STATES OF MATTER

             1. Introduction

Most of the observable  characteristics of chemical systems with which we are 

familiar represent bulk properties of matter, i.e., the  properties associated with a collection of a large number of atoms, ions or molecules. For example, an individual molecule of a liquid does not boil but the bulk boils. Collection of water molecules have wetting properties; 

individual molecules do not wet. Water can exist as ice,  which is a solid; it can exist as liquid; or it can exist in the gaseous state as water vapour or steam. Physical properties of ice, water and steam are very different. In all the three states of water chemical composition of water remains the same i.e., H2O. 

Characteristics of the three states of water depend on the energies of molecules and on the manner in which water molecules aggregate. Same is true for other substances also. Chemical properties of a substance do not change with the change of its physical state; but rate of chemical reactions do depend upon the physical state. Many times 

in calculations while dealing with data of experiments we  require knowledge of the state of matter. Therefore, it becomes necessary for a chemist to know the physical laws which govern the behaviour of matter in  different states. In this unit, we will learn more about these three physical states of matter particularly liquid and gaseous states. 

To begin with, it is necessary to understand  the nature of intermolecular forces, molecular interactions and effect of thermal energy on the motion of particles because a balance between these determines the state of a substance.

          2. INTERMOLECULAR FORCES

Intermolecular forces are the forces of

attraction and repulsion between interacting  particles (atoms and molecules). This term does not include the electrostatic forces that exist between the two oppositely charged ions and the forces that hold atoms of a molecule 

together i.e., covalent bonds.  Attractive intermolecular forces are known 

as van der Waals forces, in honour of Dutch  scientist Johannes van der Waals (1837- 1923), who explained the deviation of real gases from the ideal behaviour through these forces. We will learn about this later in this unit. van der Waals forces vary considerably in magnitude and include dispersion forces or London forces, dipole-dipole forces, and dipole-induced dipole forces. A particularly 

strong type of dipole-dipole interaction is

hydrogen bonding. Only a few elements can participate in hydrogen bond formation, therefore it is treated as a separate category. At this point, it is important to note that attractive forces between an ion and a dipole are known as ion-dipole forces and these are 

not van der Waals forces. We will now learn  about different types of van der Waals forces.

» Dispersion Forces or London Forces

» Dipole - Dipole Forces

» Dipole–Induced Dipole Forces

 

» Hydrogen bond

           3. THERMAL ENERGY

Thermal energy is the energy of a body arising from motion of its atoms or molecules. It is directly proportional to the temperature of the substance. It is the measure of average kinetic energy of the particles of the matter and is thus responsible for movement of particles. This movement of particles is called

thermal motion.

           4. THE GASEOUS STATE

This is the simplest state of matter.

Throughout our life we remain immersed in  the ocean of air which is a mixture of gases. We spend our life in the lowermost layer of the atmosphere called troposphere, which is held to the surface of the earth by gravitational force. The thin layer of atmosphere is vital to 

our life. It shields us from harmful radiation and contains substances like di oxygen, di nitrogen, carbon dioxide, water vapour, etc.