Question

A colourless lead salt, when heated, produces a yellow residue and brown fumes. Write a chemical equation of the reaction involved.

Answer 1

The question talks about a colourless lead salt. Think about the salts formed by lead which on heating evolves brown fumes. Recollect the inorganic qualitative salt analysis and think about the anions which on heating evolve brown fumes. Then write the chemical reaction.

Complete step by step solution:
- A salt which produces brown fumes on heating can either contain nitrates or bromide as anions. Nitrates and bromides on strong heating will form nitrogen dioxide and bromine gas respectively which are seen as brown coloured fumes.
- Here the question says a yellow residue is formed. This indicates that lead oxide is formed, so the colourless salt of lead which undergoes decomposition is lead nitrate.
- Decomposition reaction is the process in which a compound under the presence of strong heat forms two or more simpler compounds.
- Lead nitrate on strong heating undergoes decomposition to form lead oxide which is yellow in colour and nitrogen dioxide and oxygen gas are evolved.
- Nitrogen dioxide gas is brown in colour.
- The reaction is represented as,
$2Pb{{\left( N{{O}_{3}} \right)}_{2}}\to 2PbO+4N{{O}_{2}}+{{O}_{2}}$
- Therefore, the chemical reaction which is involved when a colourless lead salt is heated is,
$2Pb{{\left( N{{O}_{3}} \right)}_{2}}\to 2PbO+4N{{O}_{2}}+{{O}_{2}}$

Note: Remember, lead nitrate is colourless, lead oxide is yellow in colour and nitrogen dioxide gas is brown in colour. Lead nitrate on decomposition forms lead oxide with the evolution of nitrogen dioxide gas and oxygen gas. Nitrates on heating evolve nitrogen gas which is brown in colour. This is one of the tests used to detect the presence of nitrate.