What are cohesion and adhesion?

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‘Cohesion’ and ‘adhesion’ are properties demonstrated by certain particles, substances, or surfaces when they are combined or mixed with each other. The main difference is that when two different particles or substances are combined, the term used for the particle’s tendencies to stick together is called adhesion.  On the other hand, ‘cohesion’ refers to the particles’ attraction or tendency to stick with each other when they have similar components.

Substances or particles that stick together through either adhesion or cohesion depend on a lot of forces or factors.  One force that makes substances display adhesion or cohesion is by mechanical means. This is especially true for particles that have obvious gaps or voids in their surfaces. When combined with another particle, the tendency is that the other particle will fill the gaps and somewhat cling to the original particle by means of mechanical interlocking.  Other particles may also cling with other particles simply because of their chemical properties.  Some chemicals simply connect with others by way of mixing, and this happens because of the way their atoms interact with other chemicals.  Adhesion and/or cohesion may also occur through some electrical attraction between various substances or particles. When some molecules of a substance have either a positive or negative charge, these will also attract other molecules causing the adhesion and/or cohesion.

Adhesion and cohesion may also depend on which force is stronger between the two. This is especially noticeable when two dissimilar substances are combined or exposed to the same substance or particle.  In the case of water and mercury, for example, both will behave differently when put in a glass of water. When the water is poured onto the glass, it will instantly make the glass surface wet.  This is due to the fact that water has a greater adhesive tendency towards glass compared to its internal adhesive properties.  On the other hand, mercury molecules have a greater internal cohesive property compared to its adhesive attraction to glass.  With this property, mercury molecules are able to stick together and form lumps on the glass rather than dispersing across the entire surface.

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