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Matter and its properties

Matter consists of atoms and molecules. The innumerable substances that occur in nature – needles, men, mountains, stars, everything we can think of – consist of atoms. Atoms combine to form molecules. For example, two atoms of hydrogen combine with one atom of oxygen to form a water molecule. A molecule can be as simple as a hydrogen molecule, which consists of two hydrogen atoms, or as complicated as a DNA (deoxyribonucleic acid) molecule, which consists of millions of atoms of hydrogen, carbon, nitrogen, oxygen and phosphorous.


States of Matter

Matter commonly exists in three states: the solid state, the liquid state and the gaseous state. In all states, the molecules are perpetually moving. In a solid, molecules vibrate about fixed positions. Molecules in a liquid also vibrate but simultaneously they move freely throughout the material.

All substances can be transformed from one state to another by changing temperature. Ice, water and steam are different states of water.

The conversion of solid into liquid is called melting. Change of liquid to solid is said to be vaporization. The conversion of solid directly to vapour is called sublimation.


Molecular properties of matter


        ✦ It is that property of the material of a body by virtue of which the body opposed any change in its shape or size when deforming forces one applied to it, and recovers its original state as soon as the deforming forces are removed.

        ✦ Steel is more elastic than rubber.

         Elastic limit the maximum limit of the external force upto which elasticity of the body is maintained.


         ✦ The property of a body by virtue of which it does not regain its original configuration even after the removal of deforming force is called plasticity.

          Putty, Paraffin wax are nearly perfectly plastic bodies.


         ✦ The internal restoring force acting per unit area of cross-section of the deformed body is called stress.

         ✦ Its unit is Pascal


          The change in length, volume, shape of the body per unit of the original value the application of the deforming force is called strain.

          Strain is unit less quantity.


Hooks law and modules of Elasticity:

The ratio of stress to strain is a constant for the material and is called modules of elasticity.

It is also called Hookes la, which states that within the limit of  elasticity the strain produced in a body in directly proportional to the stress applied to it.


Young’s modules

It strain is longitudinal , their the modules of elasticity is called Young’s modules.


It is defined as force acting normally on unit area of the surface.

Its unit is N/m2 also called Pascal. It is a scalar quantity.

Pressure is a liquid is given by P=hsg, where h is the height, s is the density of the liquid, and g is acceleration due to gravity.

Atmospheric pressure of 1 atm = 1.01x105 N/m2=760 torr

Atmospheric pressure decreases with attitude. This is why

           1)It is difficult to cook on the  mountain

           2)The fountain pen of a passenger leaks in aero plane at height.

Atmospheric pressure is measured by barometer the slow rise in the barometric reading is the  indication of clear weather.

Sudden fall in barometer reading is the indication of storm

Slow fall in barometer reading is the indication of rain


Density and Relative density

The density of a substance (S) is defined as the ratio of its mass (M) to its volume (V).

Its unit is kg/m3

Density of water is maximum at 4oC.

The relative density is defined as the ratio of the density of the substance to the density of water at -4oC.


Relative density has no unit.

Relative density is measured by hydrometer.

Ice floats on water surface as its density (0.92g/cm3) is less than the density of the water (1g,cm3)

The density of human body is less than the density of water but the density of head is greater than the density of water.

Therefore, during swimming a person displaces the water with hands and legs and thus total weight of displaced  water becomes equal to the weight of the body.

It ice floating in water in a vessel melts, the level of water is in the vessel does not  change.

The  density of sea water is more than that of normal water. This explains why it is easier to swim in sea water.


Pascal’s law:

The pressure exerted anywhere at a point of confirmed fluid is transmitted equally and undiminished in all directions throughout the liquid.

Hydraulic lift, hydraulic press, hydraulic break work on the basis of Pascal’s law.



The upward force exerted by a fluid on the immersed body is called buoyant-force or up thrust.

The up thrust acts at the centre of gravity of the liquid displaced by the submerged part of the body that is called the centre of the buoyancy.

Buoyancy force depends on the density of the fluid and not on the density of body and acts on centre of gravity of displaced fluid.


Archimedes principle:

When a solid  body is immersed wholly or partially in a liquid, then there is some apparent loss in its weight. This loss in weight in equal to the weight of the liquid displaced by the body.


Law of floatation

Whenever a solid body in dipped into a fluid the fluid exerts force of buoyancy on it, if the force of buoyancy equals to weight of the solid, the solid will remain in equilibrium this is called floatation.

It density of material of body is equal to density of liquid, the body floats fully submerged in liquid in neutral equilibrium.

When body floats in neutral equilibrium, the weight of body is equal to the weight of displaced liquid.


(i) Diffusion:

Diffusion is the mixing up of molecules of different gases, liquids and even solids. When a bottle of perfumes is opened in a room, its molecules mix with molecules of air and smell soon spreads even to the far corner of the room because of diffusion of the molecules of perfume in air.


(ii) Surface Tension:

It is the force (F) acting normally to an unit-length (l) of an imaginary line drawn on the surface of liquid.


Its unit is N/m.

Cohesive  force: It is the intermolecular force of attraction acting between the molecules of the same substrance.

Adhesive force: It is the intermolecular force of attraction acting between the modecules of different substance.

The  surface tension decreases with rise in temperature and becomes nero at the critical temperature.

Due to surface tension, rain drops are spherical in shape.

Warm soup is tasty because at high temperature its surface tension is low and consequently the soup spreads on the all part of the tongue.


Some Phenomena based on surface tension:

1)Small drops of mercury are spherical, while large once are flat

2)Formation of lead shots

3)Floatation of needle on water

4)Dancing of Camphor on water

5)Bigger bubbles can be formed from the soap  solution easily than from water.

6)Detergent – helps in cleaning the cloths.

When water is poured on a surface, the water collects itself on the surface forming a spherical shape. Raindrop is spherical in shape. This all happens because of a property known as surface tension exhibited by liquid molecules.

Liquid drops, such as raindrops, oil drops, drops of molten metal’s, dewdrops, etc. are all spherical because their surface tend to contract in order to have minimum surface area. For a given volume, a sphere has the minimum surface area.


(iii) Capillary:

 If a clean glass tube having a small inside diameter (called a capillary tube) is dipped in water, the water rises in the tube and the level of water in the tube is higher than that of the height of the surrounding liquid. This phenomenon is called capillarity.

The force of attraction between unlike molecules is called adhesion and that between like molecules cohesion.


(iv) Viscosity:

Viscosity is another molecular property of fluids (gases and liquids). It is measured in terms of the coefficient of viscosity, h. Its SI unit is pas (Pascal second). Liquids have higher coefficient of viscosity than gases. Some liquids are more viscous than others. For example, honey is more viscous than water.

As the falling object gains velocity (due to the downward force of gravity acting on it), the opposing viscous force also increases. A stage comes when the viscous force equals the gravitational force and, therefore, the net force on the falling object becomes zero. The object than stops accelerating and falls with constant velocity, known as the terminal velocity.

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