Dalton's law of partial pressures states that the total
pressure exerted by a mixture of gases is equal to the
sum of the pressures which the different gases would exert
if each separately occupied the whole space afforded to the mixture.
This is called after
postulates are the tenets of the atomic theory and
are as follows :
- All matter is composed of extremely
small indivisible particles called atoms,
- An element is composed of only one type of atom,
- A compound is a form of matter which is composed of atoms
of two or more elements combined in fixed proportions.
The Daniell cell, which is a primary voltaic cell having a
positive electrode of copper and a negative electrode of zinc amalgam,
(Zn in alloy with Hg), was invented by the British chemist
John Daniel in 1836AD.
The zinc amalgam electrode is placed in an electrolyte of dilute
sulphuric acid solution or zinc sulphate solution in a porous
earthenware pot. This porous pot stands in a solution of
copper sulphate in which the copper electrode is immersed.
The zinc electrode acts as a source of electrons, which flow through
an external wire which connects the two electrodes, while the
zinc ions from the electrode go into solution.
Cathode reaction : Zn ==> Zn(++) + 2 e(-)
On reaching the copper electrode, these electrons combine with copper
ions in solution and the discharged copper ions are deposited on
the copper electrode as copper metal.
Anode reaction : Cu(++) + 2 e(-) ==> Cu
An equation for the overall chemical process is obtained by adding
together the two half-cell reactions in such a way that the
electrons "cancel out".
Zn + Cu(++) ==> Zn(++) + Cu
While the reaction takes place ions move through the porous
pot, but when it is not in use the cell should be dismantled to
prevent the diffusion of one electrolyte into the other.
A dative bond is formed between two different molecules, each
of which can exist independently as a stable molecule. One
molecule has a lone pair of electrons. The other molecule has a vacant sub-orbital in its outer electronic shell. Then, one
molecule "donates" its lone pair of electrons and the other
molecule "receives" the lone pair, so that the electrons of the
lone pair are shared in the dative molecule.
To be able to form a dative bond between two molecules, the
central atom in one molecule must have a lone pair of electrons
and the central atom in the other molecule must be short a pair
of electrons to have a complete outer shell. Thus, these molecules
can share the lone pair of electrons, giving rise to a dative bond.
Deca- (symbol da) is a prefix used in the metric system to
denote ten times.
Decahydrate is a crystalline hydrate containing ten molecules of
water of crystallisation per molecule of compound.
Deci- (symbol d) is a prefix used in the metric system to
denote one tenth.
Decomposition reactions are chemical reaction in which a
compound breaks down into simpler compounds or into the
elements of which it is composed.
A degree is a division on a temperature scale.
The dehydration reactions of alcohols results in their conversion into
an Alkene, and involves the
elimination of a molecule of water.
Dehydration requires the presence of an acid and the application of
heat. It is generally carried out in either of two ways
- by heating with sulphuric acid or with phosphoric acid to
temperatures as high as 200 degrees, or
- by passing the alcohol vapour over alumina at 350-400 degrees,
when the alumina serves as a lewis
Dehydration reactions involve the removal of water from
a substance. Examples of dehydrating agents include
concentrated sulphuric acid which can
remove water from a molecule, because of its high affinity for water.
Deliquescence is the absorption of water from the atmosphere by
a hygroscopic solid to such an extent that a concentrated solution
of the solid eventually forms.
The delocalisation of electrons in conjugated
results in the increased stabilisation of this type of structure.
Density of a substance is the mass of a substance per unit of volume.
The density of water is one gram per milliliter.
Depression of freezing point of a pure solvent is observed
when substances are dissolved in it. The amount by which the
freezing point is depressed is proportional to the number of
molecules of solute and independent of their nature.
The molecular depression of freezing point is the depression of
the freezing point produced when 1 gram-molecular weight
of solute is dissolved in 100 gm. of solvent.
See also Raoult's Law
A derivative is a compound that is prepared from another compound
which maintains its general structure in the reaction.
A desiccator is a container for drying substances, or for
keeping substances free from moisture.
Most metal halides are soluble in water, and on addition of a
solution of silver nitrate, the insoluble silver halide salt is
For example, when a solution of silver nitrate is added to a
solution of sodium chloride, a white precipitate of silver chloride
NaCl + AgNO3 ==> AgCl + NaNO3
- A precipitate of silver chloride is soluble in dilute aqueous
- A precipitate of silver bromide is very pale yellow precipitate
and is only slightly soluble in dilute aqueous ammonia solution.
- A precipitate of silver iodide is yellow and insoluble in
Hence, by noting the appearance of the precipitated silver
halide and its behaviour with aqueous ammonia, the original
halide ion (i.e. chloride, bromide or iodide) can be identified.
A detergent is a substance added to water to improve its
Deuterated compound are those in which some or all of the hydrogen
atoms in the molecule are replaced by deuterium atoms.
The vapour density of a gas is defined as its density
relative to hydrogen, oxygen or air. Taking hydrogen as
the reference substance, the vapour density is the ratio of
the mass of a particular volume of a gas to the mass of an
equal volume of hydrogen under identical conditions of pressure
and temperature. Taking the density of hydrogen as unity, this
ratio is equal to half the relative molecular mass of the gas.
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