GASES AND LIQUIDS
Chapter 12
CHARACTERISTICS OF GASES
The properties of gases can be explained by a
model called the kinetic molecular theory. This
theory is based on some major assumptions about
a theoretical gas often called as an ideal gas.
 An ideal gas is defined as one in which all
collisions between atoms or molecules are
perfectly eleastic and in which there are no
intermolecular attractive forces.
 The equation for ideal gas law:
 PV=nRT
 n = number of moles V= volume; T=
temperature in K.
 R = universal gas constant = 8.3145 J/mol K

PROPERTIES OF GASES
Gases are fluids: They flow just as liquids do.
Gases transmit and exert pressure equally in all
directions. Gas molecules are in constant motion
and frequently collide with one another and with
the walls of the container.
 Gases have low density: Compared with
liquids and solids gases have a very low density.
 Gases are highly compressible: If you fill a
syringe with liquid and try to push the plunger in
when the opening is plugged . Nothing happens.
It takes enormous pressure to reduce the volume
of a liquid or a solid. However if there is gas or
mixtures

of gases, such as air, in the syringe, it does not
take much pressure to move the plunger down
and compress the gases.
 Gases completely fill a container and exert
pressure equally in all directions: A solid has
a distinctive shape and volume. A liquid has a
distinctive volume but assumes the shape of its
container. In contrast, a gas fills the entire
container and exerts pressure on the container in
all directions. This pressure created is because of
the molecules colliding with each other.


The temperature of a gas determines the
average kinetic energy of its particles: The
kinetic energy of a gas molecule can be expressed
using the kinetic energy equation that relates
kinetic energy to one half the mass, m,multiplied
by the square of the speed,v,

KE = ½ mv²
WHAT IS PRESSURE?
It is the force on a surface divided by the area of
that surface. Pressure= force/area
 In SI unit force is measured in newtons (N) and
the unit of area is (m²)Therefore the unit of
pressure is Pascal(Pa). 1 pascal= 1N/m²

WHAT IS STP?
STP is Standard conditions of temperature and
pressure. To study the effects of changing
temperature and pressure on a gas, it is useful to
have a standard for comparison. Scientists have
specified a set of standard conditions called
standard temperature and pressure or STP.
 STP= 0⁰C and 1 atm

HOME WORK
Define the terms:
 A) Newtons
 B) Pascal
 C)Pressure

THE GAS LAWS
Boyle’s Law: Boyle's law describes the
relationship between the product, pressure and
volume within a closed system as constant when
temperature remains at a fixed measure; both
entities remain inversely proportional.The law
was named for chemist and physicist, Robert
Boyle who published the original law in 1662.
The law itself can be defined as: P1V1=P2V2
 “ For a fixed amount of gas kept at a fixed
temperature, P and V are inversely proportional
(while one increases, the other decreases). ”

CLASS PRACTICE

Page 435 sample problem 12A
Dalton’s Law Of Partial Pressure: The total
pressure in a gas mixture is the sum of the
partial pressures of the individual components,
each behaving as the other gases were absent.
 Ptotal = PA+PB +Pc
 Ptotal is the total pressure of the mixture. PA,PB
and Pc are the partial pressures of the mixture’s
component gases, A,B, and C. If A, B, and C are
the components of a gas mixture, then the mole
fraction of gas A is represented mathematically
as mol fraction= mol A/mol A+ B+C

Charles Law: The volume of a sample of gas at
constant pressure is directly proportional to the
absolute temperature. V/T=k
 V1/T1=V2/T2

CLASS PRACTICE

A sample of gas occupies 24m3 at 175 K. What
volume would the gas occupy at 400K?

Avogadro’s Law: Avogadro proposed that equal
volumes of different gases under the same
conditions of temperature and pressure have the
same number of molecules.

Gay Lussac’s law: At constant temperature and
pressure, gases react in volume proportions that
are whole numbers . For example, consider the
formation of gaseous hydrogen chloride from the
reaction of hydrogen gas and chlorine gas. One
volume of chlorine reacts with one volume of
hydrogen to form two volumes of hydrogen
chloride.
EFFUSION AND DIFFUSION
The passage of a gas through a small opening,
called effusion. Effusion occurs when there is a
small leak in the tire. The air in the tire effuses
through the hole. At constant temperature and
pressure the rate of effusion of a gas is inversely
proportional to the square root of the molar mass,
M. This is stated mathematically when
comparing the speed of effusion,v, of two gases, A
and B,
 vA/vB =√MB/MA
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CLASS PRACTICE
Page 444
 Sample 5,6,7,8
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HOW DO THE GAS LAWS FIT TOGETHER?


The gas laws for a given quantity of gas that is
changed from one set of P, V, or T conditions to
another.
A sample of carbon dioxide with a mass of 0.250 g
is placed in a 350 ml container at 400K. What is
the pressure exerted by the gas?
CLASS PRACTICE
Page 447
 Practice samples do all.


A weather balloon containing helium with a
volume of 410 L rises in the atmosphere and is
cooled from 27⁰C to -27⁰C. The pressure on the gas is
reduced from 110 kPa to 25 kPa. What is the volume of
the gas at lower temperature and pressure?
HOME WORK
Page 449
 2 and 4.
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GAS STOICHIOMETRY
How many liters of hydrogen gas will be
produced at 280 K and 96 kPa if 40 g of sodium
react with excess water according to the following
equation?
 2Na(s) + 2H202NaOH(aq)+H2(g)

CLASS PRACTICE
Page 451
 10,11, 13.

PHASE DIAGRAM
What is phase?
 Any part of a system that has uniform
composition and properties.
 A phase diagram is a graph that shows the
temperatures and pressures at which a substance
exists in different phases. The phases are in
equilibrium with each other along the lines of the
diagram.

Normal boiling point: The temperature at
which a substance boils at 1,0000 atm pressure.
 Critical point: The temperature and pressure at
and above which the properties of the vapor
phase of a substance cannot be distinguished
from those of the liquid phase.
 Sublimation: A change in state where a solid is
changed into a gas without going through the
liquid state.
 Triple point: the temperature and pressure at
which three phases of a substance exist in
equilibrium.

HOME WORK
Page 463
 47, 49,52 and 53.
 Test prep all
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