Free Solids MCQs with Answers

1. A crystalline solid differs from an amorphous solid in that a crystalline solid

• A. has a long range ordered arrangement of particles and a sharp melting point
• B. has no definite shape
• C. softens gradually over a range of temperature
• D. is always transparent

Explanation: In a crystal every particle occupies a regular repeating position, so all the bonds are identical and break at one definite temperature, giving a sharp melting point. An amorphous solid such as glass or rubber has only short range order and therefore softens over a range instead. Glass is sometimes called a supercooled liquid for exactly this reason.

2. The property by which a crystalline solid shows different values of a physical property such as refractive index in different directions is called

• A. isomorphism
• B. anisotropy
• C. polymorphism
• D. allotropy

Explanation: Because the arrangement of particles differs along different axes of a crystal, properties measured along those axes differ too, and this direction dependence is anisotropy. Amorphous solids and liquids are isotropic, giving the same value in every direction. Polymorphism is the existence of one compound in more than one crystalline form, and allotropy is the same idea applied to an element.

3. Diamond and graphite are both made only of carbon atoms but have very different properties. They are

• A. isomers
• B. isotopes
• C. allotropes
• D. isomorphs

Explanation: Allotropes are different structural forms of the same element in the same physical state, and here diamond is a rigid three dimensional network while graphite is made of layers with delocalised electrons between them. That is why diamond is the hardest natural substance and an insulator, while graphite is soft, slippery and conducts electricity. Isotopes differ in neutron number, not in bonding.

4. The unit cell of sodium chloride is

• A. simple cubic
• B. hexagonal
• C. body centred cubic
• D. face centred cubic

Explanation: Sodium chloride crystallises in a face centred cubic arrangement in which each sodium ion is surrounded octahedrally by six chloride ions and each chloride by six sodium ions, giving a coordination number of six for both. Caesium chloride, whose larger cation allows eight neighbours, adopts a body centred arrangement instead. The unit cell is simply the smallest repeating block from which the whole lattice can be built.

5. The coordination number of an ion in a crystal lattice is

• A. the number of oppositely charged ions immediately surrounding it
• B. the charge on the ion
• C. the number of unit cells it belongs to
• D. the number of electrons it has lost or gained

Explanation: Coordination number counts the nearest neighbours of opposite charge, so it is six in sodium chloride and eight in caesium chloride, the difference arising from the relative sizes of the ions. A larger cation can accommodate more anions around it before they begin to touch each other. The number of electrons transferred determines the charge, which is a separate quantity.

6. Ionic crystals such as sodium chloride are hard and have high melting points because

• A. their ions are held by weak van der Waals forces
• B. strong electrostatic forces act between oppositely charged ions throughout the lattice
• C. they contain covalent bonds between the ions
• D. the ions are free to move at room temperature

Explanation: Every ion attracts all the oppositely charged ions around it, and a great deal of energy is needed to break this extended three dimensional network, so the melting point of sodium chloride is 801 degrees Celsius. The ions are locked in place in the solid, which is why solid sodium chloride does not conduct electricity while the molten salt and its solution do.

7. Molecular crystals such as solid iodine have low melting points because

• A. the covalent bonds within the molecules are weak
• B. they contain no bonds at all
• C. only weak intermolecular forces have to be overcome on melting
• D. their molecules are very heavy

Explanation: Melting separates molecules from one another, and in a molecular crystal they are held only by dispersion forces or dipole interactions, which are far weaker than ionic or covalent bonds. The strong covalent bonds inside each iodine molecule are not broken at all when the solid melts, which is the distinction the question turns on. This is also why such solids are soft and often volatile.

8. Which of the following is a covalent network solid?

• A. Ice
• B. Sodium chloride
• C. Solid carbon dioxide
• D. Diamond

Explanation: In diamond every carbon is covalently bonded to four others in a continuous three dimensional network, so melting it means breaking covalent bonds and the melting point is above 3500 degrees Celsius. Ice and dry ice are molecular solids held by hydrogen bonds and dispersion forces respectively, and sodium chloride is ionic. Silicon dioxide and silicon carbide are the other standard network examples.

9. The smallest repeating portion of a crystal lattice that shows the full symmetry of the crystal is called the

• A. unit cell
• B. crystal face
• C. lattice point
• D. domain

Explanation: Stacking the unit cell repeatedly in three dimensions generates the entire crystal, so knowing its dimensions and contents describes the whole structure. A lattice point is a single position within that cell rather than the repeating block itself. The seven crystal systems are classified by the shapes and angles the unit cell can take.

10. The number of crystal systems into which all crystals are classified is

• A. four
• B. seven
• C. fourteen
• D. twenty

Explanation: There are seven crystal systems, defined by the relative lengths of the unit cell axes and the angles between them, the cubic system being the most symmetrical. These give rise to fourteen Bravais lattices when the different centring arrangements are counted, which is why fourteen is offered as a distractor. Sodium chloride belongs to the cubic system.