The State of Matter
Characteristics of matter:
It has mass and it takes up space.
Matter has the property of inertia.
It takes force to change the state of motion of matter.
The states of mater
There are three different States (forms) of matter:
Solid : Solids have definite volume and shape.
Liquid : liquids have definite volume but no definite shape. Liquids take on the shape of their container.
Gas : Gases has neither definite volume nor definite shape. They fill up whatever space they are put in.
The fourth state of matter is called plasma. Although this state is not common in earth and other planets most of the matter of the universe in the form of plasma. Plasma consists of high energy electrically charged particles and are some what like gases—they do not have definite volume or shape.
They state of matter depends on its temperature. At cold enough temperatures, almost all materials become solids. As they warm they can become liquids. At still higher temperatures they may become gases. At very high temperature they may become plasmas.
The kinetic theory of matter
According to kinetic theory of matter, all matter is made of tiny particles. These particles are in constant motion. The higher the temperature, the faster the motion. The motion and spacing of the particles determines the state of matter.
The particles of solid are packed very close together. Forces between the particles (intermolecular forces) hold them together in fixed arrangement. In solids, the particles move back and forth, but do not change positions.
Solid Liquid Gas
The particles of liquid are also very close to one another. However these particles have enough energy, so that the forces between them cannot hold them fixed in any position. The particles are free enough to slip past each other.
The particles in a gas have so much energy that the forces between them are insignificant. As a result the particles move in straight lines flying all over the container. A gas is mostly empty space. The particles change their direction only when they strike the walls of their containers or other particles.
Thermal expansion :
When heat is added or work is done on matter, its thermal energy increases. The kinetic and potential energy of its particles increase. As a result the particles move faster and collide with other particles more violently. Thus the particles move farther apart, and the volume of matter increases. The increase of volume of matter as the temperature goes up is called thermal expansion.
The nature of gas
Pressure of a gas :
According to kinetic theory of gases, the particles of a gas are in constant motion. As they move about, they strike the surface of their container. Every time they hit the surface and bounce off again, they exert a tiny force. The combined effect of very large number of particles (~ 2.64 x 1021 per liter) constantly bouncing off the surface causes the pressure of the gas.
Kinetic theory of gas:
The kinetic theory makes the following assumptions about a perfect gas.
(i) Molecules of a perfect gas move in straight lines at very great velocities until they collide with each other or with the wall of the container. The number of collision per unit time being very great, the pressure appears constant (at constant temperature).
(ii) The total volume of actual gas molecules is negligible compared to the capacity of the container.
(iii) The forces of attraction between the gas molecules are negligible.
(iv) The average kinetic energy of the gas molecules measures the temperature of the gas.
The gas laws
Boyle’s law: (1662,Relation between volume and pressure of gas)
“ The volume of a given mass of gas is inversely proportional to the pressure, if temperature remains constant.”
That is, at a constant temperature if the pressure on a gas is increased its volume will decrease in the same proportion.
In mathematical form
Or [ where K1 is a constant of proportionality]
Or,
Or,
Let us consider a cylindrical vessel of volume V contains a gas which is exerting a pressure P on the piston of the cylinder.
Now suppose that the pressure on the piston is suddenly reduced to ½ P, without temperature change. The piston will move upward. As it does so the gas will fill the greater volume of the now available and volume will be doubled. Therefore the following relation will hold good
Boyle’s law can be explained in terms of the kinetic theory of gas. If the volume of given sample of gas is reduced at constant temperature, the average kinetic energy of the molecules remains constant so they collide more frequently with the walls of the smaller container. The more frequent collisions cause higher pressure.
Charles’ law : (1787, effect of temperature on the volume of a gas)
“The volume of a given mass of a gas is directly proportional to its Kelvin temperature if pressure remains constant.”
That is, at constant temperature, the volume of a gas will increase in the same proportion as its Kelvin temperature increases.
Or [ where K1 is a constant of proportionality]
Or,
Or,
Standard temperature and pressure Standard temperature 0 0C (273 K)
Standard pressure 760 mm of Hg
Combination of Boyle’s and Charles’ law
From Boyle’s law
[at constant T]
From Charles’ law
[at constant P ]
Gay-Lussac's law (1809, relating pressure and temperature):
The pressure of a fixed amount of gas at fixed volume is directly proportional to its temperature in kelvins. i.e. P/T = K
These three laws were combined to form the combined gas law
Ideal Gas Law
With the addition of Avogadro's law, this gave way to the ideal gas law-
,
i.e. “The state of an amount of gas is determined by its pressure, volume, and temperature”
where, P is the pressure (SI unit: pascal)
V is the volume (SI unit: cubic metre)
n is the number of moles of gas
R is the ideal gas constant (SI: 8.3145 J/(mol K))
T is the temperature (SI unit: kelvin).
The ideal gas law is the most accurate for monoatomic gases at high temperatures and low pressures. This follows because the law neglects the size of the gas molecules and the
Ideal gases
Ideal gas is a hypothetical gas that obeys the gas laws exactly. An ideal gas would consist of molecules that occupy negligible space and have negligible forces between them.
Avogadro’s law (1811)
Equal volumes of all gases contain equal numbers of molecules (Avogadro’s no., 6.023 1023) at the same pressure and temerature. The law, often called Avogadro’s hypothesis, is true only for ideal gases.
S.T.P.- Standard temperature and pressure (or formerly known as N.T.P.- normal temperature and pressure) are used when comparing the properties of gases. They are 273 K (or 00C) and 101 Pa (or 760 mmHg).
Exercise 1. A certain mass of a gas occupies 211 cm3 at 180C and 740 mm Hg pressure. What volume will it occupy at – 20 0C and 770 mm Hg pressure?
Exercise 2. A certain mass of a gas occupies 146 cm3 at 20 0C and 754 mm Hg pressure. Calculate its volume at S.T.P. ( standard temperature and pressure) ( ( s.t. = 273 K , s.p. = 760 mm Hg)
