Water remains as drops on the polythene surface but, it forms a thin layer on the surface of a properly cleaned glass plate. Explain.
Dear Student,
Please find below the solution to the asked query:
Adhesive force is the attractive force between two molecules of two different substances.
Cohesive force is the attractive force between the molecules of the same material.
In the question above the adhesive forces operate between water and the two surfaces. Cohesive forces are operative between water molecules. If adhesive forces between the two surfaces in contact are stronger than cohesive forces the water molecules will spread out. On the other hand, if the adhesive forces are weaker than the cohesive forces the water molecules will be present as drops.
Water is a polar solvent and glass is a polar while polyethylene (a polymer of organic molecule ethylene) is non-polar in nature. Like molecules interact with each other while unlike molecules do not. The adhesive forces between water and glass are strong and can overcome the cohesive forces between water molecules and this helps water to spread over the glass in a thin layer.
In the interaction between water and polyethylene surface, the adhesive forces between water and polyethylene cannot overcome the cohesive forces between water molecules and hence water does not spread out but is present as drops on polyethylene surface.
Hope this information will clear your doubts about adhesive and cohesive forces.
If you have any more doubts just ask here on the forum and our experts will try to help you out as soon as possible.
Regards
Please find below the solution to the asked query:
Adhesive force is the attractive force between two molecules of two different substances.
Cohesive force is the attractive force between the molecules of the same material.
In the question above the adhesive forces operate between water and the two surfaces. Cohesive forces are operative between water molecules. If adhesive forces between the two surfaces in contact are stronger than cohesive forces the water molecules will spread out. On the other hand, if the adhesive forces are weaker than the cohesive forces the water molecules will be present as drops.
Water is a polar solvent and glass is a polar while polyethylene (a polymer of organic molecule ethylene) is non-polar in nature. Like molecules interact with each other while unlike molecules do not. The adhesive forces between water and glass are strong and can overcome the cohesive forces between water molecules and this helps water to spread over the glass in a thin layer.
In the interaction between water and polyethylene surface, the adhesive forces between water and polyethylene cannot overcome the cohesive forces between water molecules and hence water does not spread out but is present as drops on polyethylene surface.
Hope this information will clear your doubts about adhesive and cohesive forces.
If you have any more doubts just ask here on the forum and our experts will try to help you out as soon as possible.
Regards