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Module 8: Applying Chemical Ideas
Quick questions on Tests for unsaturation, hydroxyl and carboxyl groups explained: HSC Chemistry Module 8
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is tests for C=C (unsaturation)?Show answer
Bromine water. Shake the unknown with a few drops of orange bromine water. An alkene rapidly decolourises the orange to colourless by adding $Br_2$ across the double bond:
What is tests for hydroxyl $-OH$ (alcohol)?Show answer
Sodium metal. Drop a small piece of clean sodium into the unknown (dried). An alcohol releases hydrogen gas and forms a sodium alkoxide:
What is tests for carboxylic acid $-COOH$?Show answer
Sodium carbonate (or sodium bicarbonate). Add solid $Na_2CO_3$ or $NaHCO_3$ to the unknown dissolved in water (or to the neat liquid). Brisk effervescence of $CO_2$ indicates a carboxylic acid:
What is a flowchart for an unknown organic liquid?Show answer
Suppose you suspect an alkane, an alkene, an alcohol or a carboxylic acid:
What is bromine water?Show answer
Shake the unknown with a few drops of orange bromine water. An alkene rapidly decolourises the orange to colourless by adding $Br_2$ across the double bond:
What is selectivity caveat?Show answer
Permanganate also oxidises aldehydes, primary and secondary alcohols, and some aromatic side chains. So a positive permanganate test alone does not prove an alkene. Bromine water at room temperature is more specific.
What is sodium metal?Show answer
Drop a small piece of clean sodium into the unknown (dried). An alcohol releases hydrogen gas and forms a sodium alkoxide:
What is ester formation?Show answer
Heat the unknown with a carboxylic acid (ethanoic acid) and a few drops of concentrated $H_2SO_4$. A sweet, fruity smell indicates ester formation, confirming an alcohol. Reverse the reagents to confirm a carboxylic acid.
What is sodium carbonate?Show answer
Add solid $Na_2CO_3$ or $NaHCO_3$ to the unknown dissolved in water (or to the neat liquid). Brisk effervescence of $CO_2$ indicates a carboxylic acid:
What is litmus or universal indicator?Show answer
A carboxylic acid solution has pH around 3 to 4 (depending on concentration). Litmus turns red. Alcohols are essentially neutral.
What is reactive metal?Show answer
Magnesium ribbon dissolves in a carboxylic acid solution with hydrogen evolution:
What is esterification?Show answer
Warm with an alcohol and concentrated $H_2SO_4$; the fruity ester smell confirms the carboxyl group.
What is treating bromine water and permanganate as equivalent unsaturation tests?Show answer
Bromine water is more specific; permanganate also reacts with alcohols and aldehydes.
What is forgetting to do the carbonate test before the sodium test?Show answer
A sample of acid plus alcohol would give hydrogen with sodium and confuse the identification. The carbonate test discriminates first.
What is using wet sodium?Show answer
Sodium reacts with water more vigorously than with most alcohols, and any water in the sample will mask the alcohol test. Dry the sample first.