CIE IGCSE Biology: 复习笔记:1.1.4 Diffusion

CIE IGCSE Biology: 复习笔记:1.1.4 Diffusion


  • This is the process by which different gases or different liquids mix and is due to the random motion of their particles
  • Diffusing particles move from an area of high concentration to an area of low concentration
  • Eventually the concentration of particles is even as they spread out to occupy all of the available space
  • Diffusion happens on its own and no energy input is required although it occurs faster at higher temperatures


Diffusion of potassium manganate(VII), KMnO, in water. After a few hours the concentration of KMnOis the same throughout the solution


Diffusion & Molecular Mass


  • Diffusion occurs much faster in gases than in liquids as gaseous particles move much quicker than liquid particles
  • At the same temperature, different gases do not diffuse at the same rate.
  • This is due to the difference in their relative molecular masses
  • Lighter gas particles can travel faster and hence further, therefore the lower its relative mass the faster a gas will diffuse
  • This can be demonstrated in the reaction between ammonia, NH3, and hydrogen chloride gas, HCl, inside a long glass tube
  • Where the two gases meet a white smoke of ammonium chloride, NH4Cl, is formed
  • This does not occur in the middle of the tube as you might expect, but much closer to the end with the hydrogen chloride (Mr = 36.5) and the ammonia (M= 17) molecules are smaller and lighter

Relative-molecular-mass-effect-on-diffusion-of-gasesNH3 molecules have less mass than the HCl molecule, so diffuse faster, hence the product (a white smoke of NH4Cl) forms closer to the end where the HCl is