What do we call a difference in the concentration of a substance across a distance?

The concept of a difference in the concentration of a substance across a distance is referred to as a concentration gradient. This term describes how the concentration of a substance varies from one area to another, highlighting the uneven distribution of that substance.

For example, if a solute is added to one end of a container filled with solvent, the area where the solute is concentrated will have a higher concentration than the areas farther away from it. This difference creates a gradient. The concentration gradient is crucial in many biological processes, including diffusion, where molecules move from an area of higher concentration to an area of lower concentration in an attempt to reach equilibrium.

Understanding concentration gradients is important in fields such as biology because they drive processes like cellular respiration, nutrient uptake, and the movement of ions across cell membranes.