Question

In a chemistry lab, Rina took some mercury and water in two test tubes A and B respectively. Then she drained off both the liquids and on observing the empty test tubes, found that test tube A is completely dry and some water droplets are stuck to test tube B. What could be the reason for this?
A) Cohesive force between water and glass is more.
B) Adhesive force between mercury and glass is more.
C) Adhesive force between water and glass is more.
D) Cohesive force among mercury atoms is weak.

Answer 1

We know that Water droplets are found on the inner surface of one test tube because in case of water, adhesive force is greater than cohesive forces while in case of mercury cohesive forces are greater than adhesive forces.

Complete step by step answer:
We also remember that in the case of a liquid surface-active agent, due to adhesive force the liquid is held tight to the surface on which it rests. A skinny, uniform glaze is formed over the glass surface due to the fact that the adhesive forces between water and glass are strong enough to drag the water which reduces the repulsion between the same molecules. While within the case of mercury and glass, cohesive forces of mercury are lesser than adhesive forces between mercury and glass, so tube A becomes completely dry.

Hence option c is correct.

Additional information:
We also remember that the intermolecular forces between the atoms determine the boiling and melting point of the compounds stronger the intermolecular forces higher will be the boiling and melting point.
In both the hydrocarbons there is a weak London dispersion force but the strength the intermolecular forces increases with the number of atoms in the molecule.
Example:
Intermolecular forces in ethane:
Ethane has London forces like all covalent compounds.
Ethane has a polar bond, so it exhibits dipole-dipole interactions.
Ethane has hydrogen atom but there is no oxygen or fluorine or nitrogen atom to form intermolecular hydrogen bonding.
Intermolecular forces present in methanol:
Methanol has London forces like all covalent compounds.
Methanol has a polar bond, so it exhibits dipole-dipole interaction.
Methanol has Hydrogen, or an Oxygen, nitrogen or Fluorine so it has intermolecular hydrogen bonding.
We must remember that the hydrogen bonds are relatively strong intermolecular forces and thus it takes a lot of energy to break the intermolecular forces thus the molecules with stronger hydrogen bonds have higher boiling and melting point. Thus, methanol has a high boiling point and melting point because it has a strong hydrogen bond.

Note: Now we discuss about cohesive forces as,
Cohesive forces are the intermolecular forces that cause bent in liquids to resist separation. These attractive forces exist between molecules of an equivalent substance.
We see the adhesive forces as,
The attractive forces between different molecules are called adhesive forces. They’re caused by forces acting between two substances, like mechanical forces and electrostatic forces.