Mix Examples - Carbon and its Compounds Questions in English

(N/A) Detergents are preferred over soaps for washing clothes in hard water because:
$(i)$ They do not form insoluble precipitates (scum) with calcium and magnesium ions present in hard water.
$(ii)$ They are effective in both soft and hard water and produce more lather.
$(iii)$ They have a stronger cleansing action compared to soaps.

(B) When $C_{2}H_{5}OH$ reacts with $CH_{3}COOH$,i.e.,ethanol with ethanoic acid,in the presence of concentrated $H_{2}SO_{4}$,a sweet-smelling ester is formed. This reaction is known as esterification.
The chemical equation is:
$CH_{3}COOH + C_{2}H_{5}OH \xrightarrow{Conc. H_{2}SO_{4}} CH_{3}COOC_{2}H_{5} + H_{2}O$
In this reaction,$CH_{3}COOC_{2}H_{5}$ is ethyl ethanoate,which is an ester.

(N/A) $(i)$ The functional group $-COOH$ is known as the Carboxylic acid group.
$(ii)$ The functional group $-COOR'$ is known as the Ester group.
$(iii)$ The functional group $-OH$ is known as the Alcohol group.
$(iv)$ The functional group $-X$ (where $X = F, Cl, Br, I$) is known as the Halide group,and the compound is called an Alkyl halide or Haloalkane.

(N/A) Hydrogenation is the chemical reaction involving the addition of hydrogen to unsaturated hydrocarbons (such as alkenes or alkynes) in the presence of a catalyst,such as palladium $(Pd)$,platinum $(Pt)$,or nickel $(Ni)$,to produce saturated hydrocarbons.
Example: $CH_2=CH_2$ (Ethene) $+ H_2 \xrightarrow{Ni} CH_3-CH_3$ (Ethane).
Industrial Application: This reaction is widely used in the food industry for the hydrogenation of liquid vegetable oils to produce solid fats like vanaspati ghee.

(N/A) $(i)$ Methane reacts with excess chlorine in the presence of diffused sunlight to form carbon tetrachloride and hydrogen chloride:
$CH_{4} + 4Cl_{2} \xrightarrow{\text{Diffused sunlight}} CCl_{4} + 4HCl$
$(ii)$ Ethanol undergoes complete combustion to produce carbon dioxide,water,and energy (heat and light):
$CH_{3}CH_{2}OH + 3O_{2} \longrightarrow 2CO_{2} + 3H_{2}O + \text{Heat and light}$
$(iii)$ Ethyl alcohol (ethanol) is oxidized by acidified potassium dichromate $(K_{2}Cr_{2}O_{7})$ to form ethanoic acid:
$CH_{3}CH_{2}OH + 2[O] \xrightarrow{\text{Acidified } K_{2}Cr_{2}O_{7}} CH_{3}COOH + H_{2}O$

(N/A) $(i)$ All members of a homologous series contain the same functional group.
$(ii)$ They exhibit similar chemical properties due to the presence of the same functional group.
$(iii)$ There is a regular gradation in physical properties such as melting point,boiling point,and density with an increase in molecular mass.
$(iv)$ Any two successive members of the series differ by a $-CH_2$ unit or a molecular mass of $14u$.

(N/A) To form an ionic bond,carbon would either need to lose $4e^-$ to form a $C^{4+}$ ion or gain $4e^-$ to form a $C^{4-}$ ion.
$1$. Losing $4e^-$ requires a very large amount of energy to overcome the strong nuclear attraction,which is not energetically favorable.
$2$. Gaining $4e^-$ would result in $10e^-$ being held by only $6$ protons in the nucleus,which creates significant instability due to the difficulty of the nucleus in holding such a large number of electrons.
Therefore,carbon does not form ionic compounds and instead forms covalent bonds by sharing electrons.

(N/A) Soaps are defined as the sodium or potassium salts of long-chain carboxylic acids (higher fatty acids).
When soap is added to hard water,the calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$ ions present in the water react with the soap molecules.
This reaction results in the formation of insoluble precipitates known as scum (calcium or magnesium stearate).
Because these insoluble salts do not dissolve,they interfere with the cleaning action of soap and form a sticky residue.

(N/A) The three chemical reactions for the conversion of ethanoic acid $(CH_{3}COOH)$ to sodium ethanoate $(CH_{3}COONa)$ are as follows:
$1$. Reaction with Sodium Hydroxide (Neutralization):
$CH_{3}COOH + NaOH \rightarrow CH_{3}COONa + H_{2}O$
Reactant: Sodium hydroxide $(NaOH)$; Product: Water $(H_{2}O)$.
$2$. Reaction with Sodium Carbonate:
$2CH_{3}COOH + Na_{2}CO_{3} \rightarrow 2CH_{3}COONa + H_{2}O + CO_{2}$
Reactant: Sodium carbonate $(Na_{2}CO_{3})$; Products: Water $(H_{2}O)$ and Carbon dioxide $(CO_{2})$.
$3$. Reaction with Sodium Bicarbonate:
$CH_{3}COOH + NaHCO_{3} \rightarrow CH_{3}COONa + H_{2}O + CO_{2}$
Reactant: Sodium bicarbonate $(NaHCO_{3})$; Products: Water $(H_{2}O)$ and Carbon dioxide $(CO_{2})$.

(N/A) Three physical properties of ethanol:
$(i)$ Ethanol is a liquid at room temperature.
$(ii)$ It is a good solvent.
$(iii)$ It is a colourless liquid with a pleasant smell.
Three uses of ethanol:
$(i)$ It is used as an active ingredient in all alcoholic drinks.
$(ii)$ It is used in medicines such as tincture of iodine,cough syrups,and tonics.
$(iii)$ It is used as a solvent in the manufacturing of perfumes and paints.

(A) $X$ is Ethanol $(C_{2}H_{5}OH)$ and $Y$ is Hydrogen gas $(H_{2})$.
The chemical equation for the reaction is:
$2C_{2}H_{5}OH + 2Na \longrightarrow 2C_{2}H_{5}ONa + H_{2} \uparrow$
When $X$ (ethanol) is heated with excess concentrated sulphuric acid $(H_{2}SO_{4})$ at $443 \ K$,it undergoes dehydration to form Ethene $(C_{2}H_{4})$.
The name of the product is Ethene.
The structure of Ethene is $CH_{2}=CH_{2}$.

(N/A) $(i)$ Oxidation: Alcohols are converted into carboxylic acids in the presence of an oxidizing agent such as acidified $K_2Cr_2O_7$.
$CH_3CH_2OH \xrightarrow[Acidified]{K_2Cr_2O_7} CH_3COOH$
$(ii)$ Hydrogenation: Unsaturated hydrocarbons (like ethene) react with hydrogen in the presence of a catalyst like $Ni$ or $Pd$ to form saturated hydrocarbons.
$CH_2 = CH_2 + H_2 \xrightarrow[\Delta]{Ni} CH_3 - CH_3$
$(iii)$ Substitution: Saturated hydrocarbons (like methane) react with chlorine in the presence of sunlight to form chloroalkanes.
$CH_4 + Cl_2 \xrightarrow{Sunlight} CH_3Cl + HCl$

(N/A) The reaction is known as $Esterification$.
The chemical reaction is:
$CH_3COOH + C_2H_5OH \xrightarrow{Conc. H_2SO_4} CH_3COOC_2H_5 + H_2O$
The product obtained is $Ethyl \ ethanoate$ (an ester).

(N/A) The electron-dot structure of ethyne $(C_2H_2)$ shows a triple bond between the two carbon atoms and a single bond between each carbon and hydrogen atom.
In pure oxygen,ethyne undergoes complete combustion,which releases a large amount of heat,attaining the high temperature required for welding.
In contrast,air contains a lower percentage of oxygen compared to pure oxygen. When ethyne is burnt in air,it undergoes incomplete combustion,which produces a sooty flame and does not generate the high temperature necessary for welding.

(C) homologous series is a group of organic compounds having the same functional group and similar chemical properties,in which the successive members differ by a $-CH_{2}$ group.
$1$. Alkynes: These follow the general formula $C_{n}H_{2n-2}$. The compounds $C_{3}H_{4}, C_{4}H_{6}, C_{5}H_{8}$ belong to this series.
$2$. Alkenes: These follow the general formula $C_{n}H_{2n}$. The compounds $C_{3}H_{6}, C_{4}H_{8}, C_{5}H_{10}$ belong to this series.

(A) The hydrocarbons in which an addition reaction with hydrogen is possible are known as $Alkene$s. Their general formula is $C_nH_{2n}$.
The essential condition for an addition reaction is the presence of a multiple bond (double or triple bond) between carbon atoms and the use of a catalyst such as $Ni$,$Pd$,or $Pt$.
Chemical equation (Hydrogenation of Ethene):
$CH_2=CH_2 + H_2 \xrightarrow{Ni/Pd/Pt} CH_3-CH_3$
Reactant: $Ethene$
Product: $Ethane$

(N/A) Vegetable oils generally contain unsaturated fatty acids,which are considered healthier for the human body compared to animal fats,which are rich in saturated fatty acids. Saturated fats are associated with higher risks of cardiovascular diseases.
The hydrogenation reaction of vegetable oils is represented as:
$R_2C = CR_2 + H_2 \xrightarrow{Ni/Pd/Pt} R_2CH - CR_2H$
The catalysts used in this reaction are typically finely divided $Ni$ (Nickel),$Pd$ (Palladium),or $Pt$ (Platinum).

(N/A) The conversion of $CH_{4}$ to $CCl_{4}$ occurs through the following successive steps in the presence of sunlight:
$1$. $CH_{4} + Cl_{2} \xrightarrow{\text{Sunlight}} CH_{3}Cl + HCl$
$2$. $CH_{3}Cl + Cl_{2} \xrightarrow{\text{Sunlight}} CH_{2}Cl_{2} + HCl$
$3$. $CH_{2}Cl_{2} + Cl_{2} \xrightarrow{\text{Sunlight}} CHCl_{3} + HCl$
$4$. $CHCl_{3} + Cl_{2} \xrightarrow{\text{Sunlight}} CCl_{4} + HCl$
This reaction is referred to as a substitution reaction because one or more hydrogen atoms of the hydrocarbon are replaced (substituted) by another atom or group of atoms (in this case,chlorine atoms).

(N/A) $(i)$ Carboxylic acid: Ethanoic acid $(CH_{3}COOH)$. Structure: $CH_{3}-C(=O)OH$.
$(ii)$ Alcohol: Ethanol $(CH_{3}CH_{2}OH)$. Structure: $CH_{3}-CH_{2}-OH$.
$(iii)$ Compound $'X'$: Ethyl ethanoate $(CH_{3}COOC_{2}H_{5})$. Structure: $CH_{3}-C(=O)-O-CH_{2}-CH_{3}$.
Explanation: The reaction between ethanoic acid and ethanol in the presence of an acid catalyst is an esterification reaction,producing the ester ethyl ethanoate $('X')$.

(N/A) An atom or a group of atoms that determines the chemical properties of organic compounds is called a functional group. They confer specific properties to the compound,regardless of the length and nature of the carbon chain.

Organic compound	Functional group and Structure
$(i)$ Ethanol	Alcohol group $(-OH)$: $CH_{3}-CH_{2}-OH$
$(ii)$ Ethanoic acid	Carboxylic acid group $(-COOH)$: $CH_{3}-C(=O)OH$

(N/A)

Saturated hydrocarbons	Unsaturated hydrocarbons
$1.$ Saturated hydrocarbons burn with a clean blue flame because they have a higher hydrogen-to-carbon ratio,allowing for complete combustion.	$1.$ Unsaturated hydrocarbons burn with a yellow,sooty flame due to a higher carbon-to-hydrogen ratio,which leads to incomplete combustion.
$2.$ They undergo complete combustion,producing only $CO_{2}$ and $H_{2}O$ as products.	$2.$ They undergo incomplete combustion,producing unburnt carbon particles (soot) in addition to $CO_{2}$ and $H_{2}O$.

$(ii)$ $(a)$ The chemical name of $C_{4}H_{10}$ is Butane. Its structure is: $CH_{3}-CH_{2}-CH_{2}-CH_{3}$.
$(b)$ The chemical name of $C_{2}H_{2}$ is Ethyne. Its structure is: $H-C\equiv C-H$.

(N/A) $(i)$ Substances that are capable of adding oxygen to other substances are known as oxidising agents.
$(ii)$ Alkaline potassium permanganate $(KMnO_4)$ or acidified potassium dichromate $(K_2Cr_2O_7)$ act as oxidising agents because they provide oxygen to oxidise alcohols into carboxylic acids.
$(iii)$ The oxidation reaction of ethanol is:
$CH_3CH_2OH \xrightarrow[\text{Acidified } K_2Cr_2O_7]{\text{Alkaline } KMnO_4} CH_3COOH$

(N/A) $(i)$ Vegetable oils contain unsaturated fatty acids,whereas vegetable ghee contains saturated fatty acids,which are considered harmful to health.
$(ii)$ Vegetable oils are converted into vegetable ghee by the addition of hydrogen in the presence of catalysts such as palladium $(Pd)$ or nickel $(Ni)$.
$(iii)$ The process involved is known as Hydrogenation (an addition reaction). It is defined as the chemical process of adding hydrogen atoms to the carbon-carbon double bonds of an organic compound to convert it into a saturated compound.

(N/A) $(i)$ Structural isomerism is the phenomenon in which compounds have the same molecular formula but different structural arrangements of atoms.
$(ii)$ Propane $(C_{3}H_{8})$ cannot exhibit structural isomerism because it contains only three carbon atoms. With only three carbon atoms,there is only one possible way to arrange them in a chain,meaning no alternative structural arrangement can be formed.
$(iii)$ Butane $(C_{4}H_{10})$ has two structural isomers:
$(a)$ $n$-butane: $A$ straight-chain structure where four carbon atoms are linked in a single row.
$(b)$ Isobutane ($2$-methylpropane): $A$ branched-chain structure where three carbon atoms form the main chain and one carbon atom is attached as a branch to the central carbon atom.

(N/A) $(i)$ $A$ $5-8 \%$ solution of acetic acid in water is known as vinegar. Its use: It is commonly used as a preservative in pickles and various food items.
$(ii)$ When dilute ethanoic acid is added to sodium bicarbonate,carbon dioxide $(CO_2)$ gas is produced. Test: This gas turns lime water milky and extinguishes a burning splinter.

(N/A) homologous series is a group of organic compounds having the same functional group and similar chemical properties,in which all successive members differ by a $-CH_2$ unit.
Two consecutive members of the homologous series of alcohols are:
$(i)$ Methanol: $CH_3OH$
(ii) Ethanol: $C_2H_5OH$
$(i)$ The physical properties (such as melting point,boiling point,and solubility) of the compounds in a homologous series are determined by the length of the carbon chain (molecular mass).
(ii) The chemical properties of the compounds in a homologous series are determined by the functional group present in the molecule.

(N/A) $(a) (i)$ $2CH_{3}COOH + 2Na \rightarrow 2CH_{3}COONa + H_{2} \uparrow$
$(ii)$ $2CH_{3}COOH + Na_{2}CO_{3} \rightarrow 2CH_{3}COONa + H_{2}O + CO_{2} \uparrow$
$(iii)$ $CH_{3}COOH + C_{2}H_{5}OH \xrightarrow{\text{conc. } H_{2}SO_{4}} CH_{3}COOC_{2}H_{5} + H_{2}O$
$(b)$ Concentrated sulphuric acid acts as a dehydrating agent. It removes the water molecule formed during the reaction,which shifts the equilibrium forward,thereby increasing the yield of the ester.
$(c)$ Ethanoic acid is used as a preservative in pickles and for making vinegar.

(N/A) Hard water contains calcium $(Ca^{2+})$ and magnesium $(Mg^{2+})$ salts. These ions react with soap molecules to form an insoluble substance known as scum,which prevents the soap from forming a lather.
$(b)$ Detergents are generally the ammonium or sulphonate salts of long-chain carboxylic acids.
$(c)$ The main advantage is that detergents do not form insoluble precipitates (scum) with the calcium and magnesium ions present in hard water,allowing them to remain effective for cleaning even in hard water.

(N/A) soap molecule consists of two parts: a long hydrocarbon chain which is hydrophobic (water-repelling) and an ionic head which is hydrophilic (water-attracting).
When soap is dissolved in water,the hydrophobic tails attach themselves to the oil or dirt particles,while the hydrophilic heads point outwards into the water.
This arrangement forms a spherical structure known as a micelle.
The hydrophobic tails trap the oil droplet in the center,and the hydrophilic heads,being negatively charged,repel each other,keeping the dirt suspended in the water as an emulsion.
This allows the dirt to be easily rinsed away from the clothes during washing.

(A) is ethene $[CH_{2}=CH_{2}]$.
$B$ is ethane $[CH_{3}-CH_{3}]$.
$C$ is chloroethane $[CH_{3}-CH_{2}-Cl]$.
$(b)$ The chemical equation for the conversion of $A$ to $B$ is:
$CH_{2}=CH_{2} + H_{2} \xrightarrow{Ni/Pd/Pt} CH_{3}-CH_{3}$
Type of reaction: Hydrogenation (Addition reaction).

(N/A) $(i)$ $CH_{2}=CH_{2} + H_{2}O \longrightarrow C_{2}H_{5}OH$ (Ethanol)
$(ii)$ $CH_{3}COOH + NaOH \longrightarrow CH_{3}COONa$ (Sodium acetate) $+ H_{2}O$
$(iii)$ $CH_{3}CH_{2}OH \xrightarrow{H_{2}SO_{4}} CH_{2}=CH_{2}$ (Ethene) $+ H_{2}O$
$(iv)$ $HCOOH + Na \longrightarrow HCOONa$ (Sodium formate) $+ \frac{1}{2}H_{2}$
$(v)$ $C_{2}H_{5}OH \xrightarrow{\text{Alk. } KMnO_{4}} CH_{3}COOH$ (Ethanoic acid)

(A) Soaps are $Na^+$ or $K^+$ salts of long-chain carboxylic acids.
Detergents are ammonium or sulphonate salts of long-chain hydrocarbons.
Cleansing action of soap: $A$ soap molecule has two parts: a hydrophilic (water-loving) ionic head and a hydrophobic (water-repelling) hydrocarbon tail. The tail attaches to oily dirt,while the head remains in water. This forms a structure called a $micelle$. When rinsed,the $micelle$ is washed away,carrying the dirt with it.
Soaps do not form lather in hard water because they react with $Ca^{2+}$ and $Mg^{2+}$ ions present in hard water to form an insoluble precipitate known as $scum$.
Problems due to the use of detergents:
$1$. Most detergents are non-biodegradable,leading to water and soil pollution.
$2$. They can cause skin irritation or allergic reactions in some individuals.

(N/A)

$Soaps$	$Detergents$
$(i)$ $Soaps$ are sodium or potassium salts of long-chain carboxylic acids.	$(i)$ $Detergents$ are ammonium or sulphonate salts of long-chain carboxylic acids.
$(ii)$ $Soaps$ have lesser cleansing action compared to detergents.	$(ii)$ $Detergents$ have better cleansing action compared to soaps.
$(iii)$ $Soaps$ are made from animal or plant fats.	$(iii)$ $Detergents$ are made from petrochemicals.
$(iv)$ $Soaps$ are biodegradable.	$(iv)$ $Detergents$ are generally non-biodegradable.

$(b)$ Hard water contains $Ca^{2+}$ and $Mg^{2+}$ ions. These ions react with the long-chain fatty acid part of the soap to form insoluble precipitates known as scum,which hinders the cleaning process. Detergents,however,form soluble salts with these ions,so they do not form scum and work effectively in hard water.
Chemical reaction: $2C_{17}H_{35}COONa + Ca^{2+} (Mg^{2+}) \longrightarrow (C_{17}H_{35}COO)_{2}Ca (Mg) + 2Na^{+}$

(D) Vinegar is made by the fermentation of ethanol using bacteria like $Acetobacter$ $aceti$,which produces a $5-8\%$ aqueous solution of acetic acid.
$(b)$ Pure ethanoic acid (acetic acid) is known as glacial acetic acid because it freezes into an ice-like solid at temperatures below $290 \ K$.
$(c)$ Carboxylic acids are called weak acids because they do not ionize completely in an aqueous solution; they only partially dissociate into ions.
$(d)$ When the ester $CH_{3}COOC_{2}H_{5}$ (ethyl ethanoate) undergoes saponification,it reacts with a base (like $NaOH$) to form:
$1.$ Sodium ethanoate $(CH_{3}COONa)$
$2.$ Ethanol $(C_{2}H_{5}OH)$

(A) $(i)$ Ethanol $(CH_3CH_2OH)$ undergoes oxidation with alkaline $KMnO_4$ to form Ethanoic acid $(A = CH_3COOH)$.
$(ii)$ Ethanol reacts with Ethanoic acid $(A)$ in the presence of concentrated $H_2SO_4$ to form Ethyl ethanoate $(B = CH_3COOCH_2CH_3)$ and water.
$(iii)$ Ethyl ethanoate $(B)$ undergoes saponification with $NaOH$ to form Sodium ethanoate $(C = CH_3COONa)$ and Ethanol.
$(iv)$ Ethanoic acid $(A)$ reacts with $NaHCO_3$ to form Sodium ethanoate $(C)$,Carbon dioxide $(D = CO_2)$,and water.
$(v)$ Ethanol reacts with Sodium $(E = Na)$ to form Sodium ethoxide $(CH_3CH_2ONa)$ and Hydrogen gas.

(N/A) Chemical Nature: Soaps are the sodium or potassium salts of long-chain fatty acids (e.g.,stearic acid).
Difference from Synthetic Detergents: Soaps are effective only in soft water and form scum in hard water. Synthetic detergents are ammonium or sulphonate salts of long-chain carboxylic acids and are effective in both soft and hard water.
Cleansing Action (Micelle Formation): Soap molecules have two parts: a hydrophilic ionic head and a hydrophobic hydrocarbon tail.
When soap is dissolved in water,the hydrophobic tails attach to the dirt (oil/grease) particles,while the hydrophilic heads remain in the water.
This forms a spherical structure called a micelle,with the dirt trapped in the center.
The ionic heads on the surface of the micelle repel each other due to like charges,keeping the micelles suspended in water as a colloid,which is then washed away with the water.

(N/A) Hydrocarbons are organic compounds consisting entirely of carbon and hydrogen atoms linked by covalent bonds.
$(b)$ The structural difference is that saturated hydrocarbons contain only single covalent bonds between carbon atoms (e.g.,alkanes),whereas unsaturated hydrocarbons contain at least one double or triple covalent bond between carbon atoms (e.g.,alkenes or alkynes).

(N/A) Hard water contains dissolved salts of calcium and magnesium. When soap is added to hard water,it reacts with these ions to form an insoluble precipitate known as scum. This prevents the formation of lather,making it ineffective for cleaning.
$(b)$ Soap molecules have a dual nature: a hydrophilic (water-loving) head and a hydrophobic (water-repelling) hydrocarbon tail. When added to water,the hydrophobic tails cluster together to avoid water,while the hydrophilic heads face outward toward the water,resulting in the formation of spherical structures called micelles.
$(c)$ Saponification is the chemical process in which an ester is reacted with an alkali (like sodium hydroxide) to produce soap (a salt of a fatty acid) and alcohol (glycerol).

(A) $(i)$ Combustion of methane in excess oxygen produces carbon dioxide, water, heat, and light: $CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O + \text{Heat} + \text{Light}$.
$(ii)$ Esterification reaction between ethanoic acid and ethanol in the presence of an acid catalyst produces ethyl ethanoate (ester) and water: $CH_3COOH + C_2H_5OH \xrightarrow{\text{Acid}} CH_3COOC_2H_5 + H_2O$.
$(iii)$ Dehydration of ethanol using concentrated sulfuric acid at $443 \text{ K}$ produces ethene and water: $CH_3CH_2OH \xrightarrow[\text{443 K}]{\text{Conc. } H_2SO_4} CH_2=CH_2 + H_2O$.

(N/A) homologous series is a group of organic compounds having the same functional group and similar chemical properties,where each successive member differs by a $CH_{2}$ unit.
$1$. For $CH_{3}OH$ (Methanol),the next homologue is obtained by adding a $CH_{2}$ group,which results in $C_{2}H_{5}OH$ (Ethanol).
$2$. For $HCOOH$ (Methanoic acid),the next homologue is obtained by adding a $CH_{2}$ group,which results in $CH_{3}COOH$ (Ethanoic acid).

(A) As the molecular mass of hydrocarbons increases,the intermolecular forces of attraction (van der Waals forces) between the molecules also increase.
Due to these stronger intermolecular forces,more energy is required to overcome them,which leads to an increase in both the boiling point and the melting point of the hydrocarbons.

(N/A) homologous series is a group of organic compounds having the same functional group and similar chemical properties,in which the successive members differ by a $CH_2$ unit.
Characteristics:
$(i)$ All members of a homologous series can be represented by the same general molecular formula.
(ii) Any two adjacent members of the series differ by one carbon atom and two hydrogen atoms,i.e.,a $CH_2$ unit in their molecular formula.

(N/A) To distinguish between ethanol and ethanoic acid,the Sodium Hydrogen Carbonate $(NaHCO_3)$ test is used.
Procedure: Take a small amount of both substances in separate test tubes and add a pinch of solid $NaHCO_3$ to each.
Observation: Ethanoic acid reacts with $NaHCO_3$ to produce brisk effervescence due to the evolution of $CO_2$ gas.
Conclusion: Ethanol does not react with $NaHCO_3$ and shows no effervescence,thus allowing us to distinguish between the two.

(C) The suffix '-one' in the name 'Pentanone' indicates the presence of a ketone functional group. $A$ ketone is characterized by a carbonyl group $(C=O)$ bonded to two carbon atoms.

(N/A) Ethanol undergoes dehydration in the presence of hot concentrated sulfuric acid $(H_2SO_4)$ at $443 \ K$ to form ethene:
$CH_3CH_2OH \xrightarrow[Conc. H_2SO_4]{443 \ K} CH_2=CH_2 + H_2O$
$(b)$ Ethanol undergoes oxidation in the presence of alkaline potassium permanganate $(KMnO_4)$ or acidified potassium dichromate $(K_2Cr_2O_7)$ to form ethanoic acid:
$CH_3CH_2OH \xrightarrow[alk. KMnO_4]{Oxidation} CH_3COOH$

(N/A) Substitution reaction: Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride.
$CH_4 + Cl_2 \xrightarrow{\text{Sunlight}} CH_3Cl + HCl$
$(b)$ Saponification reaction: Ethyl acetate reacts with sodium hydroxide to form ethanol and sodium acetate.
$CH_3COOC_2H_5 + NaOH \longrightarrow C_2H_5OH + CH_3COONa$
$(c)$ Combustion reaction: Methane burns in oxygen to produce carbon dioxide,water,and heat energy.
$CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O + \text{Heat} + \text{Light}$

(N/A) The alcohol with three carbon atoms is $Propanol$ (specifically $Propan-1-ol$).
The chemical formula is $C_{3}H_{7}OH$.
The structural formula is:
$CH_{3}-CH_{2}-CH_{2}-OH$

(B, D) The functional groups that always occur at the terminal position of a carbon chain are the Aldehydic group $(-CHO)$ and the Carboxyl group $(-COOH)$.
In these groups,the carbon atom is already bonded to an oxygen atom via a double bond and to a hydrogen atom (in aldehydes) or a hydroxyl group (in carboxylic acids),leaving only one valency available for the carbon chain,which forces them to be at the end of the chain.

(A) The general formula $C_{n}H_{2n-2}$ represents the homologous series of alkynes.
For the $1^{st}$ member,$n = 2$,which is Ethyne $(C_{2}H_{2})$.
For the $2^{nd}$ member,$n = 3$,which is Propyne.
The chemical formula for Propyne is $C_{3}H_{4}$.

(N/A) In a homologous series,as the molecular mass increases,the following characteristics change:
$(a)$ Melting and boiling points increase gradually due to the increase in intermolecular forces.
$(b)$ Physical properties such as density and solubility in a particular solvent show a regular gradation.
$(c)$ Molecular mass increases,which leads to a change in the physical state (e.g.,from gas to liquid to solid).
$(d)$ The chemical properties remain similar,but the intensity of reactions may vary slightly due to the change in the carbon chain length.