Which of the following pairs of enthalpy changes is always negative?

6.
The correct option is D.

• Enthalpy of combustion ($\Delta H_c$) is always negative because combustion reactions are always exothermic, releasing heat.
• Enthalpy of neutralization ($\Delta H_{neut}$) is always negative because the reaction between an acid and a base is always exothermic.
• Enthalpy of formation ($\Delta H_f$) can be positive (endothermic) or negative (exothermic).
• Enthalpy of solution ($\Delta H_{sol}$) can be positive (endothermic) or negative (exothermic).

7.
The correct option is D.

• Covalent compounds are formed between two non-metal atoms by sharing electrons.
• HCl consists of hydrogen (non-metal) and chlorine (non-metal), forming a covalent bond.
• MgCl$_2$, KCl, and NaCl are all ionic compounds formed between a metal (Mg, K, Na) and a non-metal (Cl).

8.
The correct option is A.

• A soapy touch is a characteristic property of basic solutions.
• Basic solutions have a pH greater than 7. Among the given options, 12 is the only pH value that indicates a strongly basic solution.

9.
The correct option is C.

• Element X has a valency of +3, meaning it forms an ion X$^{3+}$.
• Chlorine (Cl) has a valency of -1, meaning it forms an ion Cl$^{-}$.
• To form a neutral compound, one X$^{3+}$ ion will combine with three Cl$^{-}$ ions.
• Therefore, the formula is XCl$_3$.

10.
The correct option is B.

• According to Avogadro's Law, equal volumes of all gases at the same temperature and pressure contain the same number of molecules.
• CH$_{4(g)}$ and NH$_{3(g)}$ are both gases, and if they have equal volumes at the same temperature and pressure, they must contain the same number of molecules.
• They would not have the same number of atoms because CH$_4$ has 5 atoms per molecule, and NH$_3$ has 4 atoms per molecule.
• Their rates of diffusion would be different due to their different molar masses (CH$_4$ = 16 g/mol, NH$_3$ = 17 g/mol).