Fill a glass with water and place it under a running tap. Water will flow out of the glass at the same rate that it flows into it. It's not the same molecules that are in the glass at any given moment, but we maintain the same volume of water. We have what's known as a steady state.

Nitrogen is the most common gas in the atmosphere. In terms of molecules or volume--they are proportional to each other under the same conditions of pressure and temperature--about 78% of air is N₂. Air's other main component, diatomic oxygen, has a bond dissociation energy of 494 kJ per mole. But to break nitrogen's triple bond takes almost twice as much energy: 942 kJ per mole. This makes nitrogen unreactive within the atmosphere's normal temperature range.

Over the years, economic and even medical considerations have influenced the composition of pennies. At one point they were almost pure copper, but presently copper is only a veneer used to maintain an illusory link to the past.

Le Chatelier's Principle is a neat concept, but too often it's expressed in a not so wonderful way. Here is a  textbook definition from a popular college textbook, fifth edition:

A change in any of the factors that determine the equilibrium conditions of a system will cause the system in such a manner as to reduce or counteract the effect of the change.

With such a definition, students often imagine the system as almost having a consciousness or a purpose. Wikipedia's definition also leaves something to be desired:

Perfluorinated Polar Bears!

 
No, this is not an exasperated exclamation by Captain Haddock, but might well be a shout of surprise at learning that Canadians have been searching for compounds of that nature in these snowy animals.  But why should Scott Mabury and his group at the University of Toronto be looking for them?
 
The simple answer is that they are terribly persistent in the environment.  Bit odd, one might link, considering that Fluorine is the most reactive of all the elements in the periodic table.  So reactive[1], in fact, that
 

Isotopes were discovered 100 years ago. Without knowledge of them, we would have no nuclear accidents, weapons, or waste, but we would also be without a medical probe, a weapon against cancer and a way of generating electricity. We would have no clue about the age of the earth or about the intricacies of photosynthesis and respiration. There would be no isotope signatures that reveal diets of the past and sources of illegal drugs.

Combine the two Greek words for equal and place, isos and topos, respectively, and you get the word isotope, so-chosen because isotopes of a given element have an equal number of protons and therefore occupy the same place in the periodic table.

Assuming countries continue to comply with the 1989 (enforced) Montreal Protocol, the ozone layer is expected to recover in coming decades due to declining chlorofluorocarbon (CFCs)concentrations. Total recovery, however, is not predicted before 2050 or 2060.  Although in the past year (from 2010 to 2011) the hole has increased in size by five million kilometers, for most of the last two decades, the hole seems to have stabilized, oscillating between 20 and 25 million square kilometers---about three times the size of continental United States.

Champagne, unlike other wines, undergoes a second fermentation in the bottle to trap carbon dioxide gas, which dissolves into the wine and forms the fabled bubbles in the bubbly. More than 600 different chemical compounds join carbon dioxide in champagne, each lending its own unique quality to the aroma and flavor of champagne. 

But even with all of that flavor, champagne would be just another white wine without those tiny bubbles. As the bubbles ascend the length of a glass in tiny trains, they drag along molecules of those 600 flavor and aroma substances. They literally explode out of the surface as the bubbles burst, tickling the nose and stimulating the senses.

Glow-in-the-dark stickers are nothing new; they emit visible light after being exposed to sunlight.  A paper just published in Nature Materials emits a long-lasting, near-infrared glow after a single minute of exposure to sunlight.

Why is that good? It has the potential to revolutionize medical diagnostics, give the military and law enforcement agencies a 'secret' source of illumination - because the near-infrared range can only be seen with the aid of night vision devices - and maybe even provide a foundation for solar cells that aren't complete rubbish.