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Monday, December 5, 2011

Water properties- Adhesion and Cohesion

If we talk about water we must refer cohesion and adhesion.
We already talk about another water properties the surface tension, but without the first two the triad wouldn’t be complete.
Most of the times water molecules are attracted to each other, in this case we are dealing with cohesion, on the other hand, if water molecules are attracted to other materials then we are dealing with adhesion.

What makes it possible?
As we said in other articles water molecules are made by 2 H atoms and one O atom, this gives water molecule a neutral charge, but its structure allows the water molecule to establish hydrogen bonds between H atoms from different molecules, this bonds make molecules closer from each other, that phenomenon is called cohesion. If the attraction between the H atoms is weaker then the attraction between water molecules and other material molecules surrounding, the water molecules will bond to this surrounding material instead, we call this adhesion.

All hydrophobic surfaces will repels water, this means the water molecules are not attracted to it and they form drops, this is cohesion, like wax paper or any waxed surface. Like the shower after being cleaned with specific soap, in fact a thin hydrophobic layer is formed to repel water- Cohesion.
It is also due to cohesion that the rain falls in droplets rather than molecules, otherwise we would call rain to something completely different

When water molecules are attracted to other surrounding molecules the phenomenon is called adhesion. We saw it here, remember? The water "climbed" the paper against gravity, this happened because water molecules were strongly attracted by cellulose molecules on the paper, this attraction was stronger then the attraction between H2O molecules.

Note this phenomenon only occur because one of the forces is stronger then the other.

In short:
Cohesion- when attraction forces between water molecules are stronger then the attraction forces between water and surrounding material molecules.
Adhesion- when attraction forces between water molecules are weaker then the attraction forces between water and surrounding material molecules.

The following demonstration its very simple and works very well to show cohesion

What we need:
  1. 2 styrofoam cups,
  2. water,
  3. sharp pencil or any sharp object like a nail,
  4. a bowl.
How to:
  1. Choose an appropriate location;
  2. With the pencil make two holes in the first cup, as close as possible but not touching, don't make them too large;
  3. Fill the second cup with water;
  4. Hold the punched cup above the bowl;
  5. Pour the water from the second cup into the first;
  6. The water should start to fall through the holes you made with the pencil;
  7. Quickly, "hold" the two "threads" of water with your fingers (thumb and index).
The 2 "threads" become one.

The water molecules are attracted to each other.

Please note:
  • Probably you will not make it work at first time, try again.
  • If you are lucky you will achieve the optimal distance between the holes, in that scenario you will not need to use your fingers, the "threads" will become one naturally.


Et voil√°!
Don’t get wet!

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