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Moment of truth
More than 25 million people died in Europe from the bubonic plague (the black death) in five years, from 1347 to 1352.

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fool's gold Min.
See pyrite.

formalin Chem.
A colorless solution containing about 40% by volume of methanal (formaldehyde) in water with some methanol (6-13%) added as a stabilizer. When kept below 25C a white polymer of methanal seperates out. It is used as a disinfectant, antibacterial agent and preservative for biological specimens. It is, however, ineffective as a fungicide or insecticide.

formula weight Chem.
The relative molecular mass of a compound as calculated from its molecular formula. For example, the molecular formula of water is H2O, which consists of two hydrogen atoms, each of relative atomic mass of 1 and an oxygen atom with a relative atomic mass of 16. Hence, the formula weight of water is (2x1) + 16 = 18.

Fourier space Chem., Phys.
See reciprocal space

frame dragging Phys.
See Lense-Thirring effect.

freezing mixture Chem.
A mixture of components that produces a low temperature. For example, a mixture of ice and sodium chloride (table salt) gives a temperature of -20C. A mixture of ice and calcium chloride can reach down to -55C.

Frenkel defect Chem., Phys.
A defect or disorder in the crystal lattice due to some of the ions (normally the smaller cations) being displaced from their normal positions and move to interstitial positions (in between crystal planes). This can easily occur especially with crystals that consist of very small cations when compared with the much larger anions. For example, the silver cation, Ag+ in silver bromide or chloride.

frenkel defect on silver chloride

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