Chemistry

Molality is defined as moles of solute per kilogram of solvent and does not change with temperature because mass remains constant. In contrast, molarity, normality, and formality depend on volume, which varies with temperature.

The statement that each electron in an atom must have its own unique set of quantum numbers is the Pauli exclusion principle, which prohibits two electrons from having identical quantum states.

Laughing gas is the common name for nitrous oxide (N2O), which is used as an anesthetic and analgesic. The other options are different nitrogen oxides with distinct chemical properties and uses.

A promoter is a substance that increases the efficiency or activity of a catalyst without itself being consumed in the reaction. This distinguishes it from activators or retarders, which have different roles, and 'super catalyst' is not a standard term.

Plutonium is an element that does not occur naturally in significant amounts and is primarily produced artificially in nuclear reactors. Uranium, radium, and thorium occur naturally in the Earth's crust.

The second ionization energy is always greater than the first ionization energy because after removing one electron, the atom becomes a positively charged ion, which holds its remaining electrons more tightly, requiring more energy to remove the next electron.

Covalent network crystals have very strong covalent bonds extending throughout the structure, resulting in much higher melting points compared to molecular crystals, which are held together by weaker intermolecular forces.

Honey's greater viscosity is primarily due to hydrogen bonding between its sugar molecules (glucose and fructose) and water, which creates strong intermolecular attractions, increasing resistance to flow. The irregular shape of molecules or molecular size is less significant in this context.

An atom is the smallest particle of an element that can take part in a chemical reaction, making option B correct. It is not indivisible (A is outdated), and while it is a building block (C), the defining feature is its chemical reactivity. Option D is true but less precise than B.

Metallic substances are good conductors of electricity in both solid and liquid states because they have free electrons that can move throughout the structure in both states. Ionic substances conduct electricity only in the molten (liquid) state or when dissolved in water because their ions are free to move in these states, but they do not conduct electricity in the solid state as the ions are fixed in the lattice and cannot move. Molecular solids and covalent network solids generally do not conduct electricity in either state. Therefore, the correct answer is metallic substances.