1. Air contains mostly nitrogen (78%) and oxygen (21%). It also contains other gases, including carbon dioxide and helium, and various pollutants (both gases and solids). Write a brief explanation about why air is a mixture, whether it is homogeneous or heterogeneous, and how it can be separated into its components.

It also contains other gases, including carbon dioxide and helium, and various pollutants (both gases and solids).

the best answer is d because the strong cohesion is caused by the ability of water molecules to undergo hydrogen bonding.

a is incorrect because when water solidifies, it becomes   ice, which expands and occupies a bigger volume for the same mass of water in its liquid state.

b is incorrect because water has high surface tension. besides bonding to itself it hydrogen bonds to other molecules at the top decreasing possibility for breaking of attractive forces.

c is incorrect because the water molecule has oxygen which is highly electronegative and so pulls the electron pair towards itself creating a dipole moment, making the molecule polar.

e is incorrect because water has strong adhesion, ability to stick to other molecules such as glass because of this molecules having stronger hydrogen bonding than water.