Science Exam: 12th Grade Quiz!

1.

Which of the following condition favors the existence of a substance in the solid-state?

A.

High temperature
B.

Low temperature
C.

High thermal energy
D.

Weak cohesive forces

Correct Answer
B. Low temperature

Explanation
A low temperature favors the existence of a substance in the solid-state because at lower temperatures, the kinetic energy of the particles decreases, causing them to move slower and have less energy to overcome the attractive forces between them. This results in a more ordered and tightly packed arrangement of particles, leading to the solid-state. On the other hand, high temperatures provide more thermal energy to the particles, enabling them to overcome the cohesive forces and transition into a liquid or gaseous state. Weak cohesive forces alone do not determine the existence of a substance in the solid-state, as other factors like temperature and thermal energy also play a significant role.

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2.

Which of the following is not a characteristic of a crystalline solid?

A.

Definite and characteristic heat of fusion
B.

Isotropic nature
C.

A regular periodically repeated pattern of arrangement of constituent particles in the entire crystal
D.

A true solid

Correct Answer
B. Isotropic nature

Explanation
Isotropic nature refers to the property of a material being uniform in all directions. Crystalline solids, on the other hand, have a regular and repeating pattern of arrangement of constituent particles. This means that their properties, including their physical properties like melting point and heat of fusion, are not the same in all directions. Therefore, isotropic nature is not a characteristic of a crystalline solid.

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3.

Which of the following is an amorphous solid?

A.

Graphite
B.

Quartz glass
C.

Chrome alum
D.

Silicon carbide

Correct Answer
B. Quartz glass

Explanation
Quartz glass is an amorphous solid because it lacks a regular crystal structure. Unlike crystalline solids, which have a repeating pattern of atoms, amorphous solids have a disordered arrangement of atoms. Quartz glass is made by melting pure quartz and then rapidly cooling it, preventing the atoms from forming a regular crystal lattice. This results in a solid with a random arrangement of atoms, giving it its amorphous nature.

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4.

Which of the following is true about the value of the refractive index of quartz glass?

A.

Same in all directions
B.

Different in different directions
C.

Cannot be measured
D.

Always zero

Correct Answer
A. Same in all directions

Explanation
The correct answer is "Same in all directions." Refractive index is a property of a material that describes how light bends when it passes through it. In the case of quartz glass, the refractive index is the same in all directions, meaning that light will bend at the same angle regardless of the direction it is traveling through the glass. This property makes quartz glass useful in applications where consistent and predictable refraction is required.

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5.

Which of the following statement is not true about amorphous solids?

A.

On heating they may become crystalline at certain temperature
B.

They may become crystalline on keeping for a long time
C.

Amorphous solids can be moulded by heating
D.

They are anisotropic in nature

Correct Answer
D. They are anisotropic in nature

Explanation
Amorphous solids are characterized by a lack of long-range order in their atomic or molecular structure. Unlike crystalline solids, they do not have a definite shape or a specific melting point. When heated, amorphous solids may become crystalline at a certain temperature, which contradicts the statement that they are anisotropic in nature. Anisotropy refers to the property of having different physical properties in different directions, whereas amorphous solids have isotropic properties because of their disordered structure. Therefore, the statement that they are anisotropic is not true about amorphous solids.

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6.

The sharp melting point of crystalline solids is due to

A.

A regular arrangement of constituent particles observed over a short distance in the crystal lattice
B.

A regular arrangement of constituent particles observed over a long distance in the crystal lattice
C.

Same arrangement of constituent particles in different directions
D.

Different arrangement of constituent particles in different directions

Correct Answer
B. A regular arrangement of constituent particles observed over a long distance in the crystal lattice

Explanation
The sharp melting point of crystalline solids is due to a regular arrangement of constituent particles observed over a long distance in the crystal lattice. This regular arrangement allows for strong intermolecular forces between the particles, resulting in a high melting point. When heat is applied, the regular arrangement is disrupted, causing the solid to transition into a liquid state. The sharp melting point occurs because the disruption of the regular arrangement happens abruptly at a specific temperature, leading to a distinct melting point.

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7.

Iodine molecules are held in the crystals lattice by:

A.

London forces
B.

Dipole-dipole interactions
C.

Covalent bonds
D.

Coulombic forces

Correct Answer
A. London forces

Explanation
Iodine molecules are held in the crystal lattice by London forces. London forces, also known as van der Waals forces or dispersion forces, are weak intermolecular forces that result from temporary fluctuations in electron distribution. These forces occur between all molecules, including non-polar molecules like iodine. In the crystal lattice, the iodine molecules are attracted to each other through these temporary fluctuations, creating a weak bond that holds the lattice together.

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8.

Which of the following is a network solid?

A.

S0₂
B.

I₂
C.

Diamond
D.

H₂O

Correct Answer
C. Diamond

Explanation
Diamond is a network solid because it has a three-dimensional network of covalent bonds. In a network solid, each atom is covalently bonded to its neighboring atoms, forming a continuous network throughout the entire solid. This results in a very strong and rigid structure, giving diamond its hardness and high melting point. In contrast, SO2, I2, and H2O are not network solids as they consist of discrete molecules held together by intermolecular forces.

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9.

The total number of tetrahedral voids in the face-centered unit cell is:

A.

6
B.

10
C.

8
D.

12

Correct Answer
B. 10

Explanation
In a face-centered unit cell, there are 4 tetrahedral voids associated with each corner atom and 2 tetrahedral voids associated with each face-centered atom. Since there are 8 corner atoms and 6 face-centered atoms in a face-centered unit cell, the total number of tetrahedral voids can be calculated as 4 * 8 + 2 * 6 = 32 + 12 = 44. However, each tetrahedral void is shared by 4 adjacent unit cells, so we need to divide the total by 4. Therefore, the correct answer is 44/4 = 11. Since the options provided are 6, 10, 8, and 12, the correct answer is 10.

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10.

Which of the following point defects are shown by AgBr (s) crystals? (A) Schottky defect (B) Frenkel defect (C) Metal excess defect (D) Metal deficiency defect

A.

A and B
B.

C and D
C.

A and C
D.

B and D

Correct Answer
A. A and B

Explanation
AgBr (silver bromide) crystals show both Schottky defect and Frenkel defect. Schottky defect occurs when some cations and anions are missing from their lattice positions, creating vacancies. Frenkel defect occurs when an ion is displaced from its lattice position and occupies an interstitial site. Therefore, both A (Schottky defect) and B (Frenkel defect) are correct answers.

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11.

The percentage of empty space in a body-centered cubic arrangement is:

A.

74
B.

68
C.

32
D.

26

Correct Answer
C. 32

Explanation
In a body-centered cubic arrangement, each corner of the cube is occupied by an atom, and there is one atom in the center of the cube. This arrangement leaves 32% of the space empty, as the atoms only occupy 68% of the total volume.

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