(a) Dry hydrogen chloride gas does not turn blue litmus red whereas hydrochloric acid does. Give one reason.

(b) An organic compound burns with a sooty flame. Is it a saturated or an unsaturated compound? Give reason.

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Solution

(a) Blue litmus is an indicator which changes colour only in the presence of ions like H⁺ or H₃O⁺.
Hydrochloric acid in its aqueous form dissociates and produces H⁺ions which turn the blue litmus red. However, dry hydrogen chloride gas cannot dissociate and produce H⁺ ions, as a result of which it cannot turn blue litmus red.

(b) An organic compound which burns with a sooty flame will be an unsaturated compound.
Saturated hydrocarbons contain lower percentage of carbon and get completely oxidised in the presence of oxygen.
Whereas unsaturated hydrocarbons contain higher percentage of carbon as a result of which they are not completely oxidized in the presence of oxygen. These unburnt carbon particles produce a lot of smoke and soot.

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