Free Crystalline 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. Two different compounds that have the same crystalline form and analogous chemical formulae are said to be

• A. polymorphic
• B. allotropic
• C. isomorphic
• D. anisotropic

Explanation: Isomorphous substances such as potassium sulphate and potassium chromate crystallise in the same shape because their ions are of similar size and charge, and they can even form mixed crystals. Polymorphism is the opposite situation, one substance existing in more than one crystal form, as calcium carbonate does as calcite and aragonite. Allotropy is polymorphism restricted to elements.

5. Graphite conducts electricity while diamond does not because in graphite

• A. each carbon forms four single covalent bonds
• B. the carbon atoms are ionised
• C. the layers are held by strong covalent bonds
• D. each carbon uses only three of its four valence electrons in bonding, leaving one delocalised

Explanation: In graphite each carbon is bonded to three others in a flat hexagonal sheet, and the fourth electron is delocalised between the layers, so it can carry current along the sheets. In diamond all four electrons are locked into single bonds, leaving none free. The weak forces between graphite layers also let them slide, which is why graphite is used as a lubricant and in pencils.

6. The melting point of a solid is a measure of

• A. the mass of its particles
• B. the strength of the forces holding its particles together
• C. the size of its crystals
• D. its electrical conductivity

Explanation: Melting requires enough thermal energy to overcome the forces locking the particles in the lattice, so covalent network and ionic solids melt very high, metals vary widely and molecular solids melt low. Comparing melting points is therefore a quick way to infer the type of bonding in an unknown solid. Crystal size affects appearance but not the temperature at which melting occurs.

7. Which one shows anisotropic behavior?

• A. Wood
• B. Gemstone
• C. Coke
• D. Graphite

Explanation: This is the option marked correct in the official UHS answer key. Graphite is crystalline with a strongly layered structure, so properties such as electrical conductivity differ enormously along the layers compared with across them, which is the essence of anisotropy. Coke is amorphous and therefore isotropic. Note that gemstones are crystalline and anisotropic too, and wood is anisotropic in the everyday sense of grain, so this question rests on the crystalline definition.

8. Pick the crystalline solid:

• A. Cement
• B. Ceramics
• C. Concrete
• D. Copper

Explanation: Copper is a metal whose atoms sit in a regular repeating lattice, so it is crystalline and melts at one sharp temperature. Cement, ceramics and concrete are amorphous or mixed materials with only short range order, so they soften over a range rather than melting cleanly. A sharp melting point is the practical test for crystallinity.