Rain Water

Lessening raindrops are often depicted in popular culture as "teardrop-shaped" — round at the bottom and narrowing towards the top — but this is untrue. Only drops of water dripping from some sources are tear-shaped at the moment of creation. Small raindrops are nearly globular. Larger ones become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes. The shape of raindrops was studied by Philipp Lenard in 1898. He found that small raindrops (less than about 2 mm diameter) are approximately spherical.

As they get larger (to about 5 mm diameter) they become more doughnut shaped. Beyond about 5 mm they become unsteady and fragment. On average, raindrops are 1 to 2 mm in breadth. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm. The large size is explained by concentration on large smoke particles or by collisions between drops in small regions with particularly high pleased of liquor water.

Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm trickle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.

Generally, rain has a pH slightly under 6. This is because special carbon dioxide dissolves in the dewdrop to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of rainfall, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.

Plastic

Plastic can be classified in many ways, but most generally by their polymer backbone. Other classifications include thermoplastic, thermoset, elastomer, engineering plastic, addition or compression or polyaddition, and glass transition temperature.

Some plastics are partly crystalline and partially amorphous in molecular composition, giving them both a melting point and one or more glass transitions (temperatures above which the extent of localized molecular is considerably increased). So-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are entirely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets.

Plastics are polymers: long chains of atoms bonded to one another. Common thermoplastics range from 20,000 to 500,000 in molecular weight, while thermosets are tacit to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone, (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This customization by repeat unit's molecular structure has allowed plastics to become such an essential part of twenty first-century life by fine tuning the properties of the polymer.