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Moment of truth
The Sun, with a diameter of 1,392,000 km, is made of 94% hydrogen atoms.

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albedo Astron.
A measure of the reflecting power of a non-luminous surface, defined as the amount of radiation reflected in all direction to total it receives. A perfectly black object that adsorbs all radiation has an albedo of zero, while a completely white reflecting object has an albedo of one.

alicyclic Chem.
An aliphatic compound that has a carbocyclic ring structure which may be saturated (singly bonded) or unsturated but does not have other ring structure such as a benzoid (aromatic) system. One difference between alicyclic and aromatic compounds is that, in the former case, the ring system is generally easier to break to form a chain structure with atoms joined in series, especially for low-membered ring such as 3-membered ring compound.

aliphatic Chem.
An organic compound with a molecular structure that does not contain ring structures. This includes alkanes, alkenes, alkynes and their derivatives. See also alicyclic.

alkalimetry Chem.
Volumetric analysis using accurately known solutions (standard solutions) of alkali to determine the amount of acid present.

Alkaloid Chem.
A group of nitrogen-containing organic compounds that derive mostly from plants such as dogbanes, lupins, poppies, tobacco etc. They are alkaline and generally have complex molcular structures. The role of alkaloids in plants is still not well understood but they usually have strong and diverse physiological effects on humans and animals. Some common examples of alkaloids are morphine, cocaine, caffeine, atropine and quinine, most of which are used as anesthetics or analgesics (pain relievers). Some alkaloids, such as colchicine, are poisonous that inhibits cell devision. Other strong physiological effects including stimulants, constricting blood vessels and hallucinogenics.

alkanes Chem.
Simplest homologous series of the organic compounds with the general formula CnH2n+1. They are aliphatic hydrocarbon compounds. The simplest alkane is methane (CH4), with n=1.

alkenes Chem.
Homologues series of the hydrocarbon organic compounds with the general formula CnH2n where n > 1. They are called unstaurated hydrocarbons because they contained a carbon-carbon double bond (C=C), where addition of other atoms are readily to occur. The simplest alkene is ethene (C2H4) with n=2.

allosteric Biol., Chem.
An effect of a change in the behavior or structure of a certain part of a molecule due to a change in another part of molecule. Allosteric effect is common especially in complex biomolecular systems such as protein enzymes.

allotrope Chem.
An element that exists in one of a form. See allotropy.

allotropy Chem.
The existence of elements in two or more different forms (allotropes). Usually allotropy occurs because of different atom arrangements in the crystalline solids. In some cases, an allotrope is stable over a range of temperature beyond which one changes to the other allotrope. For example, tin has two alloptropes: white tin is stable above 13.2C and grey tin below 13.2C. This type of allotropy is called enantiotropy.

However, some elements do not have definite transition temperature to revert one allotrope to the other. For instance, carbon can exist as diamond or graphite of which the latter form is stable at all temperature. This type of allotropy is called monotropy.

Some elements exist in allotropes of difference molecular structure. For example, oxygen can exist in two forms or allotropes: normal oxygen, O2 and ozone, O3.

alpha-naphthol test Biochem.
See Molisch's test.

alpha particle Phys.
A bare helium nucleus (He2+) which consists of two protons and two neutrons.

aluminothermic process Eng.
Also called Thermit process. See Goldschmidt process for details.

amalgam Chem.
A solution of metal dissolves in mercury. Amalgam is used in many applications. For example, gold amalgam in separation of gold from its ore. Gold dissolves out of its ore and the amalgam is heated in order to vaporize mercury, leaving gold behind. Other examples such as silver amalgam, as used in dentistry.

amorphous Chem.
Describing a solid that does not have a long-range order or repeating, regular 3-dimensional arrangement of atoms or molecules. These solids are also called non-crystaline. Examples are glasses, wheat flour, wood, etc.

amphoteric oxide Chem.
Oxides of weakly electropositive metals (or metalloids) such as aluminium oxide, zinc oxide and tin oxide. Normal metal oxides are basic that can react with an acid and non-metal oxides are usually acidic that can react with a base. However, amphoteric oxides can react with both acids and alkalis. For instance, aluminium can react with both hydrochloric acid and sodium hydroxide:

amylose Biochem.
A constituent of starch made up of 100-3000 or more linked glucose molecules. It gives a characteristic dark blue coloration with iodine water which serves as a test for amylose. The digestive enzyme amylase breaks down the amylose molecule into maltotriose and maltose.

amylum Bot., Biochem.
Storage carbohydrate of plants in the form of starch. It appears as form of grains in plastids such as chloroplasts.

amyotrophic lateral sclerosis Med.
A neurodegenerative disorder characterized by the destruction of large motor neurons in the spinal cord and brain (lower and upper neurons, respectively). This results in progressive muscle weakness, causing paralysis. The patient typically dies with 2 to 5 years of symptom onset. It was first discovered by the French neurologist Jean-Martin Charcot in 1869. Symptoms include lose of muscle control and strength, difficulty speaking, swallowing and breathing. However, the mind is thought to function normally. The disease is known by several names: Lou Gehrig's in the United States, motor neuron disease in England and maladie de Charcot in France.

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