All Free Chemistry MCQs with Answers

81. Which pair of liquids is most likely to be completely miscible?

• A. Water and petrol
• B. Water and ethanol
• C. Oil and water
• D. Mercury and water

Explanation: Ethanol has a hydroxyl group that hydrogen bonds with water, so the two mix in all proportions, illustrating the rule that like dissolves like. Petrol and oil are non polar hydrocarbons that cannot form such interactions with water and therefore separate into layers. Mercury is a liquid metal held by metallic bonding and is immiscible with almost everything.

82. The meniscus of mercury in a glass tube is convex because

• A. mercury is a metal
• B. mercury evaporates rapidly
• C. cohesive forces between mercury atoms exceed the adhesive forces to glass
• D. glass is polar and mercury is non polar

Explanation: Mercury atoms are held to each other by strong metallic bonding far more strongly than they are attracted to the glass, so the liquid pulls away from the walls and the surface bulges upwards. Water shows a concave meniscus for the opposite reason. When reading a burette or a thermometer the eye must be level with the bottom of a concave meniscus and the top of a convex one.

83. A pressure cooker cooks food faster because increasing the pressure inside

• A. lowers the boiling point of water
• B. removes all the air from the vessel
• C. increases the amount of steam produced
• D. raises the boiling point of water above 100 degrees Celsius

Explanation: A liquid boils when its vapour pressure equals the external pressure, so raising that external pressure means a higher temperature is needed, and water inside a pressure cooker can reach about 120 degrees Celsius. The food cooks faster because the reaction rate rises with temperature, not because there is more steam. At high altitude the reverse problem arises and cooking takes longer.

84. Which statement about liquids is INCORRECT?

• A. Liquids diffuse more slowly than gases
• B. Liquids are almost incompressible
• C. The molecules of a liquid are held in fixed positions
• D. Liquids have a definite volume but no definite shape

Explanation: Molecules in a liquid are in constant random motion and slide freely past one another, which is exactly what allows a liquid to flow and to diffuse, however slowly. Fixed positions belong to the solid state. The other three statements are all correct, and incompressibility follows from the molecules already being nearly in contact.

85. 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.

86. 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.

87. 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.

88. 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.

89. 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.

90. 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.