## Explanation

### Variables that Affect the Rate of Diffusion

### Density of Media

The **density of the medium** through which diffusion occurs significantly impacts the rate of diffusion. Higher density media slow down the diffusion process because molecules encounter more resistance to their movement. In contrast, diffusion occurs more quickly in less dense media.

### Size of Molecule

The **size of the molecule** diffusing is another crucial variable. Smaller molecules diffuse faster because they can move more easily through the medium. Larger molecules experience more collisions and resistance, leading to a slower diffusion rate.

### pH

The **pH** of the medium can indirectly affect the rate of diffusion. Changes in pH can alter the charge and, in turn, the solubility of the diffusing molecules, impacting how easily they can move through the medium.

### Concentration Gradient

The **concentration gradient** is perhaps one of the most significant factors. Diffusion occurs more rapidly when the difference in concentration between two regions is greater. This can be expressed with Fick's First Law of Diffusion:

Where:

- $J$ is the diffusion flux,
- $D$ is the diffusion coefficient,
- $\frac{dC}{dx}$ is the concentration gradient.

### Membrane Permeability

**Membrane permeability** also plays a critical role in diffusion, especially when dealing with biological contexts. Higher permeability means that molecules can pass through the membrane more easily, increasing the rate of diffusion. This can be modeled by the equation:

Where:

- $P$ is the permeability,
- $D$ is the diffusion coefficient,
- $K$ is the partition coefficient,
- $L$ is the membrane thickness.