(a) (i) A compound 'X' having two carbon atoms in its molecule turns blue litmus red and $5-8\%$ solution of 'X' in water is widely used as a preservative. Identify the compound 'X' and write its structure.

(ii) Compare its pH nature with a mineral acid.

(iii) 'X' on reacting with alcohols produces sweet smelling compounds, used in making perfumes. Name the reaction and write its chemical equation.

(iv) When sodium carbonate is added to 'X', a colourless gas is produced which turns lime water milky. Write the chemical equation for the reaction giving the name of the salt produced.

OR

(b) (i) Differentiate between saturated and unsaturated hydrocarbons by giving one example each, with a structural formula.

(ii) Write the method of converting an unsaturated hydrocarbon into a saturated hydrocarbon. Name the industry where this reaction is commonly used.

(iii) Write the name and structure of a hydrocarbon having double bond and four carbon atoms in its one molecule.

(a)

(i) Identifying 'X': The compound 'X' is ethanoic acid (acetic acid), $\mathrm{CH_3COOH}$. As stated in the NCERT text, a $5$–$8\%$ solution of acetic acid in water is called vinegar and is widely used as a preservative in pickles; being a carboxylic acid it turns blue litmus red.

Structure:

$$\mathrm{CH_3-COOH}$$

(the $-\mathrm{COOH}$ carboxyl group consists of a carbon doubly bonded to one oxygen, $=\mathrm{O}$, and singly bonded to a hydroxyl group, $-\mathrm{OH}$, on that same carbon atom)

(ii) Comparison of pH nature with a mineral acid: Ethanoic acid belongs to the class of carboxylic acids, which — unlike mineral acids such as HCl — are only weak acids; that is, they ionise only partially in water, releasing relatively few $\mathrm{H^+}$ ions. A mineral acid like HCl is a strong acid that ionises almost completely, releasing a much larger concentration of $\mathrm{H^+}$ ions for the same molar concentration. Hence, for equal concentrations, the pH of ethanoic acid solution is higher (closer to $7$, i.e. less acidic) than that of a mineral acid like HCl (lower pH, more acidic), even though both turn blue litmus red — this is exactly what Activity 4.7 of the chapter demonstrates: both acids are indicated by the litmus test, but the universal indicator shows them as not equally strong.

(iii) Reaction with alcohols (Esterification): 'X' reacts with alcohols in the presence of an acid catalyst (concentrated $\mathrm{H_2SO_4}$) to form a sweet-smelling ester. This reaction is called esterification.

$$\mathrm{CH_3COOH} + \mathrm{CH_3CH_2OH} \xrightarrow[\text{catalyst}]{\text{Conc. } H_2SO_4} \mathrm{CH_3COOC_2H_5} + \mathrm{H_2O}$$

$$\text{(Ethanoic acid)} \qquad \text{(Ethanol)} \qquad\qquad \text{(Ethyl ethanoate — an ester)}$$

Esters are used in making perfumes and as flavouring agents, and on treatment with NaOH they undergo saponification, being converted back to alcohol and the sodium salt of the acid.

(iv) Reaction with sodium carbonate: When sodium carbonate is added to 'X' (ethanoic acid), brisk effervescence of a colourless gas, $\mathrm{CO_2}$, occurs; this gas turns lime water milky (forming a white precipitate of calcium carbonate), confirming it is carbon dioxide. The salt produced is sodium ethanoate (sodium acetate).

$$2\mathrm{CH_3COOH} + \mathrm{Na_2CO_3} \rightarrow 2\mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}\uparrow$$

$$\text{(Sodium ethanoate)}$$

$$\text{Test: } \mathrm{CO_2} + \mathrm{Ca(OH)_2} \rightarrow \mathrm{CaCO_3}\downarrow(\text{milky}) + \mathrm{H_2O}$$

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OR (b)

(i) Saturated vs unsaturated hydrocarbons:

(ii) Converting an unsaturated to a saturated hydrocarbon: This is done by an addition reaction — the unsaturated hydrocarbon is treated with hydrogen gas ($\mathrm{H_2}$) in the presence of a catalyst such as nickel (Ni) or palladium (Pd), which adds hydrogen across the double/triple bond to give the corresponding saturated compound:

$$\mathrm{R{-}CH{=}CH{-}R'} + \mathrm{H_2} \xrightarrow{\text{Ni catalyst}} \mathrm{R{-}CH_2{-}CH_2{-}R'}$$

This reaction (hydrogenation) is commonly used in the vegetable oil / edible fat (vanaspati) industry, where unsaturated vegetable oils (with long unsaturated carbon chains) are hydrogenated using a nickel catalyst to convert them into saturated fats.

(iii) Hydrocarbon with a double bond and four carbon atoms: This is butene (but-1-ene), molecular formula $\mathrm{C_4H_8}$.

$$\mathrm{CH_2=CH-CH_2-CH_3}$$