Boiling
point
The boiling point of an
element or a substance is the temperature at which the vapor pressure of the
liquid equals the environmental pressure surrounding the liquid.
The boiling point of a liquid varies dependent
upon the surrounding environmental pressure. Different liquids (at a given
pressure) boil at different temperatures. A liquid in a vacuum environment has
a lower boiling point than when the liquid is at atmospheric pressure. A liquid
in a high pressure environment has a higher boiling point than when the liquid
is at atmospheric pressure.
The normal boiling point of a
liquid is the special case in which the vapor pressure of the liquid equals the
defined atmospheric pressure at sea level, 1 atmosphere. At that temperature,
the vapor pressure of the liquid becomes sufficient to overcome atmospheric
pressure and lift the liquid to form bubbles inside the bulk of the liquid. The
standard boiling point is now defined by IUPAC as the temperature at
which boiling occurs under a pressure of 1 bar.
The heat of vaporization is the
amount of energy required to convert or vaporize a saturated liquid (i.e., a
liquid at its boiling point) into a vapor.
Liquids may change to a vapor at
temperatures below their boiling points through the process of evaporation.
Evaporation is a surface phenomenon in which molecules located near the
liquid's edge, not contained by enough liquid pressure on that side, escape
into the surroundings as vapor. On the other hand, boiling is a process in
which molecules anywhere in the liquid escape, resulting in the formation of
vapor bubbles within the liquid.
Determination of Boiling Points
· Boiling point is determined by Capillary tube method.
· In this method, a few drops
of liquid are placed in a thin walled
small test tube.
· A capillary tube sealed at about 1 cm from one end, is dropped in to it.
· A glass tube containing the liquid and capillary, is then tied along a
side of thermometer so that the liquid stands just near the bulb.
· The thermometer is then lowered in a beaker containing paraffin oil.
· The beaker is heated and the bath liquid stirred continuously using with
stirrer.
· When the boiling point reached, bubbles come from lower end of capillary.
· The read the temperature from thermometer when the evaluation of bubbles
just stop.
Melting Point
The melting
point of a substance is the temperature at which the solid phase converts to
the liquid phase under 1 atmosphere of pressure.
The melting
point is one of a number of physical properties of a substance that is useful
for characterizing and identifying the substance.
To measure
the melting point of a substance, it is necessary somehow to gradually heat a
small sample of the substance while monitoring its temperature with a
thermometer. The temperature at which liquid is first seen is the lower end of
the melting point range. The temperature at which the last solid disappears is
the upper end of the melting point range. A pure substance normally has a
melting point range no larger than 1-1.5 oC.
Although
many substances melt cleanly and can be melted, crystallized, and remelted
repeatedly without chemical decomposition, others chemically decompose before
they melt, forming substances of lower molecular weight.
The
temperature at which the color change is first observed signals that the
substance is approaching the decomposition temperature.
Determination of Melting Points
· Melting point is also determined by Capillary tube method.
· A glass capillary tube which is 5-6
cm long and 1mm diameter, normally used to contain the sample for a melting
point determination.
· The tube must have one open end into
which the sample can be loaded, and one sealed end so that the capillary will
retain the solid sample.
· The substance should stand in the capillary 3-4 mm from the bottom when
thoroughly packed.
· The capillary is wetted with liquid in the bath and then tied along a
side of thermometer fixed in an iron stand.
· The thermometer is then lowered in
a beaker containing paraffin oil.
· The beaker is heated and the bath liquid stirred continuously using with
stirrer.
· When the substance in the capillary just shows sign of melting , the
burner is removed and stirring continued.
· The read the temperature from thermometer when the substance melts and
become transparent. This is the melting point range of that substance.
Solubility
Ø
A
solution is a homogeneous mixture of two or more substances.
Ø
A
solute is defined as the substance that dissolves in a solution.
Ø
A
solvent is defined as the material that dissolves the other substance(s) in a
solution. It is the dissolving medium.
Ø
Solubility
is defined as the maximum amount of a substance (solute) which will dissolve in
a given amount of solvent at a specific temperature.
Solubility is the property of a
solid, liquid, or gaseous chemical substance called solute to dissolve
in a liquid or gaseous solvent to form a homogeneous solution of the solute in
the solvent.
The solubility of a substance
depends on the used solvent as well as on temperature and pressure. The extent
of the solubility of a substance in a specific solvent is measured as the
saturation concentration where adding more solute does not increase the
concentration of the solution.
The solvent is generally a liquid,
which can be a pure substance or a mixture. The extent of solubility ranges
widely, from infinitely soluble such as ethanol in water, to poorly soluble,
such as silver chloride in water. The term insoluble is often applied to
poorly or very poorly soluble compounds.
Under certain conditions the equilibrium
solubility can be exceeded to give a so-called supersaturated solution, which
is metastable.
According to an IUPAC definition,
solubility is the analytical composition of a saturated solution expressed as a
proportion of a designated solute in a designated solvent. Solubility may be
stated in units of concentration, molality, mole fraction, mole ratio, and
other units.
Factors affecting solubility
The solubility of one substance in
another is determined by the balance of intermolecular forces between the
solvent and solute, and the entropy change that accompanies the solvation.
Factors such as temperature and pressure will alter this balance, thus changing
the solubility.
Solubility may also strongly depend
on the presence of other species dissolved in the solvent, for example, complex-forming
anions (ligands) in liquids.
Solubility will also
depend on the excess or deficiency of a common ion in the solution, a
phenomenon known as the common-ion effect. Solubility will depend on the ionic
strength of solutions.
Temperature
The solubility of a given solute in
a given solvent depends on temperature. For many solids dissolved in liquid
water, the solubility increases with temperature up to 100 °C. In liquid water
at high temperatures, the solubility of ionic solutes tends to decrease due to
the change of properties and structure of liquid water; the lower dielectric
constant results in a less polar solvent.
The chart shows solubility curves
for some typical solid inorganic salts (temperature is in degrees Celsius).
Many salts behave like barium nitrate and disodium hydrogen arsenate, and show
a large increase in solubility with temperature. Some solutes (e.g. NaCl in
water) exhibit solubility which is fairly independent of temperature. A few,
such as cerium(III) sulfate, become less soluble in water as temperature
increases. This temperature dependence is sometimes referred to as
"retrograde" or "inverse" solubility.
The solubility of organic compounds
nearly always increases with temperature. The technique of recrystallization,
used for purification of solids, depends on a solute's different solubilities
in hot and cold solvent. A few exceptions exist, such as certain cyclodextrins.
Pressure
For condensed phases (solids and
liquids), the pressure dependence of solubility is typically weak and usually
neglected in practice. Assuming an ideal solution, the dependence can be
quantified as:
The pressure dependence of
solubility does occasionally have practical significance. For example, precipitation
fouling of oil fields and wells by calcium sulfate (which decreases its
solubility with decreasing pressure) can result in decreased productivity with
time.
Polarity
A very polar (hydrophilic) solute
such as urea is very soluble in highly polar water, less soluble in fairly
polar methanol, and practically insoluble in non-polar solvents such as benzene.
In contrast, a non-polar or lipophilic solute such as naphthalene is insoluble
in water, fairly soluble in methanol, and highly soluble in non-polar benzene.
The solubility is favored by entropy
of mixing and depends on enthalpy of dissolution and the hydrophobic effect.
Applications
Solubility is of fundamental
importance in a large number of scientific disciplines and practical
applications, ranging from ore processing, to the use of medicines, and the
transport of pollutants.
Solubility is often said to be one
of the "characteristic properties of a substance," which means that
solubility is commonly used to describe the substance, to indicate a
substance's polarity, to help to distinguish it from other substances, and as a
guide to applications of the substance. For example, indigo is described as
"insoluble in water, alcohol, or ether but soluble in chloroform,
nitrobenzene, or concentrated sulfuric acid".
Solubility of a substance is useful
when separating mixtures. For example, a mixture of salt (sodium chloride) and
silica may be separated by dissolving the salt in water, and filtering off the
un dissolved silica.
Another example of this is the
synthesis of benzoic acid from phenyl magnesium bromide and dry ice. Benzoic
acid is more soluble in an organic solvent such as dichloromethane or diethyl
ether, and when shaken with this organic solvent in a separatory funnel, will
preferentially dissolve in the organic layer. The other reaction products,
including the magnesium bromide, will remain in the aqueous layer, clearly
showing that separation based on solubility is achieved. This process, known as
liquid-liquid extraction, is an important technique in synthetic chemistry.
Solubility of ionic compounds
in water
Some ionic compounds (salts)
dissolve in water, which arises because of the attraction between positive and
negative charges. For example, the salt's positive ions (e.g. Ag+)
attract the partially-negative oxygens in H2O. Likewise, the salt's
negative ions (e.g. Cl−) attract the partially-positive hydrogens in
H2O.
AgCl(s)
Ag+(aq) + Cl−(aq)
However, there is a
limit to how much salt can be dissolved in a given volume of water. This amount
is given by the solubility product, Ksp. This value depends on the
type of salt (AgCl vs. NaCl), temperature, and the common ion effect.
Solubility of organic
compounds
The principle of polarity, that like
dissolves like, is the usual guide to solubility with organic systems. For
example, petroleum jelly will dissolve in gasoline because both petroleum jelly
and gasoline are non-polar hydrocarbons. It will not, dissolve in ethyl alcohol
or water, since the polarity of these solvents is too high. Sugar will not
dissolve in benzene, since sugar is too polar in comparison with benzene.
Solubility in non-aqueous
solvents
Non polar solutes are soluble in non
aqueous solvents. Most available solubility values are those for solubility in
water. The reference also lists some for non-aqueous solvents.
Quantification of solubility
Solubility is commonly expressed as
a concentration, either by mass (g of solute per kg of solvent, g per dL
(100mL) of solvent, molarity, molality, mole fraction or other similar
descriptions of concentration.
Density
Density is the measure of
the mass per unit volume of a material (density = mass/volume). Density is a
characteristic of a substance. Mass and volume vary with size but density will
remain constant. Temperature will affect the density of a substance.
Molarity
A useful way to express exact
concentrations of solutions is molarity. Molarity is defined as moles of
solute per litre of solution. Molarity is symbolized by the capital letter M.
It can be expressed mathematically as follows.
Molarity
(M) = moles
of solute (n)
Liters
of solution (V)
Notice that the moles of solute are
divided by the liters of solution not solvent. One litre of one molar solution
will consist of one mole of solute plus enough solvent to make a final volume
of one litre.
Normality
The normal concentration is another
method for expressing the concentration of solutions.
Normality (N) is defined as
the number of equivalents of solute dissolved in one litre of solution.
One equivalent of acid is the amount
of acid necessary to give up one mole of hydrogen ions in a chemical reaction.
One equivalent of base is the amount of base that reacts with one mole of
hydrogen ions. When expressing the concentrations of bases, normality refers to
the number of available hydroxyl ions. Because hydrogen and hydroxyl ions
combine on a one-to-one basis, one OH- is equivalent to one H+ ion.
ppm
expresses the concentration of a solution in units of one part of solute to one
million parts solvent. One ppm equals one milligram of solute per litre of
solution.
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