A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group.
For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series.

According to Dobereiner, all elements occurred in groups of three, when arranged in increasing order of atomic masses. He referred to these groups as triads. In a traid the elements had similar chemical properties. 

Traids of the Dobereiners classification:
Traid1:

Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements.
Example: In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23.

Environment : The physical, chemical and ‘ biological conditions of the region in which an organism lives is called its Environment. It includes air, light, soil, temperature, water and the presence or absence of other organisms, i.e., the conditions for development or growth.
Environment has three main components, viz :
(a) Physical surroundings [soil, air and water bodies]
(b) Living organisms [plants, animals, decomposers (bacteria and fungi)]
(c) Meteorological factors (or climatic factors) . [sunlight, temperature, rainfall, humidity, pressure and wind speed].

We produce Hydrogen Chloride in the laboratory by treating sodium chloride with concentrated sulphuric acid. We, then, heat this mixture up to 420K.

                              420K
NaCl    +  H2SO4        →       NaHSO4    +   HCl

We get Sodium bisulphate as a by-product. This is insoluble. Therefore, we further mix it with more sodium chloride. This mixture has to be further warmed to a higher temperature of around 823K. It gives dissolvable sodium sulfate and HCl gas.

                                 823K
NaHSO4    +    NaCl     →        Na2SO4     +    HCl

We dry this HCl by treating it with concentrated sulphuric acid. HCl is not dried over phosphorus pentoxide or brisk lime. This is because it reacts with both of these compounds.

Magnetic field is an effect around a permanent magnet or a moving charge due to which ferromagnetic objects like some metals get attracted, and diamagnetic substances gets repelled, when placed in the magnetic field. The strength of magnetic field is called one tesla, if a charge of one coulomb, when moving with a velocity of 1 ms^-1 along a direction perpendicular to the direction of the magnetic field experiences a force of one newton. CGS units of magnetic field are called gauss or oersted.