The correct answer is B. This statement perfectly captures the dynamic nature of the autoionization equilibrium. Even in pure water, there is constant activity at the molecular level, with forward and reverse reactions occurring at equal rates. A is incorrect, as ions are present. C is incorrect as the reverse reaction is crucial for establishing equilibrium. D is incorrect, as the concentration of water is vastly greater than the very small concentrations of the ions.

The correct answer is B. This statement accurately describes the dynamic equilibrium in terms of the forward reaction (proton donation by the acid) and the reverse reaction (proton acceptance by the conjugate base). A is incorrect because at equilibrium, all species in the equation are present. C is only true at the half-equivalence point of a titration, not for a general equilibrium state. D is incorrect because both reactions occur simultaneously and continuously.

The correct answer is B. The core concept of chemical equilibrium is that it is dynamic, not static. At the molecular level, forward and reverse reactions continue to occur. The macroscopic stability (no change in concentrations) is a result of these opposing rates being equal. A is false; equilibrium is typically studied in closed systems. C and D describe irreversible reactions or reactions not yet at equilibrium.

The correct answer is B. At equilibrium, the concentrations (and therefore partial pressures) of all species become constant. Since the total pressure is the sum of the partial pressures, it also remains constant. A is incorrect because mass is always conserved in a closed system, whether at equilibrium or not. C is incorrect because individual partial pressures are constant at equilibrium, but this describes the equilibrium condition itself rather than a consequence. D is incorrect because at equilibrium the rates are equal, not that their relative values remain constant.

The correct answer is C. Constant concentrations are the key macroscopic evidence that a reversible system has reached equilibrium. The reason concentrations are constant is that the forward and reverse rates have become equal, leading to no net change. A and B are possible explanations for an irreversible reaction, but for a general chemical process, equilibrium is the most appropriate conclusion for constant concentrations. D is incorrect because if the reverse rate were greater, the product concentration would decrease, not remain constant.

The correct answer is C. A saturated solution in contact with excess solid represents a dynamic equilibrium. The solid dissolves at a certain rate, and the ions in solution precipitate at a certain rate. At equilibrium, these two rates are equal. A is incorrect because the ions exist in the saturated solution. B is incorrect because equilibrium is dynamic, not static. D is incorrect because while the stoichiometry of dissolution produces two chloride ions for every one lead ion, the statement is about the equilibrium condition itself, which is defined by rates, not simply concentration ratios.

The correct answer is C. Chemical equilibrium is a dynamic state where the rate of the forward reaction is exactly equal to the rate of the reverse reaction. This results in no net change in the concentrations of reactants and products, but reactions are still occurring. A is incorrect because concentrations are constant, but not necessarily equal. B is incorrect because equilibrium is dynamic; reactions do not stop. D describes a reaction that has gone to completion, not one at equilibrium.

This question tests understanding of equilibrium in decomposition reactions. At equilibrium, PCl₅ molecules continue to decompose into PCl₃ and Cl₂, while PCl₃ and Cl₂ simultaneously recombine to form PCl₅, with both the forward and reverse reactions proceeding at equal rates. This dynamic balance results in constant concentrations of all species over time. Choice A incorrectly assumes equilibrium requires equal concentrations of all species, which confuses the equilibrium condition with a specific equilibrium position. To recognize equilibrium, focus on the equality of reaction rates in both directions, not on concentration relationships or the misconception that reactions stop.

This question assesses comprehension of equilibrium in decomposition reactions. In PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), the constant amounts of gases indicate equilibrium has been reached. This happens because the forward decomposition and reverse recombination rates are equal, resulting in no net change. Dynamic equilibrium implies ongoing molecular interconversions, balanced to keep compositions stable. Choice C misleads by stating concentrations must be equal, reflecting the misconception that equilibrium requires equal quantities rather than equal rates. To tackle these questions, emphasize the equality of rates over equality of concentrations.

This question tests understanding of reaction rates at equilibrium. At equilibrium, NO and O₂ molecules continue to react to form NO₂, while NO₂ molecules simultaneously decompose back to NO and O₂, with the forward reaction rate exactly equal to the reverse reaction rate. This equality of rates maintains constant concentrations of all species. Choice B incorrectly suggests only the reverse reaction continues, which would cause concentrations to change over time. When analyzing equilibrium, remember that constant concentrations result from equal rates of opposing processes, not from one reaction stopping while the other continues.