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Imagine we put two glass tubes in a container bulged with mercury. These tubes have 2 radii which we call them : 1.Big 2.Small The height of mercury column in the big one is more than the small one. And the opposite happpens in a water container.

Why is that? MY THOUGHTS: So with water it makes sense . Since water molecules stick to glass ones ,(due to overcoming force of adhesion over cohesion ) water goes up till the mass of liquid is to heavy and be pulled down which it can be concluded smaller radius would has water raised more . Now although mercury has cohesion of its molecules overcomed its adhesion but it should not make any difference when it come to going up more in smaller tubes in terms of radius BUT it do.

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Fluid molecules here experience 2 types of forces: cohesive and adhesive forces.

Cohesive force: the force of attraction between 2 of the same molecules.

Adhesive force: the force of attraction between 2 foreign molecules.

For water, adhesive force is stronger than the cohesive forces, sticking more strongly to the capillary wall than other water molecules, hence as the tube is thinner, the higher the level. That also explains why the meniscus is concave.

On the other hand, for mercury, cohesive forces between the mercury molecules are stronger than adhesive forces between the mercury molecules and the capillary wall, sticking less strongly to the wall, and this falling lower than water. This also explains the convex meniscus of mercury.

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  • $\begingroup$ Thank you for answering but how come mercury falling more in thinner tubes than thicker ones ? $\endgroup$ – Abbas Jul 23 '18 at 15:15
  • $\begingroup$ And also Shouldn't be some gravity involved? $\endgroup$ – Abbas Jul 23 '18 at 16:36
  • $\begingroup$ There is. But the strong forces of attraction overcome the weight $\endgroup$ – QuIcKmAtHs Jul 23 '18 at 21:45
  • $\begingroup$ As I have mentioned I've read in wikipedia the reason for water raising up more in thinner than thicker tube is that the mass of water will be prohibited from rising when it gets too heavy so thinner tube would have more water going up. Isn't it true? $\endgroup$ – Abbas Jul 24 '18 at 9:30
  • $\begingroup$ You, you are right. $\endgroup$ – QuIcKmAtHs Jul 24 '18 at 12:01

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