Standard solution

The solution whose concentration is known is called standard solution. Eg: N/10 Na2CO3 N/2 oxalic acid,etc.
PRIMARY STANDARD SOLUTION
The solution obtained by dissolving known amount of substance(solute) in water and making definite solution is known as primary standard solution and the substances which are used to prepare primary standard solution is known as primary standard substance.Eg:Na2CO3,Oxalic acid,Mohr's salt(FeSO4(NH2)SO4.6H2O),etc..
To become primary standard substance,the following requirements must be fulfilled:
-It should be available readily in pure form.
-It shoul not be extremely hygroscopic in nature.
-Its composition in solid or in solution should not change for sufficiently long time.
-Its equivalent weight should be high so that weighing error can be minimized.

SECONDARY STANDARD SOLUTION
The oher substances standard solution of certain concentration are prepared at first and then it is standarized with the help of primary standard solution to get exact concentration such as standarized solution is known as secondary standard solution and substances which are used to prepare secondary standard solution is known as secondary standard substance.Eg:NaOH,HCl,H2SO4,KMnO4,C2Cr2O7,FeSO4,etc.

Rusting of iron

When iron is exposed to the moist atmosphere,a lose brown layer is deposited over the surface of iron due to formation of hydrated Ferric oxide(Fe2O3.XH2O) which is called rust and the phenomenon is called rusting.
One of the convincing mechanism of rusting is electrochemical mechanism.According to electrochemical mechanism,two different potential are developed within an iron sheet because of having unequal distribution of impurities.The lower electrical potential acts as anode and higher electrical potential acts as cathode.So electrochemical mechanism is accompanied by flow of electron.
At anode, Fe--> Fe++ + 2e-
The elctron released from anode are transferred towards the cathode.

At cathode, O2 + 2H2O + 4e- -->4OH-
The anode and cathode combines forming Ferrous hydroxide i.e.
Fe++ + 2Oh- -->Fe(OH)2
Ferrous hydroxide formed in above step further oxidize giving ferric hydrozide i.e.
2Fe(OH)2 + O2 + H2O --> 2Fe(OH)3
Fe(OH)3 --> Fe2O3 + 3H2O
Fe2O3 + XH2O --> Fe2O3.XH2O
Ferric hydroxide is unstable so decomposes giving Ferric oxide i.e.
2Fe(OH)3 --> Fe2O3 + 3H2O
And Ferric oxide formed in above step absorb requisite water molecules giving hydrated Ferric oxide which is rust.
Fe2O3 + xH2O --> Fe2O3.XH2O(Rust)

Second law of Thermodynamics

-The entropy of the universe always tends to be maximum and the energy of the universe always remains constant.
-The entropoy of the universe always increases in a spontaneous process i.e the change in entropy of the universe is positive in spontaneous process.
-The flow of heat from hot body to cold body occurs spontaneously but heat can't flow from cold body to hot body spontaneously.
-All the form of energy can be converted to heat energy but heat energy can't be converted to all the other forms of energy by any process.
-It is impossible to convert heat energy into an equivalent mechanical work wothout producing any change in other part of the system.

Mechanism of electric discharge

Cosmic rays ionize gas atoms inside the discharge tube.The electrons so producedare accelerated towards anode and positive ionsare accelerated towards cathode.The electrons collide with gas atoms.
1.At ordinary pressure
At ordinary pressure,the number of gas atoms are large.The inter atomic distance is small.In this case,the K.E of electron is insufficient to further ionizethe gas atoms.the energy of electron is insufficient to move from cathode to anode.Therefore there is no electric discharge at ordinary pressure.
2.At low pressure
At low pressure,the number of gas atoms inside the tube is low.In this case,the inter-atomic distance is sufficiently large.The K.E of electron is sufficient to further ionize the gas atoms when they collide.So number of free electrons is increased.In this case,electrons move from cathode to anode.Therefore,there is electric discharge at low pressure.
3.At very low pressure
At very low pressure,number of gas atoms inside the tube is very low.The inter atomic distance is very large.In this case,the number of atoms or electron is insufficient to make electric discharge at very low pressure.

Wavefront

Wavefront is defined as the locus of the particle of the medium vibrating in same phase.Anyline perpendicular to the wavefront represents the ray of light.Depending upon the nature of the source of light,there are three types of wavefront:

1) SPHERICAL WAVEFRONT
When the source of light is point source,the locus of the particle of the medium vibrating in same phase is spherical and hence it is called spherical wavefront.

2) CYLINDRICAL WAVEFRONT
When the source of light is linear,then the locus of the particle of the medium vibrating in same phase is cylindrical and hence it is called cylindrical wavefront.

3) PLANE WAVEFRONT
A very small part of spherical wavefront or cylindrical wavefront at a large distance from the source is nearly plane and hence it is called plane wavefront.

Laws of Photoelectric effect

1) This process is instantneous process i.e. the time differnce between incident of radiation and emission of electron is never more than 3 nanosecond.

2) For every metal, there is a minimum frequency below which there is no photoelectric effect.

3) For frequency of light greater than Treshold frequency, photoelectric current is directly proportional to intensity of light.

4) The maximum K.E or maximum velocity of photoelectron is independent to intensity of light.

Forced oscillation and resonance

We know that,all the oscillation suffers from a damping to overcome the resistive force.If we want the oscillation to be undamped,some external periodic force must be applied.The energy supplied by an external source compensate the loss of energy due to damping and hence oscillation continued becomes undiminished.Such type of oscillation in which the oscillation becomes continued due to action of external periodic force is called forced oscillation and the frequency of external source is called forced frequency.
When a body capable of oscillating is forced to vibrate with the help of forced frequency 'f',then the body vibrates with the same frequency.It also causes to increase the amplitude of vibration.This process continues in the frequency of external force i.e forced frequency becomes equal to the natural frequencyof the body.At this condition the body oscillates or vibrates with the maximum amplitude and the oscillation is known as resonance.
Eg:- If the marching frequency of the soldiers become equal to the natural frequency of the bridge then resonance occurs and the bridge vibrates with maximum amplitude which may collapse the bridge.For this reason,the soldiers are ordered to break their steps while crossing the bridge.