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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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molar heat capacity Chem.
Energy require to raise the temperature of a mole of substance by 1 K. The symbol is J mol-1 K-1. To convert to specific heat capacity in unit J g-1 K-1 divide the molar heat capacity by the molecular weight of the substance. In the case of a gaseous substance, two different types of heat capacity can be measured when energy is supplied to it. This either allows the gas to expand at constant pressure (Cp) or the volume of gas remains constant (Cv). See also heat capacity.

mole Chem.
Abbreviated as mol, it is the standard SI units that equals to the amount of substance, measures in terms of elementary units, as there are atoms in 0.012 kg (12.0 g) of carbon-12. The elementary units can be atoms, electrons, molecules etc. One mole of a compound has a mass equal to its relative molecular mass expressed in grams.

mole fraction Chem.
A measure, in fraction, of the amount of a component in a mixture. Usually abbreviated as X. For example, the mole fraction of component A is given by XA = nA/N where N is the total amount of substance and nA is the amount of substance of A. X is dimensionless (no units) and nA and N must refer to the amount of substances of similar elementary units (atoms, molecules. ions etc).

molecular dynamics Chem.
A computer simulation technique that trace out the movements of every atoms in a molecular system by the use of classical Newton's equation of motions. Generally, interactions among atoms are represented by some simplified (empirical) mathematical functional forms from which the forces are derived, and hence the velocities and positions of each atom in a given time interval.

Molisch's test Biochem.
Also known as the alpha-naphthol test. It is a biochemical test to detect the presence of carbohydrates in solution. The test is carried out by first mixing a small amount of the test solution with alpha-naphthol dissolved in ethanol (Molisch's reagent). Then, a small amount of concentrated sulfuric acid is poured slowly down the side of the test tube. A positive test is indicated by the formation of a violet ring at the liquid interface between the acid (bottom layer) and test solution (top layer). The test is named after the Austrian chemist H. Molisch (1856-1937).

Mond process Chem.
An industrial process to purify nickel by heating the impure metal (such as crushed and smelted nickel ore) in a stream of carbon monoxide at 60C to produce nickel carbonyl, Ni(CO)4. The compound is volatile and can be decomposed at 180C to give nickel and carbon monoxide. The method was invented by the German-born British chemist and industrialist Ludwig Mond (1839-1909).

Monel metal Eng.
Monel is a trademark of Special Metal Corporation, named after the company president Ambrose Monel. It is an alloy of nickel (60-67%) and copper (25-30%) with some iron and other trace metals such as titanium, aluminium and manganese. The alloy has high tensile strength and is resistant to corrosion and acid attacks. It can even withstand roasting in a fire with pure oxygen. It is used in marine applications, musical instruments and in domestic applications such as kitchen sinks and frames of eyeglass.

monotropy Chem.
See allotropy.

motor neuron disease Med.
A group of related disease affecting the motor neurons in the brain and spinal cord. Motor neurons are nerve cells that send instructions, in the form of electrical impulses, to the muscle. An example of the disease is amyotrophic lateral sclerosis.

Mpemba effect Phys.
An effect whereby, for a given set of conditions, hot water freezes faster than cold. Though not always the case, this seemingly couterintuitive effect has been noted since ancients by Aristotle as described in his work Meteorologica. However, the effect has become more widely known in 1963 when Erasto Mpemba, a high-school pupil in Tanzania realized that putting hot ice cream mixture in the fridge froze before the other cooled mixtures. Since then, experiments have been carried out world-wide to investigate the effect. Nevertheless, Mpemba effect still remains a puzzle.

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