| | If you introduce a layer of salty water in a container and make sure it rests below layers of distilled water (Fig. 1), the salt will slowly spread upward. Eventually, all the salt will be evenly distributed, meaning that the concentration of salt has become uniform throughout the body of water (Fig. 2). We can determine the concentration of salt indirectly by measuring the conductivity of the solution. A simulation based on actual measurements shows that it takes more than a month for about 1 g salt to spread in a cube of 1 liter of distilled water.
Interpretation Salt can be dissolved in liquids such as water (common salt decomposes into Na+ and Cl– ions, making the solution electrically conductive). If the concentration of the dissolved substance is higher in some places than in others, it migrates through the liquid to where there is less. When the concentration has become uniform throughout the solvent, the process—called diffusion—stops. We say that concentration differences are the driving force for diffusive transports. | |
