ACH 103: INTRODUCTION TO ANALYTICAL CHEMISTRY
UNIT1: THE GASEOUS STATE
1.1: Introduction
The State of Matter
Matter exists in any one of the following state (phases); solid, liquid and gas. The three are clearly
convertible as in ice, water and steam, which represent the same substance in different phases. But
what makes them different? In the solid state, particles are fixed in a uniform manner in definite
positions in a crystal lattice by strong forces operating between them. As a consequence, solids
are fairly rigid, have a definite shape and volume and resist compression or distortion. Only
vibrational motion is allowed for the solid particles. This vibrational motion is dependent on the
temperature, thus the higher the temperature the longer the distance of vibration hence the weaker
the forces between the particles. At a given temperature these forces become so weak that the
solid structure breaks down, i.e. melts to form a liquid.
In the liquid state the particles can move freely i.e. posses translational motion, with only weak
forces operating between the particles as compared to solids, which are however not weak enough
to allow complete separation of the particles from one another. Therefore a liquid has a definite
volume but takes the shape of the container holding it i.e. no definite shape. These weak forces
depend on the temperature too. Thus the higher the temperature the faster the particles become
and therefore the weaker the forces holding them together. As in solids, at a certain temperature
the forces become so weak that the liquid state breaks down, i.e. boils to form a gas.
In the gaseous state the restraining forces of attraction have been overcome completely such that
the particles move in a completely random manner at high speeds. Hence a gas has no definite
shape or volume; fills the container holding it and is thus highly compressible. The sensitivity to
changes in volume with changes in pressure and temperature is therefore:
Solid<Liquid<<Gas
Clearly the importance of the relationship between volume, temperature and pressure is only
important for the gaseous state and will be our subject in this chapter.
1.3: THE GAS LAWS
Experiments with a large number of gases reveal that four variables are usually sufficient to
define the state or condition, of a gas: temperature, T, pressure, P, volume, V and the quantity of
the gas, which is usually expressed as the number of moles, n. The equations that express the
relationship among P, T, V and n are known as the gas laws or equations of state.
1.3.1 The Pressure-Volume Relationship: Boyle's Law
The first person to investigate the relationship between the pressure of a gas and its volume was
the British Chemist Robert Boyle (1627-1691).
, A quantity of gas is trapped in the tube behind a column of mercury. Boyle changed the pressure
on the gas by adding mercury to the tube. He found that the volume of the gas decreased as the
pressure increased. The pressure exerted on a confined gas is varied and the change in volume is
recorded. A relationship between pressure and volume is determined. Pressure is the manipulated
variable and volume is the responding variable. The temperature and amount of gas are controlled
variables.
For example, doubling the pressure caused the gas volume to decrease to one-half its original
value. Plots of pressure against volume at a constant temperature (isotherms) at two different
temperatures are shown in Fig.1.2.
Boyle’s law states that:
'At a constant temperature, the volume of a definite mass of a gas is inversely
proportional to pressure'
When two measurements are inversely proportional, one gets smaller as the other gets larger. Thus
when volume increases, pressure decreases. To perform his gas experiment, Boyle used a J-shaped
tube like that shown in Figure 1.1
1
From the figure, as volume increase pressure decrease i.e. V
P
Boyle's law can be expressed in mathematical terms.
1
V = cons tan t x or PV = constant 1.1
P
Teaching Note: The equation PV = constant describes a hyperbola.
UNIT1: THE GASEOUS STATE
1.1: Introduction
The State of Matter
Matter exists in any one of the following state (phases); solid, liquid and gas. The three are clearly
convertible as in ice, water and steam, which represent the same substance in different phases. But
what makes them different? In the solid state, particles are fixed in a uniform manner in definite
positions in a crystal lattice by strong forces operating between them. As a consequence, solids
are fairly rigid, have a definite shape and volume and resist compression or distortion. Only
vibrational motion is allowed for the solid particles. This vibrational motion is dependent on the
temperature, thus the higher the temperature the longer the distance of vibration hence the weaker
the forces between the particles. At a given temperature these forces become so weak that the
solid structure breaks down, i.e. melts to form a liquid.
In the liquid state the particles can move freely i.e. posses translational motion, with only weak
forces operating between the particles as compared to solids, which are however not weak enough
to allow complete separation of the particles from one another. Therefore a liquid has a definite
volume but takes the shape of the container holding it i.e. no definite shape. These weak forces
depend on the temperature too. Thus the higher the temperature the faster the particles become
and therefore the weaker the forces holding them together. As in solids, at a certain temperature
the forces become so weak that the liquid state breaks down, i.e. boils to form a gas.
In the gaseous state the restraining forces of attraction have been overcome completely such that
the particles move in a completely random manner at high speeds. Hence a gas has no definite
shape or volume; fills the container holding it and is thus highly compressible. The sensitivity to
changes in volume with changes in pressure and temperature is therefore:
Solid<Liquid<<Gas
Clearly the importance of the relationship between volume, temperature and pressure is only
important for the gaseous state and will be our subject in this chapter.
1.3: THE GAS LAWS
Experiments with a large number of gases reveal that four variables are usually sufficient to
define the state or condition, of a gas: temperature, T, pressure, P, volume, V and the quantity of
the gas, which is usually expressed as the number of moles, n. The equations that express the
relationship among P, T, V and n are known as the gas laws or equations of state.
1.3.1 The Pressure-Volume Relationship: Boyle's Law
The first person to investigate the relationship between the pressure of a gas and its volume was
the British Chemist Robert Boyle (1627-1691).
, A quantity of gas is trapped in the tube behind a column of mercury. Boyle changed the pressure
on the gas by adding mercury to the tube. He found that the volume of the gas decreased as the
pressure increased. The pressure exerted on a confined gas is varied and the change in volume is
recorded. A relationship between pressure and volume is determined. Pressure is the manipulated
variable and volume is the responding variable. The temperature and amount of gas are controlled
variables.
For example, doubling the pressure caused the gas volume to decrease to one-half its original
value. Plots of pressure against volume at a constant temperature (isotherms) at two different
temperatures are shown in Fig.1.2.
Boyle’s law states that:
'At a constant temperature, the volume of a definite mass of a gas is inversely
proportional to pressure'
When two measurements are inversely proportional, one gets smaller as the other gets larger. Thus
when volume increases, pressure decreases. To perform his gas experiment, Boyle used a J-shaped
tube like that shown in Figure 1.1
1
From the figure, as volume increase pressure decrease i.e. V
P
Boyle's law can be expressed in mathematical terms.
1
V = cons tan t x or PV = constant 1.1
P
Teaching Note: The equation PV = constant describes a hyperbola.
