix on their own.
Particles of matter have force acting between them.

Which of the statements given above is/are correct ?

1 only

2 only

Both 1 and 2

Neither 1 nor 2

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UPSC CDS-2 – 2019

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Both statements about particles of matter are correct.
Statement 1: Particles of matter intermix on their own due to their constant motion. This phenomenon is known as diffusion and is responsible for mixing substances when brought into contact, e.g., sugar dissolving in water, or smells spreading in the air.
Statement 2: Particles of matter have forces acting between them. These are intermolecular forces of attraction (like Van der Waals forces) and repulsion, which hold the particles together in solids and liquids, and are weaker in gases. These forces influence the physical state and properties of matter.

Matter is composed of particles that are in constant motion (at temperatures above absolute zero) and interact with each other through forces.

The strength of intermolecular forces and the kinetic energy of particles determine the state of matter (solid, liquid, gas). Diffusion rates depend on factors like temperature, concentration gradient, and the state of matter.

39. Bose-Einstein Condensate is

Bose-Einstein Condensate is

solid state of matter

fifth state of matter

plasma

state of condensed matter

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UPSC CDS-2 – 2019

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Bose-Einstein Condensate (BEC) is considered a fifth state of matter, distinct from the common states of solid, liquid, gas, and plasma. It is formed when a gas of bosons is cooled to temperatures very near absolute zero, causing the particles to condense into the lowest quantum state, behaving as a single quantum mechanical entity.

Bose-Einstein Condensate represents a state of matter exhibiting unique quantum properties at extremely low temperatures.

The concept of BEC was predicted by Satyendra Nath Bose and Albert Einstein in the 1920s and first experimentally created in 1995 by Eric Cornell, Carl Wieman, and Wolfgang Ketterle, who shared the Nobel Prize in Physics in 2001 for this achievement. Plasma is often referred to as the fourth state of matter and exists at very high temperatures.

40. If two miscible liquids of same volume but different densities P 1 an

If two miscible liquids of same volume but different densities P₁ and P₂ are mixed, then the density of the mixture is given by

(P1 + P2) / 2

2P1P2 / (P1 + P2)

2P1P2 / (P1 - P2)

P1P2 / (P1 + P2)

This question was previously asked in

UPSC CDS-2 – 2018

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Let V be the volume of each liquid. The total volume of the mixture is V_total = V + V = 2V.
The mass of the first liquid is m₁ = density × volume = P₁V.
The mass of the second liquid is m₂ = density × volume = P₂V.
The total mass of the mixture is m_total = m₁ + m₂ = P₁V + P₂V = (P₁ + P₂)V.
The density of the mixture is ρ_mixture = m_total / V_total = (P₁ + P₂)V / (2V) = (P₁ + P₂) / 2.
This formula is valid when equal volumes of two miscible liquids are mixed.

– Density is mass per unit volume.
– When mixing, total mass is the sum of individual masses, and total volume is the sum of individual volumes (assuming no volume change upon mixing, which is typical for miscible liquids unless otherwise specified).
– The calculation is based on the given condition that the liquids have the ‘same volume’.

If two miscible liquids of *same mass* m but different densities P₁ and P₂ are mixed, the volume of the first liquid is V₁ = m/P₁, and the volume of the second liquid is V₂ = m/P₂. The total mass is 2m, and the total volume is V₁ + V₂ = m/P₁ + m/P₂ = m(P₁ + P₂)/(P₁P₂). The density of the mixture would be (2m) / [m(P₁ + P₂)/(P₁P₂)] = 2P₁P₂ / (P₁ + P₂), which is option B. The question specifies ‘same volume’, not ‘same mass’.