Although I’m not entirely sure what answer you’re looking for, I’d say because most scientific investigations require multiple trials of an experiment
Answer:
Hydrogen bond is a <em>special type of dipole-dipole interaction between the hydrogen atom in a polar bond, such as N-H, O-H, or F-H, and electronegative O,N or F atom</em>. The interaction is written:
A-B . . . B or A-H . . . .A
A and B represent O,N or F; A-H is one molecule or part of a molecule and B is a part of another molecule, and the dotted represents the hydrogen bond. So, hydrogen bond is a type of intermolecular forces (attractive forces between molecules, <u>NOT</u> between atoms of the same molecule), and only a few elements can participate on hydrogen bond formation.
<em>Note: dipole-dipole interactions are forces between polar molecules, that is, between molecules that possess dipole moments. </em>
Sand is less dense than gravel and will therefore sink to the bottom of the container while the gravel stays ontop.