Molecular Theory of Matter, Physics tutorial

Concept of Matter:

Matter is something which consists of weight and occupies space. Each and every object or substance is made up of matter. Most of the properties and behavior of substances can best be described by supposing that the entire substances are composed of small particles termed as molecules. The supposition that matter is build up of very small particles (or molecules) which are in constant motion is termed as the 'molecular theory of matter'.

Structure of Matter:

Proof of the Particle Nature of Matter:

1) Most of the substances in solid form can simply be crushed to powder form example: piece of chalk, charcoal, lump of clay and piece of stone.

2) A dry stick or dry wood is simply broken into smaller bits.

3) Solubility: Whenever you drop a cube of sugar into a cup of water and turn the water, the sugar 'disappears'. That is, it dissolves in water.

4) Whenever you scrape the surface of a piece of chalk, you will observe thousands of very small particles flake off and float via the air.

5) If a beam of light (example: sunlight) is entering a dusty room via a window, you will notice a disordered motion of the dust particles in the air.

Experimental Evidence of the Particle Nature of Matter:

Experimental proof of the atomic or molecular nature of matter is the Brownian motion termed after the botanist, Robert Brown, who introduced the phenomenon in the year 1827. As he was noticing tiny pollen grains hanged in water beneath a microscope, Brown observed that the small pollen grains moved around in zigzag paths even although the water appeared to be perfectly still. The hither and thither through the molecules of water pollen grains were being knocked about which were robustly moving about.

Description of Brownian motion:

The uneven movement of the smoke particles is due to the motion of the invisible air molecules that bombard each and every particle from all sides.

The particle is much small and the number of molecules of air hitting one side is not balanced through the number of molecules striking the opposite side at the similar instant. Thus, the particle moves in the direction of the resultant force and whenever it moves to the other place, the similar thing occurs.

Brownian motion is significant for two reasons.

1) It gives evidence for the existence of the very small particles of matter termed as molecules

2) It provides evidence that molecules are in a constant state of the arbitrary motion.

Atomic Structure:

The atom is the smallest indivisible particle of an element that can take part in the chemical change.

It comprises of two parts: The nucleus and the electrons. The nucleus is the heavy part of the atom and is build up of two kinds of particles named as protons and neutrons. The protons carry a positive charge whereas neutrons carry no charge. The electrons carry a negative charge and circle in orbits around the heavy nucleus. The numbers of orbits based on the substance, for illustration hydrogen consists of just one orbit whereas oxygen consists of two. An electron is extremely light (around 1/1840 of the mass of the proton). The negative charge of an electron is equivalent to the positive charge of a proton and the number of electrons in an atom is equivalent to the number of protons. The atom is thus electrically neutral.


A molecule is the smallest particle of a substance that can have a separate existence and still maintain the properties of that substance.

Structure, Nature and Size of Molecules:

1) Most of the substances can't exist by themselves as individual atoms; instead they join their atoms with themselves or by other atoms to form molecules. Therefore a molecule might be build up of identical atoms of the similar element or different atoms of two or more elements. For illustration a molecule of hydrogen is build up of two atoms of hydrogen however a molecule of water comprises of two atoms of hydrogen and one atom of oxygen.

2) The molecules of any pure substance are similar that they encompass the similar structure, similar mass and similar mechanical properties.

3) Molecules are made up by atoms joining in simple proportions.

4) The easiest model of a molecule is that of a rigid sphere (similar to a small billiard ball) that can move and collide with other molecules or by a wall and apply attractive or repulsive forces on neighboring molecules. The molecular forces reduce as the distance separating the molecules rise.

5) Molecules are in the constant motion. The motion is arbitrary (that is, haphazard or zigzag) in liquids and gases however oscillatory or vibration in solids.

6) The size of a molecule is very small, of the order of 10-9 to10-10 m. As a result of the small size, molecules can't be notice by the naked eyes or even by the help of a microscope: Again due to the reason of the small size, one gram of an element consists of some millions of molecules. For Illustration one gram of hydrogen consists of around 1023 molecules.

States of Matter:

Matter subsists in three main states, that is, solid, liquid and gas. Solids encompass fixed volume and shape. They can't be poured. Liquids contain fixed or constant volumes however they suppose the shape of the container. They can be poured. Gases contain no fixed shape; however for all time occupy the shape of the container. They can be poured.

1) Solids: In solids, the molecules vibrate about a mean position. The forces among the neighboring molecules are strong and the attractive and repulsive forces. Solids thus encompass characteristic crystalline structures in which the atoms are ordered in regular patterns termed lattices.

2) Liquids: In liquids, forces among the molecules are less strong than in the solids; however they are both cohesive forces. Therefore, the molecules move freely all the way through the liquid, exchanging partners or neighbors as they go. The liquid can therefore take the shape of the container and can be poured as the molecules are very free and are in continuous random motion.

3) Gases: In gases, the molecules are much further distant than such of solids and liquids. They move freely and at high speeds in the space in which they subsist. As in liquids they are in the constant random motion. The cohesive or attractive force is negligible, and therefore a gas can be poured and is perfectly free to expand and fully fill the container.

Kinetic Molecular Theory of Matter:

The theory explains that matter is build up of very small particles termed as molecules that are in constant motion.

Basic Assumptions of the Kinetic Molecular Theory:

1) Matter can exist either in the solid, liquid or gaseous state.

2) All the substances comprise of molecules, the smallest particle that can exist separately.

3) In solids, the molecules vibrate on a mean or fixed position. The forces among the molecules are strong and might be repulsive or attractive. All true solids encompass a crystalline structure in which the atoms are arranged in regular patterns termed as lattices.

4) In liquids the molecules move freely in all the directions. Moreover to vibrational energy, they contain translational energy. Kinetic energy of the liquid molecules is greater than in solids.

5) In gases the molecules are in stable motion and are further apart than in solids and liquids. They move at very high speeds and encompass translational, vibrational and further rotational energy when the molecules are building up of two or more atoms. The attractive or cohesive force is negligible, therefore, gases are perfectly free to expend and entirely fill the vessels

Basic Assumptions of the Kinetic Theory of Gases:

1) Gases are building up of molecules: We can take molecules as point masses which are perfect spheres. Molecules in a gas are much far apart, so that the space among each individual molecule is loads of orders of magnitude greater than the diameter of molecule.

2) Molecules are in constant random motion: There is no general prototype regulating either the magnitude or direction of the velocity of molecules in a gas. At any specified time, molecules are moving in several different directions at various speeds.

3) Movement of molecules governed by Newton's Laws: In accordance by the Newton's First Law, each and every molecule moves in a straight line at a fixed velocity, not interacting by any of the other molecules apart from in a collision. In a collision, molecules apply equivalent and opposite forces on one other.

4) Molecular collisions are perfectly elastic: Molecules don't lose any kinetic energy when they collide with one other.

Processes using the Kinetic Molecular Theory:


Diffusion is basically the tendency of molecules to migrate and fill an-empty space as an outcome of their random thermal motions.

Diffusion occurs in liquids and gases however more slowly in solids. Its rate based on the density and temperature of the gas. Light gases at high temperatures diffuse sooner as compare heavy gases at low temperatures.

Change of State (Melting, Vaporization):

Melting: Heat reduces the cohesion that holds molecules altogether in the crystal lattice of a solid. At melting point heat breaks the lattice fully and the molecules are free to move around arbitrarily, however still held altogether by the force of cohesion. Throughout melting the temperature is constant as heat is absorbed to break the lattice.


At boiling point the temperature of the boiling liquid is again constant. The heat absorbed is employed to overcome the force of cohesion through breaking the attractive forces holding the liquid molecules altogether. Therefore the molecules break away and become the molecules of gas. The similar amount of heat absorbed for melting is given out for fusion (just similar in case of vaporization and condensation).


In a liquid the molecules move arbitrarily having different speeds and they encompass an average Kinetic energy. A few molecules close to the surface which are moving with speed more than the average speed succeed in escaping from the attraction of their neighbors and jump out of the liquid. This is termed as evaporation. The molecules would have left the liquid to form the vapor. As they get away from the liquid by this additional energy the temperature of the liquid decreases. Therefore we have the cooling effect of evaporation.

Properties of Fluids at Rest:

1) Adhesion and Cohesion:

The Adhesion is a force of attraction between molecules of various substances, example: molecules of glass and water.

Cohesion is the force of attraction between molecules of similar substance, example: water molecules.

The diverse behavior of water and mercury whenever spilled on a clean glass surface is described by means of adhesion and cohesion:

2) Surface Tension:

Surface tension is the force acting all along the surface of a liquid, causing the liquid surface to behave similar to a stretched elastic scene.

The surface of a liquid acts as if it were a stretched elastic skin having the result which it can hold an object denser than the liquid gently placed on it. This is termed as surface tension.

3) Capillarity:

Capillarity or simply capillary action, phenomenon in which the surface of a liquid is noticed to be elevated or depressed where it comes into contact with a solid. For illustration: the surface of water in a clean drinking glass is observed to be slightly higher at the edges, where it contacts the glass, than in the middle. Capillarity can be described by considering the consequences of two opposing forces: adhesion, that is, the attractive (or repulsive) force among the molecules of the liquid and those of the container, and cohesion, that is, the attractive force among the molecules of the liquid.

4) Osmosis:

Osmosis is the propensity of a solvent to pass from the dilute solution, via a semi permeable membrane to a concentrated solution. Semi-permeable membrane is a substance like cellophane, parchment or vegetable material that would let some molecules of liquid to diffuse via them however not others. Such a membrane might let the molecules of a solvent to pass via it however not those of a solute.

5) Viscosity:

Viscosity is the friction in fluids (that is, liquids and gases). It might be stated as the internal friction among the layers of a liquid or gas in motion. Liquids which pour slowly are much viscous than those that of power faster. Therefore extremely cold honey is more viscous than extremely cold water.

Crystalline and Amorphous Substances:

Solids encompass definite shape and volume as the average distance among the molecules or atoms remain constant and don't change with time. The arrangement of molecules within a solid differs from one to other. This outcome in two kinds of solids:

  • Crystalline solids
  • Amorphous solids

In the crystalline solids, the molecules or atoms are arranged in an order, expanding over a large volume of the crystal. All bonds contain the similar bond strength. Thus, such solids encompass an accurate melting point. They as well contain a uniform chemical composition. Illustrations of crystalline solids comprise: calcite, quartz, rock salt, sugar, mica and diamonds.

Amorphous solids on the other hand, don't contain a regular and periodic arrangement of atoms. The entire bonds are not uniformly strong. Such solids don't encompass an accurate melting point. Illustrations of amorphous solids are glass, rubbers, cement, plastic and paraffin.

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