Diffusion: Molecules present in gases are in constant random motion. Due to this molecular motion, gas particles spread and intermix from an area of high concentration to low concentration. This property is called diffusion. When a bottle of perfume is opened in a corner, its sweet smell spreads throughout the room due to diffusion. Figure 14.6 (conceptually) shows diffusion of hydrogen gas from one container to another. Diffusion is a spontaneous process.
The difference in rates of diffusion of gases can be explained with kinetic particle theory. At constant temperature, molecules of all gases possess the same average kinetic energy. Since hydrogen is lighter than oxygen, its molecules move faster at a given temperature. Thus, hydrogen diffuses much faster than oxygen. Rate of diffusion increases with increase in temperature as particles gain kinetic energy and move faster, leading to rapid mixing.
Two cotton plugs soaked in hydrogen chloride (HCl) and ammonia (NH₃) solutions are placed at opposite ends of a 100 cm glass tube. They produce white dense fumes of ammonium chloride where they meet. HCl travels 40.5 cm while NH₃ covers 59.5 cm in the same duration. Thus ammonia diffuses faster than hydrogen chloride because NH₃ is lighter (molar mass ~17 g/mol) than HCl (~36.5 g/mol).
The tendency of medicine molecules to move from higher to lower concentration is called diffusion. Diffusion rates control how rapidly and effectively medicines are absorbed, distributed, and eliminated from the body.
When taken orally, diffusion rates between stomach and intestine control absorption into blood — faster diffusion means earlier absorption and faster onset of action. Once in blood, drug molecules diffuse into tissues and organs. The rate of diffusion determines how quickly the drug becomes effective. For example, lipid-soluble drugs diffuse more easily through cell membranes, making them more effective in a shorter time. Generally, a faster diffusion rate leads to higher drug concentration at target organs and a more effective response.
In short, rates of diffusion play a crucial role in drug delivery and movement throughout the body, ensuring essential substances reach target locations quickly to help treat diseases.
✓ Understand the kinetic molecular theory: at same temperature, all gases have same average kinetic energy, but lighter gases move faster.
✓ Remember the ammonia-HCl experiment: lighter NH₃ diffuses faster (59.5 cm vs 40.5 cm).
✓ Diffusion increases with temperature because particles gain kinetic energy.
✓ Medical application: rapid diffusion leads to faster drug absorption and quicker therapeutic effect.
✓ Practice the MCQ quiz below to test your understanding.
Select the best answer for each question. After choosing, correct option turns green, wrong turns red. Submit to see score + answer key.