A series of similarly constituted compounds in which the members present have the same functional group and similar chemical properties and any two successive members in a particular series differ in their molecular formula by ( - CH₂) unit, is called a homologous series.
e.g. alkane series C_nH_2n+2

(i) Reactions between unsaturated hydrocarbons with simple substances to form a single saturated product are addition reactions. 
(ii) Those reactions in which one or more hydrogen of a saturated hydrocarbon is replaced by an atom or a group of atoms are substitution reactions.

Aldehyde, ketone.

Functional group: Ketone, name of the compound: Propanone.

Thirteen covalent bonds.

(i) Ethanol, (ii) Ethanal.

Name - Ethane
Formula - C₂H₆

(i) When ethanol is heated with excess of concentrated sulphuric acid at 443K, it gets dehydrated to form ethene.
\(\underset { Ethanol }{ { CH }_{ 3 }-{ CH }_{ 2 } } OH\xrightarrow [(Dehydration) ]{ conc. H_2SO_4;443K } \underset { Ethene }{ { CH }_{ 2 }={ CH }_{ 2 } } +H_2O\)
In this reaction, concentrated sulphuric acid acts as a dehydrating agent which removes water molecules from the ethanol molecule.

Alkaline potassium permanganate of acidified potassium dichromate.
(i) Ethanol will not affect litmus paper Ethanoic acid will turn blue litmus paper red.
(ii) Ethanol will not react with sodium hydrogen carbonate. Ethanoic acid will give brisk effervescence due to colourless, odourless carbon dioxide gas.

(i)Oxidation reaction:
\(\underset{Ethanol}{CH_3CH_2OH}+2[O]\xrightarrow[KMnO_4]{Alkaline}\underset{Ethanoic\ acid}{CH_3COOH}+H_2O\)
(ii)Addition reaction
\(\underset{Ethene}{CH_2=CH_2}+H_2\xrightarrow[573K]{Bi}CH_3-CH_3\)
(iii)Substitution reaction:
\(\underset{Methane}{CH_4+CL_2}\xrightarrow{Sunlight}\underset{Methyl\ chloride}{CH_3CL}+HCl_3\)

(i) Ethanol gets oxidised to ethanoic acid.
\(\underset{Ethanol}{CH_3CH_2OH}+2[O]\xrightarrow{Alkaline\ KMnO_4}\underset{Ethanoic\ acid}{CH_3COOH}+H_2O\)
(ii)Ethene will be formed
\(\underset{Ethanol}{CH_3CH_2OH}\xrightarrow[443K]{conc. H_2SO_4}\underset{Ethane}{CH_2=CH_2}+{H_2O}\)
Uses:
(i) It is used in tonics and cough syrups.
(ii) It is used as fuel.
(iii) It is used as solvent
(iv) It is used in wine, beer and whisky

(a) Carbon forms large number of compounds since carbon is small in size and can form stable covalent bonds (catenation) and it shows tetravalency.
(b) Air holes of a gas burner are made open (adjusted) so that air can pass through, which is needed for complete combustion, so that heated vessels do not get blackened.
(c) Some synthetic detergents are non-biogradable, therefore, cause pollution of water.

C_nH_2n = Alkene
2nd member = C₃H₆ (Propene)

C₃H₆H₂C = CH - CH₃
Propene is second member of series whose general formula is C_nH_2n.

CH₃CH₂OH, ethanol

Bromine solution in CCl₄ has an orange colour. When it is added dropwise to ethene, the orange colour disappears due to the formation of the colourless ethylene dibromide. This reaction is not shown by ethane as it is addition reaction.
\(\mathrm{CH}_2=\mathrm{CH}_2+\mathrm{Br}_2 \longrightarrow \mathrm{CH}_2-\mathrm{CH}_2\)
Ethene          Bromine               |             |
                     (orange               Br           Br
                      colour)            Ethylene dibromide
                                                  (colourless)
The two can be distinguished also by subjecting them to the flame. Saturated hydrocarbon (ethane) generally gives a clear flame while unsaturated hydrocarbon (ethene) gives a yellow flame with lots of black smoke.

Drinking of methanol is lethal because in the liver, it gets oxidized to methanol which rapidly reacts with the compounds of the cell. As a result, protoplasm gets coagulated in much the same way as an egg gets coagulated on boiling or cooking. Consequently, the man dies.

Compound X is ethanol and Compound Y is acetic acid or ethanoic acid.

The atomic number of carbon is 6 which means that a neutral atom of carbon retains 6 electrons. So, the electronic configuration of carbon is KL. Since carbon has 4 electrons in its outermost shell so, it should either loss or gain 4 electrons to achieve the inert gas configuration and become stable.
(1) It could gain four electrons forming C^4- anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons due to inter electronic repulsion.
(2) It could lose 4 electrons froming C⁴⁺ cation. But it would require a large amount of energy to remove four electrons from its outermost shell. Thus, it forms compounds mainly by covalent bonds.
Two properties of carbon which lead to huge number of carbon compounds are:
(1) Catenation: Catenation is the unique property of carbon atoms to form bonds with other atoms of carbon giving rise to large molecules.
(2)Tetravalency: Since carbon has a valency of 4, it is capable of bonding with 4 other atoms of carbon or atoms of some other monovalent element.
The reason for the formation of strong bonds by the carbon atoms is their small atomic size. Due to the small size of carbon atoms their nuclei hold the shared pairs of electrons between atoms strongly, leading to the formation of strong covalent bonds. The carbon atoms also form strong covalent bonds with the atoms of other elements such as hydrogen, oxygen, nitrogen, sulphur, chlorine and other elements.

Isomerism is a phenomenon due to which some compounds have same molecular formula but different structural formulae .
Characteristics:
(i) They differ in structural formula
(ii) They differ in melting point
(iii) They differ in boiling point
(iv) They differ in solubility in same solven.

a) Compounds of carbon and hydrogen only are called hydrocarbons. For example
methane (CH₄), ethane (C₂H₆), ethene (C₂H₄), ethyne (C₂H₂), cydohexane
(C₆H₁₂), benzene (C₆H₆) etc.

b) Saturated hydrocarbons contains only C - C and C - H single covalent bonds while
unsaturated hydrocarbons contain at least one C = C double bond (besides C - C
and C - H single covalent bonds) or one triple bond.

c) That portion of the organic molecule which largely determine its chemical properties is called the functional group. For example, -OH (alcoholic group), (keto group) - CHO (aldehyde group), -COOH (carboxyl group).