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Index (H)

HA - HC

HD - HF

HG - HJ

HK - HN

HO - HR

HS - HV

HW - HZ

Moment of truth
The fastest man-made object that ever leave the Earth is the New Horizon spacecraft, launched in January 2006, and attained a speed of more than 36000 mph.


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hydrated lime Chem.
See slaked lime.

hydrocarbon Chem.
A chemical compound that contains only carbon and hydrogen. There are a large number of hydrocarbons. The main examples are the alkanes (CnH2n+1), alkenes (CnH2n) and alkynes (CnH2n-1). These compounds are linear hydrocarbon chains. Hydrocarbons can also form cyclic compound such as arenes and cycloalkanes (CnH2n).

hydrogen burning Astron.
A thermonuclear process that occurs in the core of a star, such as the Sun. It is believed to occur via the following proton-proton chain reactions:

hydrogen burning

where e+ is the positron, ne is the electron neutrino. The energy release for each 24He (helium) nucleus is about 26 MeV. The temperature must reach at least 106 K for the reaction to occur. It is essentially a fusion reaction that involves 4 protons (p) to produce one 24He nuclei (alpha particle).

hydrolysis Chem.
A chemical reaction of a compound with water. For example, hydrolysis normally occur for salts of weak acids or bases. For instance, the salt of ethanoic acid (acetic acid), sodium ethanoate, reacts with water to give the acid:

hydrolysis

hydrophilic Chem.
Having an affinity for water. Example of such are hydroxyl group and carboxylic group that can form hydrogen bond with water molecules. See also lyophilic.

hydrophobic Chem.
Lacking the affinity for water. Examples of such are non-polar aromatic group and hydrocarbon group that only weakly interacting with water. This is the reason why molecules that contain large number of non-polar groups tend to be less soluble in water. See also lyophobic.

hypertonic solution Biol.
A solution that has a higher osmotic pressure than the other solution. In other words, a hypertonic solution has a greater solute concentration than the other solutions. When an animal cell is placed in a hypertonic solution (say 10% salt solution), the osmotic gradient necessitate the water to be drawn from the cell to the surrounding salt solution. Subsequently, the cell shrinks (crenated).

hypha Bot.
The plural is hyphae.
(1) A filament or thread-like structure, usually white in color, composing the mycelium of a fungus. It is usually begun to grow from a spore of a fungus.
(2) Filamentous outgrowths and branching produced by certain group of bacteria, such as Actinomycetes that give soil its freah 'earthy' smell.

hypotonic solution Biol.
A solution that has a lower osmotic pressure than the other solution. In other words, a hypotonic solution contains less solutes than the other solutions and when an animal cell is placed in a hypotonic solution (distilled water, for example), the osmotic gradient draws water into the cell and subsequently it expands and bursts (lysed).

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