A salt is the product of the neutralisation of an acid
by a base. Salts are usually ionic solids.
A salt bridge is the linking structure, consisting of
a viscous gel of saturated sodium chloride in a
glass tube, used to connect two half cells.
A saturated compound is one in which all the atoms are linked by
single bonds. Saturated compounds
react with other compounds by undergoing
substitution reactions. If a
saturated compound undergoes an elimination reaction, an unsaturated compound is formed.
A saturated solution is a solution contains as much
solute as the
solvent can hold (i.e. dissolve) at a that
A secondary alcohol is one in which the hydroxyl group [-OH] is
attached to a secondary carbon atom (i.e. a carbon atom
which has one hydrogen atoms attached to it).
A secondary cell is an electrochemical
for the production of electricity. After use the secondary cell can be recharged
by passing an electric current through the cell in the
opposite direction to that delivered during use. Examples of
a secondary cell include the lead-acid battery used in cars, and
the nickel-cadmium cells used in portable electrical devices.
A secondary standard is a working standard material which
is used in the laboratory as a working standard, and which has
been calibrated against a primary standard
material of known composition.
Secondary treatment involves the biochemical oxidation of
a sewage or trade effluent to meet the standards before its
discharge to the aquatic environment.
Semiconductors are materials whose electrical conductivity
is intermediate between that of conductors and that of
insulators. The electronics industry is based on the
beneficial uses of the electrical properties of semiconductors.
Separation is required to isolate pure compounds from mixtures and is a prerequisite
to the analysis of mixtures. In chemical manufacturing plants, separation is normally
achieved by distillation, sublimination,
crystallisation, or by
filtration. In analytical chemistry,
separation is normally achieved by chromatographic
Ionic compounds exist in a crystalline form in the solid
state. For example, the electrostatic attractive force which exist
between the positively charged sodium ion and the negatively charged
chloride ion cause these oppositely charged ions to arrange
themselves as close as possible in a crystal lattice. The exact
structure of the unit cell of the crystal structure will depend
on the size and charge on each of the ions.
In the case of sodium chloride the lattice structure is a centered cube. In this type of lattice, a given face of the
lattice has a sodium ion at its center and four sodium
ions at the corners of the face, a chloride ion is at
the center of the face with four chloride ions at the
corners of the face and four sodium ions spaced equally between
the chloride ions.
As the forces between ions in a crystal structure is
considerable, the crystal lattice is a very stable
structure. The lattice is not easily broken by vibration of
the ions. A great deal of energy (i.e. a high temperature), is needed
to overcome these forces. Hence, ionic compound has a
high melting point.
The shape of a simple covalent compounds is related to the
number of electron pairs in the outer shell of the central
atom of the molecule. Each electron pair repels all others
and the electron pairs arrange themselves in space so that
they are as far apart as possible.
The shape of a molecule is related to the number of electron
pairs in the outer shell of the Central Atom of the
molecule (i.e. the atom to which the other atoms in the
molecule are bonded). Each electron pair in the central
atom, repels the other electron pairs in that atom. Thus, the electron
pairs will arrange themselves in space so that they are as far
apart as possible. This theory to explain the shapes of simple
covalent molecules is known as Valence Shell Electron Pair Repulsion.
Number of electron pairs Shape of molecule Example
======================== ================= =======
2 Linear BeF2
3 Triangular planar BCl3
4 Tetrahedral CH4
5 Trigonal bipyramidal PCl5
6 Octahedral SF6
7 Pentagonal bipyramidal IF7
Sigma Bonds (s Bonds) are the single covalent bonds formed between
two atoms by the end-on overlap of the atomic orbitals on the
The single bond is the covalent or coordinate bond between two
atoms that involves the sharing of a single pair of
electrons between the atoms.
Slaked lime (i.e. calcium hydroxide), is prepared by adding water
to freshly prepared quicklime.
Soap is a mixture of the sodium salt of the fatty
acids, which is obtained on hydrolysis of fats with
sodium hydroxide. Glycerol is also obtained in this reaction.
Sodium thiosulphate, Na2S2O3, is a salt used in photography for
the fixing of photographic plates.
A solid is one of the states in which matter can exist.
A solid is rigid, has a definite shape and has a fixed volume.
Solubility is the quantity of solute that dissolves a given
quantity of solvent to form a saturated solution, and is
expressed in kilograms per meter cubed, moles per kilogram of
solvent, etc. The solubility of a substance in a given
solvent depends on temperature. No simple general relation
exists between solubility and temperature, so each case
must be investigated separately. Polar solutes are more
soluble in polar solvents.
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A general property of gases is that they diffuse to fill the volume in which they are contained
(i.e. they have neither fixed shape or fixed volume). Thus, gases which do not react with
each other, are infinitely soluble in each other in all proportions, due to this power of
This behaviour is described by Graham's
and by Dalton's Law of Partial
Similarly, when a gas dissolves in a liquid with which it does not react, its behaviour, it is
uniformly distributed throughout the volume of the solvent and its behaviour is described
by Henry's Law. In general, the solubility
of a gas which is only sparingly soluble in a solvent, decreases with increasing temperature.
Hydrogen, nitrogen and oxygen are non-polar and are only
sparingly soluble in water. In the case of these three
gases, the gases continue to exist as covalent molecules
in solution and there is no significant alteration to
the structure of the molecules of gas.
However, when a polar covalent gas dissolves in polar
solvent, it often undergo chemical reaction with the solvent,
and significant changes to the structure of the molecules
of the gas occur in solution. For example, the polar
covalent gas ammonia is very soluble in water. However, in
aqueous solution, ammonia exists as an ammonium ion, NH4(+),
having extracted a hydrogen ion from a molecule of water.
NH3 + H2O ==> NH4(+) + HO(-)
Ammonia Ammonium Hydroxyl
The resulting solution is alkaline, due to the existence of
the hydroxyl ion in solution.
Similarly, the polar covalent gas hydrogen chloride is very
soluble in water. In aqueous solution, the molecule exists in
ionic form, as the positively charged hydrogen ion and
the negatively charged chloride ion.
HCl + ==> H(+) + Cl(-)
Hydrogen Hydrogen Chloride
Chloride Ion Ion
It should be noted that the hydrogen ion is stabilised by hydration
in aqueous solution, as the hydronium ion.
H(+) + H2O ==> H3O(+)
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A substance is soluble in a liquid, if it dissolves in that
liquid. A liquid can also dissolve in another liquid is it
is miscible with the liquid.
A solute is the material that dissolves in a liquid.
A solution is a mixture of a liquid, called as the solvent, with
a gas, liquid or solid known as the solute.
Solutions are homogeneous mixtures, and usually consist of
a mixture of a solid (i.e. the solute) and a liquid (i.e. the
solvent). When a solution if formed, the molecules of the solute
are evenly distributed amongst the molecules of the solvent. In a
solution, there are interactions between the molecules of the
solute and the molecules solvent, and this interaction is known
In this case, the solid is called the solute, is said to have
dissolved in the liquid. The liquid is called the solvent.
If a solid can dissolve in a given liquid, the solid is said to
be soluble in that liquid. If the solid cannot dissolve in a
liquid, that solid is insoluble in that liquid.
For example, when sugar is dissolved in water, the sugar
is the solute, the water is the solvent and the resultant
mixture is the solution.
The amount of a solute which a solvent can hold in solution
is dependent on the temperature. When a solution contains as
much solute as the solvent can hold at a particular temperature,
the solution is said to be a saturated solution at that temperature.
An unsaturated solution is one in which more solute will
go into solution.
A supersaturated solution is one in which too much solute
has been dissolved in a solvent. As a general rule, hot
solvents can dissolve more solute than the same quantity
of cold solvent. A hot saturated solution forms a
supersaturated solution when cooled to a lower
temperature. Because the solvent is unable to hold
all the solute in solution at the lower temperature, some
of solute precipitates out in the form of crystals. This
process is called crystallisation.
Crystallisation is a method used to purify solids. If one
takes a solid containing a small amount of impurities and
dissolves it in a suitable solvent to form a saturated
solution at elevated temperature, the impurities are likely
to remain in solution when the solution is cooled
and the solid crystallises out.
Solutions of electrolytes are the electrically conducting
solutions formed when salts are dissolved in water. The
high electrical conductivity of the solution is due to the
presence of ions from the dissociation of the salt.
Solvation is the interaction of ions of a
solute with the molecules of the solvent. It occurs only
with polar solvents.
Solvation is the process that causes ionic solids to
dissolve in polar solvents. The energy released during
solvation is sufficient to compensate for the
lattice energy of the ionic crystal, in which the
ions are arranged in the solid state.
A solvent is a liquid that dissolves another substance or
substances to form a solution.
The spectrum of hydrogen is obtained by an electrical discharge in a
Geissler Tube. The spectrum
contains four bright lines, due to atomic hydrogen and is used in calibrating
spectroscopes and refractomeres. The principal lines in
the spectrum of hydrogen are :
- a red line, H, at 656.2 nm (i.e. Fraunhofer's C),
- a blue line, H, at 434.0 nm,
- a greenish-blue line, H, at 486.1 nm,
- (i.e. Fraunhofer's F) and
- an indigo line, H, at 410.2 nm.
This spin quantum number, s relates to the spin on an
electron. The spin may have one of two
values, s = + 0.5. or s = - 0.5. Each sub-orbital can accommodate two electrons, and the two electrons in each sub-orbital must
have opposite spins. Allowing two electrons in each
sub-orbital and the magnetic quantum, ml explains
the number of electrons which can be accommodated in each sub-orbital.
The standard heat of combustion is the heat evolved when
one gram mole of a substance is burned in oxygen.
A standard reference electrode is required in all
In the case of the determination of standard reduction
potentials, the standard reference electrode is the hydrogen half-cell.
In the case of pH measurement, the standard reference electrode
is the calomel electrode.
A standard solution is one which contains a known weight
of reagent in a definite volume of solution.
The states of matter are the three forms (i.e. solid, liquid
and gas) in which matter is observed to exist. A fourth state of
matter (i.e. the plasma state) exists at very high
temperatures, when all materials are decomposed into their
elements and the electrons are stripped from these atoms
to form the plasma.
Steam cracking is the industrial process using steam at
high temperature to break down high molecular weight hydrocarbons
into smaller molecules.
Steam reforming is a process used to convert methane derived
from natural gas into a mixture of carbon monoxide and hydrogen,
which is then used to synthesise organic chemicals. The
reaction occurs at about 900 degC using a nickel catalyst.
CH4 + H2O ==> CO + 3H2
Stoichiometry is the quantitative relationship between the amounts
of materials involved in a chemical reaction. The total
quantity of reactants (i.e. the mass of the starting materials)
must be equal to the total mass of the products of the reaction.
The stoichiometry of electrolysis is described by
Faraday's Laws of Electrolysis.
The strength of acids is determined by the degree to which they
are ionised in aqueous solution. For example, sulphuric acid which
is a strong acid is fully dissociated, and all the displaceable
hydrogen in the acid is present in solution as hydrogen ion.
H2SO4 ==> H(+) + SO4
100% as H(+)
Similarly, the weak acids ethanoic acid is only partially ionised
in solution, and only approximately 5% of the displaceable
hydrogen in the acid is present in solution as hydrogen ion.
CH3COOH ==> H(+) + CH3COO(-)
5% as H(+)
Strong acids and bases are acids and bases which are
completely dissociated in solution.
A strong electrolyte is a substance which undergoes almost complete
dissociation into ions in solution. Solution of a
strong electrolyte consist almost entirely of positively
charged ions and negatively charged ions, which are stabilised
in solution by solvation (i.e. by being associated with aggregates of molecules of the solvent). Examples of strong electrolytes
include almost all salts, the caustic alkalis and the
mineral acids in dilute solutions.
Topic Strength of Acids
The structure of crystals is described in terms of the
geometry of a unit cell of the crystal structure.
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