Properties of liquids #

Anything which is occupying space and mass is called as matter.Matter can exist in solid,
liquid and gaseous state.Matter also undergo interconversion of states from one state to
another. Melting is a good example of that.

Liquid state of matter has the following properties:

• In chemistry, liquids are substances in a slightly mobile state of matter.

• It has fixed volume but not fix shape and size.
• Liquids are more compressible in nature than solids and less compressible than
gases.
• Liquids have tendency to flow.
• It has moderate level of intermolecular forces, space, kinetic energy. Water is an
example of liquid.
• Liquids have their boiling points above room temperature, under normal conditions.
The liquids on heating slowly changes to vapor or gaseous phase. This process is
called boiling.

Surface #
The surface of a material, whether solid or liquid, exhibits distinct physical and chemical
properties compared to its bulk due to the unique arrangement and exposure of atoms or
molecules at the surface. These surface properties are crucial in various applications, such
as catalysis, adsorption, and materials science.

Chemical Properties of Surfaces:

• Surface Reactivity:

Surfaces often have unpaired electrons, dangling bonds, or unsaturated atoms, making
them chemically reactive. This is critical in processes such as catalysis, corrosion, and
surface modification.

• Catalytic Activity:

Many reactions occur on the surface of materials, especially in heterogeneous catalysis.

The catalytic activity of a surface depends on its structure, composition, and energy,
allowing it to facilitate chemical reactions without being consumed.

• Adsorption:
Surfaces can adsorb gases, liquids, or other solids. Adsorption occurs due to physical
interactions (physisorption) or chemical bonding (chemisorption) between the surface
atoms and adsorbate molecules.Adsorption is vital for catalysis, sensors, and filtration
processes.

Physical Properties of Surfaces:

• Surface Energy:

Surfaces have higher energy than the bulk material because atoms at the surface are not
fully surrounded by neighboring atoms, leading to unsaturated bonds.This excess energy is
known as surface energy or surface tension (for liquids).Higher surface energy implies that
a surface is more reactive or prone to interactions with other substances (e.g., adsorption
or catalysis).

• Surface Roughness:

The physical texture of a surface, characterized by microscopic peaks and valleys, affects
how a surface interacts with other materials, such as adhesion, friction, and wetting
behavior.Rough surfaces can have a larger effective surface area, enhancing reactivity or
adsorption capacity.

• Surface Area:

Surface area is the total area exposed by the surface of a material. High surface area is
critical for applications like catalysis and adsorption, where more surface interaction leads
to better performance.

Determination of Surface Area :

The determination of surface area is a critical process in materials science, catalysis, and
adsorption studies, as it helps in understanding how much surface is available for
interactions such as chemical reactions or adsorption. The surface area of a material,
especially porous or powdered substances, plays a vital role in its reactivity, adsorption
capacity, and overall performance in various applications. Several methods are employed
to measure surface area, each tailored to the nature of the material under investigation.

BET (Brunauer-Emmett-Teller) Method :

The BET method is the most commonly used technique for determining the surface area of
porous and powdered materials. It extends the Langmuir theory to multilayer adsorption
and provides a more accurate estimation of surface area.
Principle: The BET method involves the physical adsorption of a gas (typically nitrogen)
onto the surface of the material at low temperatures. As the gas molecules adhere to the
surface, they form a monolayer, followed by additional layers as pressure increases. By
measuring the quantity of gas adsorbed at different pressures, the total surface area can
be calculated.The BET equation is used to model the relationship between the pressure
and the volume of adsorbed gas:

The BET equation Is used to model the relationship between the pressure and the volume
of adsorbed gas: P = 1 + C -1. . P

V(P°- P). VmC. VmC. P°

P = equilibrium pressure , P° = saturated pressure of absorbed gases , V = volume , Vm=

volume of gas required to form a monolayer of absorbent. C= bet constant.

Absorption :
Absorption is a physical or chemical phenomenon or a process in which atoms, molecules
or ions enter the liquid or solid bulk phase of a material. This is a different process from
adsorption, since molecules undergoing absorption are taken up by the volume, not by the
surface (as in the case for adsorption). A bulk phenomenon. Endothermic process.

Adsorption :
Adsorption is the adhesion of molecules (or ions and atoms) to the surface of a solid or
liquid. The molecules accumulate only at the surface and do not enter the bulk of the
adsorbing material.The substance whose molecules get adsorbed at the surface is called
the adsorbate.The substance on whose surface the process takes place is called the
adsorbent.It is a surface phenomenon and an exothermic process.

Adsorption Isotherm :Adsorption isotherms have been of immense importance to

research dealing with environmental protection and adsorption techniques. The two
primary methods used for predicting the adsorption capacity of a given material are known
as the Freundlich and Langmuir isotherms.

As we know from Le Chatelier’s principle, the direction of equilibrium in a reaction shifts in

the direction in which stress is relieved. So, here we can see that upon application of
excess pressure on the system, the equilibrium shifts in the direction where the number of
molecules decreases so that the pressure in the system decreases.An adsorption isotherm
is a graph that represents the variation in the amount of adsorbate(x) adsorbed on the
surface of the adsorbent with the change in pressure at a constant temperature
Types of Adsorption #
Due to the force of interaction between adsorbate and adsorbent, adsorption in surface
chemistry is classified into two types.

1. Physical Adsorption or Physisorption :

There exists a weak van der Waals force between adsorbate and adsorbent.Adsorption will
decrease by increasing temperature.This adsorption is a multi-layered process.Examples:
H2 and N gases adsorb on coconut charcoal.

Characteristics:

• Nature of forces: Weak van der Waals forces

• Specificity: It is not specific in nature
• Reversibility: The process is reversible
• Layer: It is a multi-layered process
• Enthalpy of adsorption: Low enthalpy of adsorption [ 20 – 40 KJ/mole ]
• The energy of activation: Very low
• Desorption: Very easy
• Factors affecting: Surface area of adsorbent nature of adsorbate, pressure,
temperature.
2. Chemical Adsorption or Chemisorption :

It is due to strong chemical forces between adsorbate and adsorbent.This type of

adsorption is almost a single-layered phenomenon.Adsorption increases by an increase in
temperature.Examples: Charcoal, Silica gel, Alumina.

Characteristics:

• Nature of forces: Strong chemical forces

• Specificity: Highly specific nature
• Reversibility: It is irreversible
• Layer: It is a single-layered process
• Enthalpy of adsorption: High enthalpy of adsorption [40 – 400 KJ/mole]
• The energy of activation: Very high
• Desorption: Very difficult
• Factors affecting: Surface area of adsorbent, nature of adsorbate temperature.

Classification of Colloids #
Colloid :
A colloid is primarily a heterogeneous mixture in which the minute particles of one
substance are dispersed in another substance, called the dispersion medium.The minute
particles here are 1 to 1000 nanometers in diameter but they still remain suspended and do
not settle at the bottom of the mixture. They are visible under an optical or an electron
(smaller particles) microscope.

Types of Colloids :
Colloids can be classified according to different properties of the dispersed phase and
medium.Firstly, based on the types of particles of the dispersed phase, colloids can be
classified as:

1. Multimolecular Colloids

When the dissolution of smaller molecules of substance or many atoms takes place, they
combine to form a species whose size is in the range of colloidal size. The species formed
is known as the multimolecular colloid.For example, the Sulphur solution contains
particles which have thousands of S8.

2. Macromolecular Colloids

In this type of colloid, the macromolecules form a solution with a suitable solvent. The size
of the particles of this macromolecular solution lies in the range of colloidal particle size.
Thus, this solution is also known as the macromolecular colloids. The colloids formed here
are similar to that of the actual solution in many respects and are very stable.Example:
Starch, proteins, enzymes, and cellulose are the naturally occurring macromolecular
colloids whereas polyethene, synthetic rubber, etc. are the synthetic macromolecules.

3. Associated Colloids

Some substances act as a strong electrolyte when they are in low concentrations, but they
react as colloidal sols when they are in high concentration. In higher concentration,
particles aggregate showing colloidal behaviour. These aggregated particles are known as
the micelles. They are also known as the associated colloids. The formation of the micelles
occurs above a particular temperature called the Kraft temperature (Tk) and also above a
specific concentration called critical micelle concentration. These colloids can be
reverted by diluting it. E.g of some associated colloids are soaps and synthetic detergents.

Preparation of Colloids :
The stable colloids are known as lyophilic sols, in these, the strong forces of attraction take
place between the dispersed phase and dispersion medium. Some of the major methods
to prepare colloids are as follows:
 3. Condensation Method :

Small solute particles are condensed in this process to create a dispersed phase particle.

Oxidation: We can obtain colloidal Sulphur by passing oxygen gas through the solution of
Hydrogen Sulphides. HNO3, H3Br2, etc are used as oxidizing agents in this process

2H2S + O2 ⇢ 2H2O + 2S

Reduction: In the process, suitable reducing agents such as formaldehyde, hydrogen

peroxide, stannous chloride, etc are reacted with the aqueous solution of these salts to
obtain metals like gold, silver, and platinum in the colloidal state.

2AuCl3 + 3SnCl2 ⇢ 3SnCl4 + 2Au

Hydrolysis: Salt solutions are hydrolyzed by boiling their respective dilute solutions. For
example, ferric hydroxide is obtained by hydrolysis of its corresponding salt.

FeCl3 + 3H2O ⇢ Fe(OH)3 + 3HCl

4. Dispersion Methods :

Large particles of material (suspension) are broken down into smaller particles using these
procedures. The procedures listed below are used:

Mechanical Dispersion: In this method, the substances are grounded to coarse particles
and mixed with a dispersion medium to get a suspension. Then, it is ground in a colloidal
mill consisting of two metallic dyes rotating in opposite directions. This method is used to
obtain colloidal solutions of black ink paint varnished dyes, etc.

Properties of Colloids :
There are many physical and chemical properties colloids have, some of those properties
are as follows:

• As the size of particles is very small (1-1000 nanometers), they can’t be filtered using
the basic filtration method. To filter colloids, we need more specialized filtration
methods, such as ultrafiltration or centrifuge.
• Colloids are very stable. And when left untouched for long periods it doesn’t show
the sign of sedimentation or precipitation.
• Colloids are heterogeneous mixtures, where the dispersed medium is suspended in
the dispersion phase in nor uniform arrangement.
 • Despite the heterogeneous nature of colloids, they seem to have a homogenous
appearance. This is because the particle is very small in colloids which can’t be
seen by the naked eye.

Surfactants :
Surfactants are a category of chemical compounds that are used in lowering the surface
tension (or interfacial tension) between different compounds, such as two liquids or
between a gas and a liquid, or it can also be between a liquid and a solid. Surfactants are
categorised as organic compounds and are amphiphilic in nature. It basically means that
they contain both hydrophobic and hydrophilic groups.In other words, a surfactant has
both a water-insoluble component and a water-soluble component. One of the common
properties of surfactants is that they will diffuse in water and adsorb at interfaces between
air and water.

Phase rule :

The phase rule, formulated by Josiah Willard Gibbs, Is a principle in thermodynamics that
provides a way to determine the number of degrees of freedom (F) in a system at
equilibrium. The rule is particularly useful in the study of multi-component systems, such
as those encountered in physical chemistry and materials science.

Gibbs Phase Rule :

Gibbs phase rule states that if the equilibrium in a heterogeneous system is not affected by
gravity or by electrical and magnetic forces, the number of degrees of freedom is given by
the equation ;

F=C-P+2

Where, C is the number of chemical components,P is the number of phases.Basically, it

describes the mathematical relationship for determining the stability of phases present in
the material at equilibrium conditions.

Phase Rule Derivation :

Gibbs phase rule on the basis of the thermodynamic rule can be derived as follows:First,
let us consider a heterogeneous system consisting of Pn number of phases and Cn number
of components in equilibrium. Let us assume that the passage of a component from one
phase to another doesn’t involve any chemical reaction. When the system is in equilibrium,
it can be described by the following parameters:

• Temperature
 • Pressure
• The composition of each phase

The total number of variables required to specify the state of the system is:

• Pressure: same for all phases

• Temperature: same for all phases
• Concentration

The independent concentration variable for one phase with respect to the C components is
C – 1. Therefore, the independent concentration variables for P phases with respect to C
components is P (C – 1).Total number of variables = P (C – 1) + 2….. (1)

The total number of equilibria:

The various phases present in the system can only remain in equilibrium when the
chemical potential (µ) of each of the component is the same in all phases, i.e ;

µ1, P1 =µ1, P2 =µ1, P3 =. …. =µ1, P

The number of equilibria for each P phases for each component is P – 1.For C components,
the number of equilibria for P phases is C ( P – 1).Hence, the total number of equilibria
involved is E = C (P – 1)… (2)

Equating eq (1) and (2), we get ;

The obtained formula is the Gibbs phase rule. Stay tuned to BYJU’S to learn more physics
derivations.

One-Component System (Theory) :

A one-component system is a thermodynamic system that contains only a single chemical
substance. In this system, the substance can exist in different phases (solid, liquid, or gas),
depending on the conditions of temperature and pressure. The phase behavior of a one-
component system can be represented on a phase diagram, which plots pressure against
temperature, highlighting the boundaries where phase changes occur.

Example of a One-Component System: Water (H₂O)

Water is a classic example of a one-component system. It can exist in three distinct
phases: ice (solid), water (liquid), and steam (gas). The phase diagram for water shows how
the system transitions between these phases depending on temperature and pressure.

• Solid Phase (Ice): Exists below 0°C at 1 atm pressure.

• Liquid Phase (Water): Exists between 0°C and 100°C at 1 atm pressure.
• Gas Phase (Steam): Exists above 100°C at 1 atm pressure.
• Triple Point: The triple point for water is at 0.01°C and 0.00604 atm. At this
condition, ice, water, and vapor can coexist in equilibrium.
• Critical Point: At 374°C and 218 atm, water reaches its critical point, where it
becomes a supercritical fluid.

Two-Component System (Theory) :

A two-component system consists of two chemically distinct substances, and the phase
behavior is determined not only by temperature and pressure but also by the composition
of the mixture. These systems are typically studied using binary phase diagrams, which
illustrate the equilibrium phases at various compositions and temperatures.The interaction
between the two components affects the system’s overall phase behavior. The two
components may form solutions, eutectic mixtures, or compounds depending on their
nature.

Example of a Two-Component System: Saltwater (H₂O + NaCl) :

• A simple example of a two-component system is saltwater, where water (H₂O) is the
solvent and sodium chloride (NaCl) is the solute.
• Effect on Freezing Point: The addition of NaCl to water lowers the freezing point of
the system. This is known as freezing-point depression.
• Eutectic Point: In the case of the saltwater system, the eutectic point occurs when
the concentration of NaCl is high enough that both the salt and water solidify at the
same temperature.