Carbon atoms have 4 electrons in their outcomes shell. So need to gain or lose electrons to attain noble gas configuration.
(i) 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.
(ii) It could lose four electrons forming C⁴⁺ cations. But it would require a large amount energy to remove four electrons from its outermost shell.
Because of these reasons, carbon shares its valence electrons to complete its octet with other atoms to form covalent bonds.
(a) Covalent compounds are bad conductors of electricity because they do not contain ions.
(b) Covalent compounds have usually low melting and boiling because they are made up of electronically neutral molecule. So the force of attraction between the molecules of a covalent compound is very weak. Only a small amount of heat energy is required to break these weak molecular forces, due to which covalent compounds have low melting and boiling points.

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.

i) Since compound C on combustion forms two moles of CO₂ and 3 moles of H₂ O, therefore compound C must contain two carbon atoms and six hydrogen atmos. Thus compound C must be C₂H₆ (ethane)
\(\underset{Ethene(C)}{2C_2H_6}+\underset{Oxygen}{7O_2}\xrightarrow{Heat}\underset{Carbon\ dioxide}{4CO_2}+\underset{Water}{6H_2O}\)
ii) Since compound (C) is obtained by addition of 1 mole of H₂ in presence of Ni to compound (B), therefore (B) must be ethane.
\(\underset{Ethene(B)}{CH2}=CH_2+H_2\xrightarrow{Ni/Heat}\underset{Ethene(C)}{CH_3-CH_3}\)
iii) Since compound (B) is formed by heating compound (A) with cone. H₂SO₄ therefore compound (A) must be ethanol.
\(\underset{Ethanol(A)}{CH_3CH_2OH}\xrightarrow{Conc. H_2SO_4,443K}CH_2=CH_2+\underset{Ethene(B)}{H_2O}\)

a)\(\underset{Ethanol}{CH_3CH_2OH}\xrightarrow{Conc.H_2SO_4.170^0C}CH_2=\underset{Ethene}{CH_2}+H_2O\)
b)\(\underset{Propanol}{CH_3CH_2CH_2OH+2[O]}\xrightarrow[or\ Aci.K_2Cr_2O_7,\Delta]{AlK.KMnO_4, \Delta}\underset{Propanoic\ acid}{CH_3CH_2COOH}+H_2O\)

Carboxylic acid reacts with alcohols to form ester. For example, when ethanoic acid is warmed with ethanol in presence of a few drops of concentrated sulphuric acid as catalyst, an ester (ethyl ethanoate, commonly called ethyl acetate) and water are formed.
\(\underset{Acetic\ or\ ethanoic\ acid}{CH_COOH}+\underset{Ethanol}{C_2H_5OH}\xrightarrow{Conc. H_2SO_4}\underset{Ethyl\ ethanoate}{CH_3COOC_2H_5}+\underset{Water}{H_2O)}\)
Ester are sweet -smelling substances. So they are widely used in making perfumes and flavoring agents. Further, the reaction between a carboxylic acid and an alcohol to form a sweet ester is used as a test for alcohols as well as carboxylic acids.

Addition of hydrogen to understand hydrocarbons in presence of a catalyst such as nickel or palladium to form saturated hydrocarbons is called hydrogenation. For example,
\(\underset { Ethyne }{ CH } \equiv CH+{ H }_{ 2 }\xrightarrow [ ]{ Ni/473K} \underset { Ethene }{ { CH }_{ 2 } } ={ CH }_{ 2 }+{ H }_{ 2 }\xrightarrow [ ]{Ni/473K } { CH }_{ 3 }-\underset { Ethane }{ { CH }_{ 3 } } \)

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).

\(\underset{Ethanoic\ acid}{CH_3COOH} + \underset {Sodium\ hydrogen\\ Carbonate}{NaHCO_3} \rightarrow \underset{sodium\ ethanoate}{CH_3COONa}+ \underset{Cabon\ dioxide}{CO_2} + \underset{Water}{H_2O}\)
Activity to show that ethanoic acid reacts with metal hydrogen carbonate to liberate carbon dioxide.
Procedure:
i) Take two test tubes and label them as A and B.
ii) Now, take few grams of sodium carbonate in test tube A.
iii) Take few grams of sodium hydrogen carbonate in test tube B.
iv) Now, add ethanoic acid to both the test tubes.
v) Note your observation.
vi) Pass the gas liberated in test tube A and B through lime water or calcium hydroxide solution.
vii) Record your observation.
Observation
a) A brisk effervescence is formed.
b) The lime water turns milky in both the test tubes.

When carboxylic acids react with sodium carbonate or sodium bicarbonates, it liberates carbon dioxide gas which turns lime water milky.
\(\underset{Calcium\ hydroxide\\ (Lime\ wate)}{Ca(OH)_2(aq)}+\underset{Carbon\ dioxide}{CO_2(g)}\rightarrow \underset{Calcium\ carbonate\\ (White \ ppt)}{CaCO_3(s)\downarrow}+\underset{Water}{H_2O(l)}\)