With equal amounts of two substances in water, why might A reach diffusion equilibrium faster than B?

The correct choice is that solute A is smaller than solute B. The size of a solute molecule plays a significant role in the rate of diffusion. Smaller molecules typically diffuse more quickly through a solvent, such as water, due to their reduced friction and the ability to move through the spaces between water molecules more easily.

In contrast, larger molecules may encounter more resistance as they navigate through the solvent, leading to a slower diffusion rate. This size-related aspect of diffusion is governed by principles such as Graham's law, which states that the rate of diffusion is inversely proportional to the square root of the molar mass of the diffusing substance. Therefore, when both substances start with the same concentration and are in a similar environment, the smaller solute can reach diffusion equilibrium faster because it can traverse the solution with greater ease compared to the larger solute.

Temperature, concentration gradient, and distance traveled are factors that can influence diffusion rates, but in this scenario, the size of the solute is the strongest determining factor for why one reaches equilibrium faster than the other.