| Directions: (Q. 6 to 10) |
| Case II: Read the passage given below and answer the following questions. |
| In an assembly of atoms or molecules, a solid phase is formed whenever the interatomic attractive forces significantly exceed the disruptive thermal forces and thus restrict the mobility of atoms, forcing them into more or less fixed positions. From energy considerations, it is evident that in such solids the atoms or molecules will always attempt to assume highly ordered structures which are characterised by symmetry. Depending on the nature of the active interatomic forces, all solids may be subdivided into the following categories: |
| Ionic solids: These solids consist of positively and negatively charged ions arranged in a regular fashion throughout the solid. These solids are very hard and brittle, have very high melting points and have high enthalpies of vaporisation, e.g., NaCl, MgO, KCl, LiCl, etc. |
| Covalent solids: In these solids, the constituent particles are atoms which are linked together by a continuous system of covalent bonds. These bonds are strong and directional in nature. The covalent crystals are hard, have high melting points, are poor conductors of electricity. Diamond is a typical example of covalent solids. |
| Metallic solids: In these solids, the constituent particles are positive ions immersed in a sea of mobile electrons. Metallic solids may be hard as well as soft. They are good conductors of heat and electricity e.g., common metals such as nickel, copper and alloys. |
| Molecular solids: In these the constituent particles are molecules. The molecules are held together by dispersion forces or London forces, dipole-dipole forces or hydrogen bonds. |
| In the following questions (Q. No. 6-10), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage |
| Assertion: Molecular solids are characterized by low melting point. |
| Reason: Molecular solids are made up of covalent molecules. |
A) Assertion and reason both are correct statements and reason is correct explanation for assertion.
B) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C) Assertion is correct statement but reason is wrong statement.
D) Assertion is wrong statement but reason is correct statement.
Correct Answer: B
Solution :
In molecular solids, molecules are held together by weak forces of attractions.
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