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Index (D)

DA - DC

DD - DF

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DK - DN

DO - DR

DS - DV

DW - DZ

Moment of truth
The Voyager I spacecraft is the most distant man-made object in the universe. It was launched in 1977 and as in 2002, it was more than 8 billions miles from the Earth.


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dolomite Min.
A common sedimentary rock-forming carbonate mineral, consisting of a mixed calcium and magnesium carbonate, Ca.Mg(CO3)2. Usually appear pinkish with pearly luster but can be greyish or brown if other contaminants are present. It crystalizes in the rhombohedral system with hardness of 3.5-4. The mineral was named after the French mineralogist Deodat de Dolomieu.

Doppler effect Phys.
Also known as Doppler shift, it is the apparent change of frequency or wavelength due to the relative motion between the source and the observer. For example, change in frequency of a siren (of an ambulance) sound to a higher pitch as it approaches towards an observer. The frequency waves are compressed, shortening the wavelength and increasing the frequency. As the siren is moving away from the observer, the sound waves are elongated and result in lower pitch. The effect was first discovered by Johann C. Doppler (1803-1853) in 1842. Diagram below illustrates the effect.

doppler shift effect


dot product Math.
Also called scalar product, denoted as ab where a and b are two vectors inclined to each other at an angle q.
dot product definition
The dot product of the two vectors is defined as
ab = |a||b|cos q
where |a| and |b| are the magnitude of vectors.

If a = x1i + y1j + z1k and b = x2i + y2j + z2k then
ab = ba = x1x2 + y1y2 + z1z2

double decomposition Chem.
A chemical reaction involving exchange of radical ions:

double decomposition

which involves exchange of nitrate (NO3-) and bromide (Br-) ions between the metal cations.

double salt Chem.
A crystalline salt that contains two different ionic compounds. For example, aluminium potassium sulphate (potash alum)is a crystalline compound with formula Al2(SO4)3.K2SO4.24H2O. Potash alum is also called a double sulfate. Double salts only exist in solid. When dissolve in water, they act as a mixture of the two separate salts. Hence, potash alum dissolves in water to give a mixture of aluminium sulfate and potassium sulfate.

dry ice Chem.
Solid, frozen carbon dioxide. Unlike normal ice, dry ice sublimes rather than melts, at -78C at standard atmospheric pressure. Hence, the word 'dry'. It is conveniently used as a refrigerant for its ease of handling and is much colder than normal ice.

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