Solution

A solution is a homogeneous mixture of two (or more) substances, the composition of which
may vary between certain limits. A solution consisting of two components is called binary
solution. The component which is present in large quantity is called solvent and the component
which is small in quantity is called solute.
Type of Solutions: The three states of matter (gas, liquid or solid) may behave either as solvent
or solute. Depending on the state of solute or solvent, mainly there may be the following seven
types of binary solutions.

Standard Solution: It is a solution of known concentration.

Concentration: It represents how much of given substance is present in given solution.
In all the techniques of quantitative analysis the use of solutions requires some basis for the
expression of solution concentration.
1) Percent Concentration 2) Parts per million 3) Parts per billion 4) Molarity 5) Normality 6)
Molality 7) Formality
1- Percent Concentration: It refers to the amount of the solute per 100 parts of the solution. It
can also be called as parts per hundred (pph).
It can be expressed by any of following four methods,
% 𝒘/𝒘 = 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆/ 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑿𝟏𝟎𝟎
%𝒗/𝒗 = 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆/ 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑿𝟏𝟎𝟎
% 𝒘/𝒗 = 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆/ 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑿𝟏𝟎𝟎
% 𝒗/𝒘 = 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆/ 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑿𝟏𝟎𝟎

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2- Parts per million (ppm): It is used to express the concentration of dilute solutions and is
expressed as
ppm = 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆 / 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑿𝟏𝟎𝟔
3- Parts per billion (ppb): It is used to express the concentration of dilute solutions and is
expressed as-
ppb = 𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒆 /𝑴𝒂𝒔𝒔 𝒐𝒇 𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑿𝟏𝟎𝟗
4- Molarity Number of moles of solute (Substance) dissolved in one liter (1000 mL) of Solution
is called as Molarity.
M = No. of moles of solute/ 1 L of solution
5- Normality Number of gram equivalent of solute (Substance) dissolved in one liter (1000 mL)
of solution is called as Normality.
N = No. of gram equivalent of solute/ 1 L of solution
6- Molality A molal solution contains 1 mole of solute per one kilogram of solution is called as
Molality.
M = no. of moles of solute/ 1 Kg of solution
7- Formal Concentration (Formality) The concentration unit, formal, is similar to the more
familiar molar concentration in that it is calculated as the number of moles of a substance in a
liter of solution.
F= Weight of solute/ Volume of solution X Formula weight
The formal Concentration (Formality) is applicable to the ionic substances.

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IDEAL SOLUTIONS
Cohesive forces: forces that exist between the two similar types of the molecules i.e. between
solute- solute or between solvent-solvent.
Adhesive forces: forces that exist between different types of molecules i.e. between solute and
solvent.
“Solutions in which the adhesive and cohesive forces are same (equal) are known as ideal
solutions” Or “Solutions that obey Raoult's law”
Raoult`s Law:
At a definite temperature, the partial pressure (PA) of component (A) in a liquid mixture is equal
to the vapor pressure of that component in the pure state (P°A ) multiplied by the mole fraction
(χ A) of that component in the solution.
PA = 𝛘A P°A
Mixtures of Volatile Liquids
BOTH liquids are volatile and contribute to the vapor; the total vapor pressure can be
represented using Dalton’s Law:
PT = PA + PB The vapor pressure from each component follows Raoult’s Law:
PT = 𝛘A P°A + 𝛘B P°B
Benzene and Toluene Consider a two solvent (volatile) system – The vapor pressure from each
component follows Raoult's Law.
Characteristics of an ideal solution:
 Ideal behavior is expected to be exhibited by the systems which comprises of the
chemical similar compounds, because it is only in such systems that the conditions of
equal intermolecular forces between components are likely to be satisfied.
Examples: solutions of ethyl alcohol- methyl alcohol, chloroform-bromoform, benzene- toluene.
 Ideal solutions have zero enthalpy change i.e. heat is neither absorbed nor evolved during
solution formation.
 The volume of the solution is exactly equal to the sum of the individual volumes of the
components.

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Non- Ideal or Real Solutions
Solutions in which cohesive and adhesive forces are not equal are known as non-ideal or real
solutions Solutions in which solute-solute, solvent-solvent and solute-solvent attractive forces
are not equal. Solutions which don’t obey Raoult’s law
Deviations from Raoult`s Law
Negative deviation from Raoult`s Law:
Real solution which showed negative deviation from Raoult`s Law are those solution in which
adhesive forces (i.e. solute-solvent) are stronger than the cohesive forces (i.e. solute-solute or
solvent-solvent) and vapor pressure of the solution is less than expected from Raoult`s law.
i.e. A-B > A-A, B-B
Reasoning of lowering of vapor pressure The attractive forces between solute and solvent (A-
B) are stronger than those exerted between solute-solute (A-A) and solvent-solvent (B-B)
molecules, then this strong mutual affinity between solute and solvent molecules results in the
formation of complex and results in lowering of escaping tendency of solvent molecules and
ultimately lowering of vapor pressure. When this occur, there may be decrease in solution
volume occur than the sum of volume of the components.
E.g. Chloroform-Ethanol, Benzene-Ethanol
Positive deviation from Raoult`s Law: Real solution which showed positive deviation from
Raoult`s Law are those solution in which cohesive forces (i.e. solute-solute or solvent-solvent)
are stronger than the adhesive forces ( i.e. solute- solvent)and vapor pressure of the solution is
greater than expected from Raoult`s law. i.e. A-B < A-A, B-B
Reasoning of elevation of vapor pressure The attractive forces between solute and solvent (A-
B) are less than those exerted between solute-solute (A-A) and solvent-solvent (B-B) molecules,
then the presence of “A” reduces the (B-B) attraction and similarly presence of “B”
molecules reduces (A-A) attraction. This results in greater escaping tendency of A and B and
ultimately partial vapor pressure of the components are greater than expected from
Raoult`s law showed positive deviation.
When this occur, there may be increase in solution volume occur than the sum of volume of the
components. E.g. Chloroform-Acetone, Pyridine-Acetic Acid

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Colligative Properties

A colligative property is a property of a solution that is dependent on the ratio between the total
number of solute particles (in the solution) to the total number of solvent particles. Colligative
properties are not dependent on the chemical nature of the solution’s components. Thus,
colligative properties can be linked to several quantities that express the concentration of a
solution, such as molarity, normality, and molality.

The word “colligative” has been adapted or taken from the Latin word “colligatus” which
translates to “bound together”.

What are Colligative Properties?

Dilute solution containing non-volatile solute exhibit some properties which depend only on the
number of solute particles present and not on the type of solute present. These properties are
called colligative properties. These properties are mostly seen in dilute solutions.

Colligative Properties Examples

If we add a pinch of salt to a glass full of water its freezing temperature is lowered considerably
than the normal temperature. Alternatively, its boiling temperature is also increased and the
solution will have a lower vapor pressure. There are changes in its osmotic pressure as well.

Similarly, if we add alcohol to water, the solution’s freezing point goes down below the normal
temperature that is observed for either pure water or alcohol.

Different Types of Colligative Properties of Solution

There are different types of colligative properties of a solution. These include vapor pressure
lowering, boiling point elevation, freezing point depression and osmotic pressure.

1. Lowering of Vapor Pressure

In a pure solvent, the entire surface is occupied by the molecules of the solvent. If a non- volatile
solute is added to the solvent, the surface now has both solute and solvent molecules; thereby
fraction of surface covered by solvent molecules gets reduced. Since the vapor pressure of the

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solution is solely due to solvent alone, at the same temperature the vapor pressure of the solution
is found to be lower than that of the pure solvent.

If P0 is the vapor pressure of pure solvent and Ps is the vapor pressure of the solution. The
difference Po – Ps is termed as lowering in vapor pressure. The ratio Po – Ps / Po is known as
the relative lowering of vapor pressure.

2. Elevation in Boiling Point

The boiling point of a liquid is the temperature at which the vapor pressure is equal to
atmospheric pressure. We know that on the addition of a non-volatile liquid to a pure solvent,
the vapor pressure of a solution decrease. Therefore, to make vapor pressure equal to
atmospheric pressure we have to increase the temperature of the solution. The difference in the
boiling point of the solution and the boiling point of the pure solvent is termed as elevation in
boiling point.

If T0b is the boiling point of the pure solvent and Tb is the boiling point of the solution then
elevation in boiling point is given as

∆Tb =T0b-Tb

As ∆Tb ∝ m

ΔTb = Kb m
Where, Kb = Boiling Point Elevation Constant

∆Tfb=T0b-Tb
As we knew
∆Tb α m so
∆Tb = Fb m
As
𝟏𝟎𝟎𝟎 𝒘
𝒎
𝒘𝟏 𝑴

x 1000 x

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3. Depression in Freezing Point

The freezing point of a substance is defined as the temperature at which the vapor pressure of
its liquid is equal to the vapor of the corresponding solid. According to Raoult’s law when a
non-volatile solid is added to the solvent its vapor pressure decreases and now it would become
equal to that of solid solvent at a lower temperature. The difference between the freezing point
of the pure solvent and its solution is called depression in freezing point.

If T0f is the boiling point of the pure solvent and Tf is the boiling point of the solution then
depression in freezing point is given as

∆Tf =T0f-Tf
As we knew
∆Tf α m so
∆Tf = Ff m
As
𝟏𝟎𝟎𝟎 𝒘
𝒎
𝒘𝟏 𝑴

x 1000 x w2

4. Osmotic Pressure

When a semipermeable membrane is placed between a solution and solvent, it is observed that
solvent molecules enter the solution through the semipermeable membrane and the volume of the
solution increases. The semi-permeable membrane allows only solvent molecules to pass through
it but prevents the passage of bigger molecules like solute. This phenomenon of the spontaneous
flow of solvent molecules through a semipermeable membrane from a pure solvent to a solution
or from a dilute to a concentrated solution is called osmosis.

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The flow of solvent molecules through the semipermeable membrane can be stopped if
some extra pressure is applied from the solution side. This pressure that just stops the flow
of solvent is called osmotic pressure of the solution.

Osmotic pressure is a colligative property as it depends on the number of solute present and not
on the nature of the solute. Experimentally it was proved that osmotic pressure (Π) is directly
proportional to molarity (C) and temperature (T).

Mathematically, Π = CRT where R is the gas constant.

As C is equal to n/v so

⇒ Π = (n2/V) RT

Here, V is the volume of solution in liters and n2 is moles of solute

If m2 is the weight of solute and M2 molar mass of solute, then n2= m2/M2

Π = W2 RT / M2V

Thus by knowing the values of ℼ, w2, T and V we can calculate the molar mass of the solute.

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Distribution law
When two immiscible solvents are shaken together, in equilibrium, both solvents are saturated
with a solute. Solubility also represents concentration, therefore, the solubility distribution law
can be expressed as C1/C2 = S1/S2, where S1 and S2 represent the solubility of the solute in the
two solvents, respectively. It is possible to calculate the solubility of the solute in the second
solvent by knowing the Distribution coefficient (KD) in one solvent and solubility in the first
solvent.
Distribution law applications
Various applications of distribution law are found both in the laboratory and in the industry.
1. Solvent extraction - In chemicals, solvent extraction is the process of separating organic
materials from aqueous solutions. Typically, organic solvents such as ether or benzene
are used to shake aqueous solutions first.
2. Partition chromatography- It is applied to a bed of silica soaked in water to make a
paste of this composition. The flow of hexane down the column is allowed. It is another
immiscible solvent. As each component of the mixture is divided into separate stationery
and mobile phases, the stationary liquid phase (water) precedes the mobile phase
(hexane).
3. Desilverization of lead- The molten lead or zinc form immiscible layers, distributing the
silver between them. Because the zinc distribution ratio at 800° C is about 300 to 1, most
silver is deposited into the zinc layer.
4. Confirmatory test for bromine and iodide- Chlorine water is used to treat the salt
solution. Consequently, bromine and iodine are released. Shake mixture with chloroform
to remove the bromine or iodine.
5. Determination of dissociation- In the case of substance X, it exists as single molecules
after being dissociated in an aqueous solution.
6. Determination of solubility - Assume that iodine is to be determined by its solubility in
benzene. Iodine and benzene are shaken together. We find experimentally the equilibrium
concentrations of iodine in benzene (Cb), and water (Cw), and calculate the distribution
coefficient based on the values.

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FACTORS THAT AFFECT SOLUBILITY

Temperature -- Generally, an increase in the temperature of the solution increases the

solubility of a solid solute. For example, a greater amount of sugar will dissolve in warm
water than in cold water. A few solid solutes, however, are less soluble in warmer solutions.

Pressure -- For solid and liquid solutes, changes in pressure have practically no effect on
solubility. For gaseous solutes, an increase in pressure increases solubility and a decrease in
pressure decrease solubility.

Nature of the solute and solvent – The amount of solute that dissolves depends on what
type of solute it is. While only 1 gram of lead (II) chloride can be dissolved in 100 grams of
water at room temperature, 200 grams of zinc chloride can be dissolved. This means that a
greater amount of zinc chloride can be dissolved in the same amount of water than lead II
chloride.

Size of the particles -- When a solute dissolves, the action takes place only at the surface of
each particle. When the total surface area of the solute particles is increased, the solute
dissolves more rapidly. Breaking a solute into smaller pieces increases its surface area and
increases its rate of solution.

Stirring -- With liquid and solid solutes, stirring brings fresh portions of the solvent in contact
with the solute. Stirring, therefore, allows the solute to dissolve faster.

Amount of solute already dissolved – When you have very little solute in the solution,
dissolving takes place quickly. When you have a lot of solute in the solution, dissolving takes
place more slowly.

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