What does a dust particle floating on a water surface illustrate?

Answer 1

This illustrates water's high surface tension. Because of water's natural cohesion, or affinity for itself (ie, it sticks to itself), items can be floated on the surface of water as long as they are not heavy enough to break the water's surface tension. This is true even when items are objectively more dense than water and should rightfully sink. The dust particle in this experiment is really more dense than water, but because it is so small, it's tiny mass can't overcome the force of water's surface tension and thus it 'floats.' This is a basic chemistry class demonstration that is often conducted even with metal filings, which will also float when fine enough, although obviously metal is more dense than water. Mosquitos exploit the same principle when perching on the water. That's the simple answer. The reason for this property is a bit involved. Read on:

Water is known in chemistry to be a highly polar compound, meaning that the individual molecules each act like a magnet, with a positive and negative pole. This is due to the internal arrangement of atoms within the molecule, two Hydrogen atoms with an Oxygen atom in between. But that's only part of the story; All stable compounds have positive and negative charges that negate each other when taken together, otherwise, they'd bond too readily with other matter (that would make them unstable). Although polar compounds are have no net charge, the internal charges are distributed asymmetrically. Thus, some parts of the molecule have a more positive charge, and some parts have a more negative charge. This is caused by a surplus of electrons for bonding, often in compounds with hydrogen, because hydrogen can't form double or triple bonds because it lacks additional space for electrons (to put it simply... saying more would open the quantum mechanical atomic model, also known as one really big can of worms). These 'extra' electrons form what are called 'lone pairs.' In the case of water, two lone pairs are attached to the Oxygen molecule, much as if they were two additional atoms. Two other electrons bond with each of the hydrogen molecules, for a total of eight electrons.

These 'extra' electron pairs exert a repulsive force against other bonding electrons that warp the actual geometry of the molecule. Whereas three bonded atoms would seem (at least according to common sense) to bond in a straight line, like for example a carbon dioxide molecule, the lone pairs exert a push against the electrons in the hydrogen bonds. This bends water's molecular geometry into a tetrahedral shape (like a pyramid with a triangular base). Thus, water's molecular geometry is classified as 'bent.'

Since these lone pairs have a strong negative charge, the end of the molecule with the lone pairs acts as the negative pole of a magnet. The end of the molecule with the Hydrogen atoms acts as the positive pole of a magnet. The positive poles strongly attract the negative poles of other nearby water molecules, and the whole mass basically clings together through magnetic attraction, on a molecular level.

This is a very important concept in chemistry; Nearly anyone can and has observed water's high surface tension, for example when water beads up on a car's hood and windows, or when small leaves or debris appear to float on its surface. But the explanation for this phenomenon is found deep within the atomic structure of water molecules, giving us a glimpse into the mysterious unobservable world around us. This is why this demonstration is ever-present in many basic chemistry classes.