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Liquid Solution JEE Notes | EduRev

JEE : Liquid Solution JEE Notes | EduRev

``` Page 1

Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
CONCEPT OF VAPOUR PRESSURE
When a pure liquid is kept in closed vessel (in which no air is present) the liquid evaporates to give the
vapours. After sometime a dynamic equilibrium is established between liquid and vapours. The pressure
that the vapours exert at equilibrium on the walls of the container or on the surface of the liquid is
called the vapour pressure of the liquid at that temperature, Figure (a).
Case 1:
What would happen to the vapour pressure of the liquid if we take a bigger container, Figure (b) or a
wider container, Figure (c).
Vapour
Liquid
Vapour
Liquid
Vapour
Liquid
Fig. (a)
Fig. (b)
Fig. (c)
Key concept:
When the liquid is in equilibrium with vapours, Liquid  Vapours, the K
p
of the system is K
p
=
P
v
, where P
v
is the vapour pressure. We know that Kp for a given chemical  equilibrium is a constant
and only depends on temperature. Therefore the vapour pressure of the liquid is a constant and
does not depend on the nature of the vessel used and that it only depends on temperature.
Case 2:
Take a vessel with the same liquid as in case 1 and cover it with a glass plate (of negligible weight)
having a hole in it. Would the vapour pressure be the same now?
Vapour
Liquid
Key concept:
Well, the liquid system is the same and the temperature is also same. Therefore the vapour
pressure should be same. The vapour pressure of a liquid is independent of the surface area
exposed on top of the liquid surface provided the surface area exposed should not be zero.
TYPE OF SOLUTIONS
Solutions can have continuously variable compositions, and they are homogeneous on a scale beyond
the size of individual molecules. This definition can be used to cover a wide variety of systems,
LIQUID SOLUTION
LIQUID SOLUTION
Page 2

Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
CONCEPT OF VAPOUR PRESSURE
When a pure liquid is kept in closed vessel (in which no air is present) the liquid evaporates to give the
vapours. After sometime a dynamic equilibrium is established between liquid and vapours. The pressure
that the vapours exert at equilibrium on the walls of the container or on the surface of the liquid is
called the vapour pressure of the liquid at that temperature, Figure (a).
Case 1:
What would happen to the vapour pressure of the liquid if we take a bigger container, Figure (b) or a
wider container, Figure (c).
Vapour
Liquid
Vapour
Liquid
Vapour
Liquid
Fig. (a)
Fig. (b)
Fig. (c)
Key concept:
When the liquid is in equilibrium with vapours, Liquid  Vapours, the K
p
of the system is K
p
=
P
v
, where P
v
is the vapour pressure. We know that Kp for a given chemical  equilibrium is a constant
and only depends on temperature. Therefore the vapour pressure of the liquid is a constant and
does not depend on the nature of the vessel used and that it only depends on temperature.
Case 2:
Take a vessel with the same liquid as in case 1 and cover it with a glass plate (of negligible weight)
having a hole in it. Would the vapour pressure be the same now?
Vapour
Liquid
Key concept:
Well, the liquid system is the same and the temperature is also same. Therefore the vapour
pressure should be same. The vapour pressure of a liquid is independent of the surface area
exposed on top of the liquid surface provided the surface area exposed should not be zero.
TYPE OF SOLUTIONS
Solutions can have continuously variable compositions, and they are homogeneous on a scale beyond
the size of individual molecules. This definition can be used to cover a wide variety of systems,
LIQUID SOLUTION
LIQUID SOLUTION
Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
including ordinary solution such as alcohol in water or HClO
4
in benzene and even solution of large
proteins in aqueous salt solutions. It is also useful sometimes to consider some colloidal suspensions
undergoing Brownian motion as solutions, and there are solid solutions where one solid is uniformly
dissolved in another.
SOME EXAMPLES OF SOLUTIONS
Solute Solvent Name of Type Heat of Solution per
Mole of Solute (kJ)
O
2(g)
N
2(g)
Gaseous  0
Toluene Benzene Ideal  â€“ 0.1
Acetone Chloroform Non ideal  5
NaCI
(s)
H
2
O
(
Ionic  â€“ 3.9
H
2
SO
4(
H
2
O
(
Ionic  95.3
Positive values are heat released.

Most solutions can be described as having a majority called a solvent and one or more minority
ingredients called solutes. The solvent is usually a liquid, whereas solutes can be solids, liquids, or
gases. Solutions can be distinguished from compounds by the kind of interaction between ingredients.
Compounds form as a result of interactions between relatively permanent partners, while the interac-
tions in solutions involve continuously variable sets of solute and solvent molecules, and this interac-
tion is widely distributed among a large and solvent molecules. Above table gives some examples of
solutions and for each we want to be able to understand why the solute dissolves in the solvent.
O
2(g)
in N
2(g)
This first example is; of course, of one gas dissolved in another all gases dissolve in each other in all
proportions. The reason for this is that solute and solvent do not interact. This is illustrated in fig.
which shows that

2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
N
2
N
2
N 2
N
2
N
2
O 2
O
2
O
2
O
2
O
2
N 2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
(a)
(b)
(c)
Fig. The expansion of gas as illustrated by a solution of N
2
and O
2
(diagrammatic). The expansions of
(a) N
2
and of (b) O
2
are shown to be equivalent to forming (c) a solution of N
2
and O
2
.
the process of two gases dissolving in each other is equivalent to the expansion of each gas into the
larger volume without any interaction with the other. As we can see from this figure and as we also
know from our daily experience, this mixing takes place spontaneously. In a gas, the high thermal
energy that the molecules posses keeps them always on the move. This motion allows them to
distribute themselves between the two bulbs in an arrangement with the greatest probability of
occurring. That distribution results in equal O
2
or N
2
partial pressures in each bulb with the N
2
and O
2
completely mixed. It is important to note that the energy of the system has not changed, only the way
in which the molecules are distributed; that is, the entropy, S of the system has changed.
On the molecular level, entropy is a measure of randomness; a maximum in entropy corresponds to the
most probable distribution on a statistical basis.
Page 3

Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
CONCEPT OF VAPOUR PRESSURE
When a pure liquid is kept in closed vessel (in which no air is present) the liquid evaporates to give the
vapours. After sometime a dynamic equilibrium is established between liquid and vapours. The pressure
that the vapours exert at equilibrium on the walls of the container or on the surface of the liquid is
called the vapour pressure of the liquid at that temperature, Figure (a).
Case 1:
What would happen to the vapour pressure of the liquid if we take a bigger container, Figure (b) or a
wider container, Figure (c).
Vapour
Liquid
Vapour
Liquid
Vapour
Liquid
Fig. (a)
Fig. (b)
Fig. (c)
Key concept:
When the liquid is in equilibrium with vapours, Liquid  Vapours, the K
p
of the system is K
p
=
P
v
, where P
v
is the vapour pressure. We know that Kp for a given chemical  equilibrium is a constant
and only depends on temperature. Therefore the vapour pressure of the liquid is a constant and
does not depend on the nature of the vessel used and that it only depends on temperature.
Case 2:
Take a vessel with the same liquid as in case 1 and cover it with a glass plate (of negligible weight)
having a hole in it. Would the vapour pressure be the same now?
Vapour
Liquid
Key concept:
Well, the liquid system is the same and the temperature is also same. Therefore the vapour
pressure should be same. The vapour pressure of a liquid is independent of the surface area
exposed on top of the liquid surface provided the surface area exposed should not be zero.
TYPE OF SOLUTIONS
Solutions can have continuously variable compositions, and they are homogeneous on a scale beyond
the size of individual molecules. This definition can be used to cover a wide variety of systems,
LIQUID SOLUTION
LIQUID SOLUTION
Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
including ordinary solution such as alcohol in water or HClO
4
in benzene and even solution of large
proteins in aqueous salt solutions. It is also useful sometimes to consider some colloidal suspensions
undergoing Brownian motion as solutions, and there are solid solutions where one solid is uniformly
dissolved in another.
SOME EXAMPLES OF SOLUTIONS
Solute Solvent Name of Type Heat of Solution per
Mole of Solute (kJ)
O
2(g)
N
2(g)
Gaseous  0
Toluene Benzene Ideal  â€“ 0.1
Acetone Chloroform Non ideal  5
NaCI
(s)
H
2
O
(
Ionic  â€“ 3.9
H
2
SO
4(
H
2
O
(
Ionic  95.3
Positive values are heat released.

Most solutions can be described as having a majority called a solvent and one or more minority
ingredients called solutes. The solvent is usually a liquid, whereas solutes can be solids, liquids, or
gases. Solutions can be distinguished from compounds by the kind of interaction between ingredients.
Compounds form as a result of interactions between relatively permanent partners, while the interac-
tions in solutions involve continuously variable sets of solute and solvent molecules, and this interac-
tion is widely distributed among a large and solvent molecules. Above table gives some examples of
solutions and for each we want to be able to understand why the solute dissolves in the solvent.
O
2(g)
in N
2(g)
This first example is; of course, of one gas dissolved in another all gases dissolve in each other in all
proportions. The reason for this is that solute and solvent do not interact. This is illustrated in fig.
which shows that

2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
N
2
N
2
N 2
N
2
N
2
O 2
O
2
O
2
O
2
O
2
N 2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
(a)
(b)
(c)
Fig. The expansion of gas as illustrated by a solution of N
2
and O
2
(diagrammatic). The expansions of
(a) N
2
and of (b) O
2
are shown to be equivalent to forming (c) a solution of N
2
and O
2
.
the process of two gases dissolving in each other is equivalent to the expansion of each gas into the
larger volume without any interaction with the other. As we can see from this figure and as we also
know from our daily experience, this mixing takes place spontaneously. In a gas, the high thermal
energy that the molecules posses keeps them always on the move. This motion allows them to
distribute themselves between the two bulbs in an arrangement with the greatest probability of
occurring. That distribution results in equal O
2
or N
2
partial pressures in each bulb with the N
2
and O
2
completely mixed. It is important to note that the energy of the system has not changed, only the way
in which the molecules are distributed; that is, the entropy, S of the system has changed.
On the molecular level, entropy is a measure of randomness; a maximum in entropy corresponds to the
most probable distribution on a statistical basis.
Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
3
www.learnershabitat.ac.in
While the mixing of ideal gases is controlled by entropy, the mixing of molecules that interact with each
other can surely be controlled by energy. In the mixing process the molecular interactions can either
cause the energy to increases or decrease. Chemical changes that are very exothermic show a large
decrease in energy and enthalpy; as a result, they are spontaneous. However, most solutes dissolve in
solvents because mixing increase the systemâ€™s entropy ( ?S > 0); that it is more probable for the
solutes to dissolve in the solvents then for them to remain separate as pure substances.
Toluene in Benzene
These two similar liquids readily dissolve in each other. At first glance we might think this solution
shows the same pattern of mixing as do two ideal gases, but this cannot be correct since the
molecules in a liquid interact with each other very strongly. The key to this solution is that the
molecules of the two liquids have similar electronic structures and similar sizes. As a result, the
benzene   â€“   benzene, benzene   â€“   toluene and toluene   â€“   toluene interactions are very similar. A
toluene molecule, as an approximation, does not know if it is surrounded by other toluene molecule or
by benzene molecules. As we might expect, there is a very negligible heat of solution. These two
liquids completely dissolve one in the other and are an ideal solution. These two liquids completely
dissolve one in what is called an ideal solution. The entropy gained by mixing toluene with benzene is
the same as that gained by mixing two ideal gases. Solvent and solute molecules have no specific
interaction with each other that do not have with molecules of their own kind. As a result the
molecules distribute randomly in the solution and the change in entropy upon mixing is the same as
that for two ideal gases.
Acetone in Chloroform
These two liquids are not very similar; the attraction of acetone and chloroform molecules for each
other is quite different from the attraction of the like molecules for each other. When acetone and
chloroform are mixed heat is given off so they do not form an ideal solution.
Even so, they are completely miscible. In some cases when the molecules in two liquids have quite
different interactions, they may not be completely miscible. For example water and acetone are
completely miscible, but water and chloroform are not. The property of liquids used to characterize
solvent   â€“   solute interactions is called polarity. Polarity measures the small separation of positive and
negative charges in molecules. Water is considered to be the most polar of the common solvents, as
attested by the fact that it is the best solvent for ionic solutes. Acetone has an intermediate value of
polarity, and chloroform is much less polar than acetone. Molecules interact very differently with
molecules of widely different degrees of polarity than they do with others of their own kind. This
difference in interaction leads to immiscibility.
NaCl(s) in H
2
O(
?
)
The compound NaCl has the high melting temperature characteristic of most  ionic solids. It consist
of Na
+
and Cl
â€“
ions. When NaCl dissolves in water, these ions interact with the water molecules
and separate. The interaction with water is particularly large for positively charged ions, and the
Na
+
is surrounded by six closely bound water molecules, Ions surrounded by closely bound water
molecules are said to be hydrated. For salts such as CaCl
2
of AlCl
3
, the water molecules interact
even more strongly with their cations and they give off heat when they dissolve. For NaCl the
energy required to separate the Na
+
and Cl
â€“
ions is nearly balanced by the energy of interaction
between the water and the ions. Salts such as NaNO
3
or NH
4
NO
3
make their solutions colder when
they dissolve to produce hydrated Na
+
,NH
4
+
,
and NO
3
â€“
ions. The entropy of solution for an ionic
solid would never be expected to be as high as its entropy of vaporization; nevertheless, salts such
as NaCl and NaNO
3
dissolve in water because there is a grater probability of finding them in the form
of separated and hydrated ions than packed neatly in a solid. Again entropy is the controlling factor.
H
2
SO
4
(
?
) in H
2
O(
?
)
Almost every chemist knows that H
2
SO
4
gives off a large amount of heat when it dissolves in water.
While pure H
2
SO
4
does not contain ions, as indicted by that fact that it is a liquid at room tempera-
ture, in dilute aqueous solutions it completely ionizes into H
+
and SO
4
2   â€“
. Its high heat of solution
Page 4

Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
www.learnershabitat.ac.in
CONCEPT OF VAPOUR PRESSURE
When a pure liquid is kept in closed vessel (in which no air is present) the liquid evaporates to give the
vapours. After sometime a dynamic equilibrium is established between liquid and vapours. The pressure
that the vapours exert at equilibrium on the walls of the container or on the surface of the liquid is
called the vapour pressure of the liquid at that temperature, Figure (a).
Case 1:
What would happen to the vapour pressure of the liquid if we take a bigger container, Figure (b) or a
wider container, Figure (c).
Vapour
Liquid
Vapour
Liquid
Vapour
Liquid
Fig. (a)
Fig. (b)
Fig. (c)
Key concept:
When the liquid is in equilibrium with vapours, Liquid  Vapours, the K
p
of the system is K
p
=
P
v
, where P
v
is the vapour pressure. We know that Kp for a given chemical  equilibrium is a constant
and only depends on temperature. Therefore the vapour pressure of the liquid is a constant and
does not depend on the nature of the vessel used and that it only depends on temperature.
Case 2:
Take a vessel with the same liquid as in case 1 and cover it with a glass plate (of negligible weight)
having a hole in it. Would the vapour pressure be the same now?
Vapour
Liquid
Key concept:
Well, the liquid system is the same and the temperature is also same. Therefore the vapour
pressure should be same. The vapour pressure of a liquid is independent of the surface area
exposed on top of the liquid surface provided the surface area exposed should not be zero.
TYPE OF SOLUTIONS
Solutions can have continuously variable compositions, and they are homogeneous on a scale beyond
the size of individual molecules. This definition can be used to cover a wide variety of systems,
LIQUID SOLUTION
LIQUID SOLUTION
Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
2
www.learnershabitat.ac.in
including ordinary solution such as alcohol in water or HClO
4
in benzene and even solution of large
proteins in aqueous salt solutions. It is also useful sometimes to consider some colloidal suspensions
undergoing Brownian motion as solutions, and there are solid solutions where one solid is uniformly
dissolved in another.
SOME EXAMPLES OF SOLUTIONS
Solute Solvent Name of Type Heat of Solution per
Mole of Solute (kJ)
O
2(g)
N
2(g)
Gaseous  0
Toluene Benzene Ideal  â€“ 0.1
Acetone Chloroform Non ideal  5
NaCI
(s)
H
2
O
(
Ionic  â€“ 3.9
H
2
SO
4(
H
2
O
(
Ionic  95.3
Positive values are heat released.

Most solutions can be described as having a majority called a solvent and one or more minority
ingredients called solutes. The solvent is usually a liquid, whereas solutes can be solids, liquids, or
gases. Solutions can be distinguished from compounds by the kind of interaction between ingredients.
Compounds form as a result of interactions between relatively permanent partners, while the interac-
tions in solutions involve continuously variable sets of solute and solvent molecules, and this interac-
tion is widely distributed among a large and solvent molecules. Above table gives some examples of
solutions and for each we want to be able to understand why the solute dissolves in the solvent.
O
2(g)
in N
2(g)
This first example is; of course, of one gas dissolved in another all gases dissolve in each other in all
proportions. The reason for this is that solute and solvent do not interact. This is illustrated in fig.
which shows that

2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
N
2
N
2
N 2
N
2
N
2
O 2
O
2
O
2
O
2
O
2
N 2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
(a)
(b)
(c)
Fig. The expansion of gas as illustrated by a solution of N
2
and O
2
(diagrammatic). The expansions of
(a) N
2
and of (b) O
2
are shown to be equivalent to forming (c) a solution of N
2
and O
2
.
the process of two gases dissolving in each other is equivalent to the expansion of each gas into the
larger volume without any interaction with the other. As we can see from this figure and as we also
know from our daily experience, this mixing takes place spontaneously. In a gas, the high thermal
energy that the molecules posses keeps them always on the move. This motion allows them to
distribute themselves between the two bulbs in an arrangement with the greatest probability of
occurring. That distribution results in equal O
2
or N
2
partial pressures in each bulb with the N
2
and O
2
completely mixed. It is important to note that the energy of the system has not changed, only the way
in which the molecules are distributed; that is, the entropy, S of the system has changed.
On the molecular level, entropy is a measure of randomness; a maximum in entropy corresponds to the
most probable distribution on a statistical basis.
Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
3
www.learnershabitat.ac.in
While the mixing of ideal gases is controlled by entropy, the mixing of molecules that interact with each
other can surely be controlled by energy. In the mixing process the molecular interactions can either
cause the energy to increases or decrease. Chemical changes that are very exothermic show a large
decrease in energy and enthalpy; as a result, they are spontaneous. However, most solutes dissolve in
solvents because mixing increase the systemâ€™s entropy ( ?S > 0); that it is more probable for the
solutes to dissolve in the solvents then for them to remain separate as pure substances.
Toluene in Benzene
These two similar liquids readily dissolve in each other. At first glance we might think this solution
shows the same pattern of mixing as do two ideal gases, but this cannot be correct since the
molecules in a liquid interact with each other very strongly. The key to this solution is that the
molecules of the two liquids have similar electronic structures and similar sizes. As a result, the
benzene   â€“   benzene, benzene   â€“   toluene and toluene   â€“   toluene interactions are very similar. A
toluene molecule, as an approximation, does not know if it is surrounded by other toluene molecule or
by benzene molecules. As we might expect, there is a very negligible heat of solution. These two
liquids completely dissolve one in the other and are an ideal solution. These two liquids completely
dissolve one in what is called an ideal solution. The entropy gained by mixing toluene with benzene is
the same as that gained by mixing two ideal gases. Solvent and solute molecules have no specific
interaction with each other that do not have with molecules of their own kind. As a result the
molecules distribute randomly in the solution and the change in entropy upon mixing is the same as
that for two ideal gases.
Acetone in Chloroform
These two liquids are not very similar; the attraction of acetone and chloroform molecules for each
other is quite different from the attraction of the like molecules for each other. When acetone and
chloroform are mixed heat is given off so they do not form an ideal solution.
Even so, they are completely miscible. In some cases when the molecules in two liquids have quite
different interactions, they may not be completely miscible. For example water and acetone are
completely miscible, but water and chloroform are not. The property of liquids used to characterize
solvent   â€“   solute interactions is called polarity. Polarity measures the small separation of positive and
negative charges in molecules. Water is considered to be the most polar of the common solvents, as
attested by the fact that it is the best solvent for ionic solutes. Acetone has an intermediate value of
polarity, and chloroform is much less polar than acetone. Molecules interact very differently with
molecules of widely different degrees of polarity than they do with others of their own kind. This
difference in interaction leads to immiscibility.
NaCl(s) in H
2
O(
?
)
The compound NaCl has the high melting temperature characteristic of most  ionic solids. It consist
of Na
+
and Cl
â€“
ions. When NaCl dissolves in water, these ions interact with the water molecules
and separate. The interaction with water is particularly large for positively charged ions, and the
Na
+
is surrounded by six closely bound water molecules, Ions surrounded by closely bound water
molecules are said to be hydrated. For salts such as CaCl
2
of AlCl
3
, the water molecules interact
even more strongly with their cations and they give off heat when they dissolve. For NaCl the
energy required to separate the Na
+
and Cl
â€“
ions is nearly balanced by the energy of interaction
between the water and the ions. Salts such as NaNO
3
or NH
4
NO
3
make their solutions colder when
they dissolve to produce hydrated Na
+
,NH
4
+
,
and NO
3
â€“
ions. The entropy of solution for an ionic
solid would never be expected to be as high as its entropy of vaporization; nevertheless, salts such
as NaCl and NaNO
3
dissolve in water because there is a grater probability of finding them in the form
of separated and hydrated ions than packed neatly in a solid. Again entropy is the controlling factor.
H
2
SO
4
(
?
) in H
2
O(
?
)
Almost every chemist knows that H
2
SO
4
gives off a large amount of heat when it dissolves in water.
While pure H
2
SO
4
does not contain ions, as indicted by that fact that it is a liquid at room tempera-
ture, in dilute aqueous solutions it completely ionizes into H
+
and SO
4
2   â€“
. Its high heat of solution
Liquid Solution â€“ Nirmaan TYCRP
LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
4
www.learnershabitat.ac.in
arises largely from the heat of hydration of H
+
. Other proton   â€“   yielding solutes like HCl(g) and
HClO
4
(
?
) also have large, positive heats of solution so they readily dissolve in water. .
RAOULTâ€™S LAW
It states that on adding a non â€“ volatile solute to a volatile solvent, forming an ideal solution,  the
vapour pressure of the solvent is lowered and it is directly proportional to the mole fraction of the
solute in the solution.
Case 1:
Let us add a non â€“ volatile solute that dissolves in the liquid and observe the effect on vapour pressure.
As the non â€“ volatile substance dissolves, some of the particles would be present on the surface of the
liquid, there by decreasing the number of solvent particles present per unit area of the surface.
Key concept:
The vapour pressure of a liquid does not depend on the surface area of the solvent exposed but
only depends on the no. of solvent molecules present per unit area on the surface (which de-
creases only on dissolving a solute).
The rate at which the solvent molecules leave the surface is proportional to the no. of solvent
molecules per unit area on the surface, which in turn is proportional to the mole fraction of the solvent.
Rate of vaporization = k X
solvent
Where k is proportionality constant and X
solvent
is the mole fraction of the solvent in the solution. The
rate with which vapour molecules condense is proportional to their concentration in the gaseous
phase, which is proportional to their partial pressure.
Rate of condensation = kâ€™ P
solvent
Where kâ€™ is proportionality constant and P
solvent
is vapour pressure of the solvent. At equilibrium,
k X
solvent
= kâ€™ P
solvent
(
?
both rates are equal)
or P
solvent
=
k
k '
X
solvent
In case of pure solvent, X
solvent
= 1 and P
solvent
=
k
k '
. If the pure vapour pressure of the solvent is P ?,
then
k
k '
= P ?.
? P = X
solvent
P ?.
This is the mathematical expression of the Raoultâ€™s law.
Case 2:
Let us mix two volatile liquids. Assuming that both of them obey Raoultâ€™s law, P
total
= X
A A
P
?
+ X
B

B
P
?
,
where X
A
is mole fraction of component A in the solution and
A
P
?
is its pure vapour pressure and
likewise for B. This also implies that P
A
= X
A

A
P
?
and P
B
= X
B

B
P
?
, where P
A
and P
B
are the partial
pressures of A and B. In this case, definition of an ideal solution changes. Since both of them obey
Raoultâ€™s law, one cannot assume any one of them to be in lesser amount.
Key concept:
An ideal solution is a solution in which A â€“ A interactions, B â€“ B interactions and A â€“ B interaction
are same (provided that ?H
mix
and ?V
mix
= 0).
Page 5

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LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
1
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CONCEPT OF VAPOUR PRESSURE
When a pure liquid is kept in closed vessel (in which no air is present) the liquid evaporates to give the
vapours. After sometime a dynamic equilibrium is established between liquid and vapours. The pressure
that the vapours exert at equilibrium on the walls of the container or on the surface of the liquid is
called the vapour pressure of the liquid at that temperature, Figure (a).
Case 1:
What would happen to the vapour pressure of the liquid if we take a bigger container, Figure (b) or a
wider container, Figure (c).
Vapour
Liquid
Vapour
Liquid
Vapour
Liquid
Fig. (a)
Fig. (b)
Fig. (c)
Key concept:
When the liquid is in equilibrium with vapours, Liquid  Vapours, the K
p
of the system is K
p
=
P
v
, where P
v
is the vapour pressure. We know that Kp for a given chemical  equilibrium is a constant
and only depends on temperature. Therefore the vapour pressure of the liquid is a constant and
does not depend on the nature of the vessel used and that it only depends on temperature.
Case 2:
Take a vessel with the same liquid as in case 1 and cover it with a glass plate (of negligible weight)
having a hole in it. Would the vapour pressure be the same now?
Vapour
Liquid
Key concept:
Well, the liquid system is the same and the temperature is also same. Therefore the vapour
pressure should be same. The vapour pressure of a liquid is independent of the surface area
exposed on top of the liquid surface provided the surface area exposed should not be zero.
TYPE OF SOLUTIONS
Solutions can have continuously variable compositions, and they are homogeneous on a scale beyond
the size of individual molecules. This definition can be used to cover a wide variety of systems,
LIQUID SOLUTION
LIQUID SOLUTION
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LEARNERS HABITAT EXPERTS Pvt. Ltd.: 97/1, IIIrd Floor, Near NCERT, Adchini, New Delhi, 011-32044009
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including ordinary solution such as alcohol in water or HClO
4
in benzene and even solution of large
proteins in aqueous salt solutions. It is also useful sometimes to consider some colloidal suspensions
undergoing Brownian motion as solutions, and there are solid solutions where one solid is uniformly
dissolved in another.
SOME EXAMPLES OF SOLUTIONS
Solute Solvent Name of Type Heat of Solution per
Mole of Solute (kJ)
O
2(g)
N
2(g)
Gaseous  0
Toluene Benzene Ideal  â€“ 0.1
Acetone Chloroform Non ideal  5
NaCI
(s)
H
2
O
(
Ionic  â€“ 3.9
H
2
SO
4(
H
2
O
(
Ionic  95.3
Positive values are heat released.

Most solutions can be described as having a majority called a solvent and one or more minority
ingredients called solutes. The solvent is usually a liquid, whereas solutes can be solids, liquids, or
gases. Solutions can be distinguished from compounds by the kind of interaction between ingredients.
Compounds form as a result of interactions between relatively permanent partners, while the interac-
tions in solutions involve continuously variable sets of solute and solvent molecules, and this interac-
tion is widely distributed among a large and solvent molecules. Above table gives some examples of
solutions and for each we want to be able to understand why the solute dissolves in the solvent.
O
2(g)
in N
2(g)
This first example is; of course, of one gas dissolved in another all gases dissolve in each other in all
proportions. The reason for this is that solute and solvent do not interact. This is illustrated in fig.
which shows that

2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
N
2
N
2
N 2
N
2
N
2
O 2
O
2
O
2
O
2
O
2
N 2
N
2
N
2
N
2
N
2
O
2
O
2
O
2
O
2
O
(a)
(b)
(c)
Fig. The expansion of gas as illustrated by a solution of N
2
and O
2
(diagrammatic). The expansions of
(a) N
2
and of (b) O
2
are shown to be equivalent to forming (c) a solution of N
2
and O
2
.
the process of two gases dissolving in each other is equivalent to the expansion of each gas into the
larger volume without any interaction with the other. As we can see from this figure and as we also
know from our daily experience, this mixing takes place spontaneously. In a gas, the high thermal
energy that the molecules posses keeps them always on the move. This motion allows them to
distribute themselves between the two bulbs in an arrangement with the greatest probability of
occurring. That distribution results in equal O
2
or N
2
partial pressures in each bulb with the N
2
and O
2
completely mixed. It is important to note that the energy of the system has not changed, only the way
in which the molecules are distributed; that is, the entropy, S of the system has changed.
On the molecular level, entropy is a measure of randomness; a maximum in entropy corresponds to the
most probable distribution on a statistical basis.
Liquid Solution â€“ Nirmaan TYCRP
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While the mixing of ideal gases is controlled by entropy, the mixing of molecules that interact with each
other can surely be controlled by energy. In the mixing process the molecular interactions can either
cause the energy to increases or decrease. Chemical changes that are very exothermic show a large
decrease in energy and enthalpy; as a result, they are spontaneous. However, most solutes dissolve in
solvents because mixing increase the systemâ€™s entropy ( ?S > 0); that it is more probable for the
solutes to dissolve in the solvents then for them to remain separate as pure substances.
Toluene in Benzene
These two similar liquids readily dissolve in each other. At first glance we might think this solution
shows the same pattern of mixing as do two ideal gases, but this cannot be correct since the
molecules in a liquid interact with each other very strongly. The key to this solution is that the
molecules of the two liquids have similar electronic structures and similar sizes. As a result, the
benzene   â€“   benzene, benzene   â€“   toluene and toluene   â€“   toluene interactions are very similar. A
toluene molecule, as an approximation, does not know if it is surrounded by other toluene molecule or
by benzene molecules. As we might expect, there is a very negligible heat of solution. These two
liquids completely dissolve one in the other and are an ideal solution. These two liquids completely
dissolve one in what is called an ideal solution. The entropy gained by mixing toluene with benzene is
the same as that gained by mixing two ideal gases. Solvent and solute molecules have no specific
interaction with each other that do not have with molecules of their own kind. As a result the
molecules distribute randomly in the solution and the change in entropy upon mixing is the same as
that for two ideal gases.
Acetone in Chloroform
These two liquids are not very similar; the attraction of acetone and chloroform molecules for each
other is quite different from the attraction of the like molecules for each other. When acetone and
chloroform are mixed heat is given off so they do not form an ideal solution.
Even so, they are completely miscible. In some cases when the molecules in two liquids have quite
different interactions, they may not be completely miscible. For example water and acetone are
completely miscible, but water and chloroform are not. The property of liquids used to characterize
solvent   â€“   solute interactions is called polarity. Polarity measures the small separation of positive and
negative charges in molecules. Water is considered to be the most polar of the common solvents, as
attested by the fact that it is the best solvent for ionic solutes. Acetone has an intermediate value of
polarity, and chloroform is much less polar than acetone. Molecules interact very differently with
molecules of widely different degrees of polarity than they do with others of their own kind. This
difference in interaction leads to immiscibility.
NaCl(s) in H
2
O(
?
)
The compound NaCl has the high melting temperature characteristic of most  ionic solids. It consist
of Na
+
and Cl
â€“
ions. When NaCl dissolves in water, these ions interact with the water molecules
and separate. The interaction with water is particularly large for positively charged ions, and the
Na
+
is surrounded by six closely bound water molecules, Ions surrounded by closely bound water
molecules are said to be hydrated. For salts such as CaCl
2
of AlCl
3
, the water molecules interact
even more strongly with their cations and they give off heat when they dissolve. For NaCl the
energy required to separate the Na
+
and Cl
â€“
ions is nearly balanced by the energy of interaction
between the water and the ions. Salts such as NaNO
3
or NH
4
NO
3
make their solutions colder when
they dissolve to produce hydrated Na
+
,NH
4
+
,
and NO
3
â€“
ions. The entropy of solution for an ionic
solid would never be expected to be as high as its entropy of vaporization; nevertheless, salts such
as NaCl and NaNO
3
dissolve in water because there is a grater probability of finding them in the form
of separated and hydrated ions than packed neatly in a solid. Again entropy is the controlling factor.
H
2
SO
4
(
?
) in H
2
O(
?
)
Almost every chemist knows that H
2
SO
4
gives off a large amount of heat when it dissolves in water.
While pure H
2
SO
4
does not contain ions, as indicted by that fact that it is a liquid at room tempera-
ture, in dilute aqueous solutions it completely ionizes into H
+
and SO
4
2   â€“
. Its high heat of solution
Liquid Solution â€“ Nirmaan TYCRP
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arises largely from the heat of hydration of H
+
. Other proton   â€“   yielding solutes like HCl(g) and
HClO
4
(
?
) also have large, positive heats of solution so they readily dissolve in water. .
RAOULTâ€™S LAW
It states that on adding a non â€“ volatile solute to a volatile solvent, forming an ideal solution,  the
vapour pressure of the solvent is lowered and it is directly proportional to the mole fraction of the
solute in the solution.
Case 1:
Let us add a non â€“ volatile solute that dissolves in the liquid and observe the effect on vapour pressure.
As the non â€“ volatile substance dissolves, some of the particles would be present on the surface of the
liquid, there by decreasing the number of solvent particles present per unit area of the surface.
Key concept:
The vapour pressure of a liquid does not depend on the surface area of the solvent exposed but
only depends on the no. of solvent molecules present per unit area on the surface (which de-
creases only on dissolving a solute).
The rate at which the solvent molecules leave the surface is proportional to the no. of solvent
molecules per unit area on the surface, which in turn is proportional to the mole fraction of the solvent.
Rate of vaporization = k X
solvent
Where k is proportionality constant and X
solvent
is the mole fraction of the solvent in the solution. The
rate with which vapour molecules condense is proportional to their concentration in the gaseous
phase, which is proportional to their partial pressure.
Rate of condensation = kâ€™ P
solvent
Where kâ€™ is proportionality constant and P
solvent
is vapour pressure of the solvent. At equilibrium,
k X
solvent
= kâ€™ P
solvent
(
?
both rates are equal)
or P
solvent
=
k
k '
X
solvent
In case of pure solvent, X
solvent
= 1 and P
solvent
=
k
k '
. If the pure vapour pressure of the solvent is P ?,
then
k
k '
= P ?.
? P = X
solvent
P ?.
This is the mathematical expression of the Raoultâ€™s law.
Case 2:
Let us mix two volatile liquids. Assuming that both of them obey Raoultâ€™s law, P
total
= X
A A
P
?
+ X
B

B
P
?
,
where X
A
is mole fraction of component A in the solution and
A
P
?
is its pure vapour pressure and
likewise for B. This also implies that P
A
= X
A

A
P
?
and P
B
= X
B

B
P
?
, where P
A
and P
B
are the partial
pressures of A and B. In this case, definition of an ideal solution changes. Since both of them obey
Raoultâ€™s law, one cannot assume any one of them to be in lesser amount.
Key concept:
An ideal solution is a solution in which A â€“ A interactions, B â€“ B interactions and A â€“ B interaction
are same (provided that ?H
mix
and ?V
mix
= 0).
Liquid Solution â€“ Nirmaan TYCRP
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Case 3:
Is the vapour pressure of a solution of a non   â€“   volatile solute in a volatile liquid also a constant?
As we know that the concentration of a solution is not a constant in this case. Therefore, this system
does not have a K
P
. It means that the vapour pressure of a solution is not a constant. To make the
calculations involving liquid solution does not effectively change the concentration of the solution.
Key concept:
The system of solution â€“  vapour also has a constant K
P
and therefore a constant vapour pres-
sure.
COLLIGATIVE PROPERTIES
These are the properties of solutions which depend upon the relative number of solute particles
(molecules or ions) but not upon their nature. These are relative lowering of vapour pressure, elevation
of boiling point, depression in freezing point and the osmotic pressure arising due to the presence of a
non   â€“   volatile solute.
Key concept:
In dilute solutions these properties depend only on the number of solute particles present and
not on their identity. For this reason they are called colligative properties (denoting depending
on the collection).
We shall assume throughout the following that.
(i) The solute is non   â€“   volatile.
(ii) The solute does not dissolve in the solid solvent (the pure solid separates when the solution is frozen).
RELATIVE LOWERING OF VAPOUR PRESSURE
According to the Raoultâ€™s law, P
solvent
= X
solvent

solvent
P
?
,
Where P
solvent
is the vapour pressure of the liquid solution, X
solvent
is its mole fraction in the solution
and
solvent
P
?
is the pure vapour pressure.
?
?
?
solvent
solvent
solvent
P
X
P
1    â€“    X
solvent
= 1    â€“    ?
solvent
solvent
P
P
=
?
?
?
solvent solvent
solvent
P P
P
or X
solute
=
?
?
?
P P
P
Above expression is called as the relative lowering of vapour pressure (fraction of the vapour pressure
lowered).  Thus, the relative lowering of vapour pressure of a solvent due to the dissolution of a non-
volatile solute in it is equal to the molefraction of the solute in the solution. This is another form of the
Raoultâ€™s law.
Key concept:
The molefraction of any substance does not change with temperature. So the molefraction of
the solute (X
solute
) does not change with temperature. Then, according to above expression the
relative lowering of the vapour pressure should be independent of temperature. This is found
to be so only for dilute solutions. Therefore, the Raoultâ€™s law, in this form, is applicable to the
dilute solutions of non ?volatile and non ?electrolytic solutes only.
OSTWALD â€“ WALKER EXPERIMENT
When air is passed through any gas, the gas diffuses into the air due to the principle of diffusion till the
pressure of the gas in the air is equal to that of the gas outside. Therefore as air is passed through the
solution, it absorbs the vapours of the solvent till the pressure of the solvent vapours in the air is P
(vapour pressure of solvent in the solution).
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