A student observes a decomposition reaction where a pale yellow powder changes into a red-brown solid and a colourless gas upon heating. Which compound was used?
Lead nitrate
On heating, lead nitrate undergoes thermal decomposition: \(2Pb(NO_3)_2 \xrightarrow{\Delta} 2PbO + 4NO_2 + O_2\).
The solid residue is lead(II) oxide, PbO, which looks reddish-brown when hot and yellow when cold. Along with it, brown fumes of nitrogen dioxide and oxygen are given off – oxygen is the colourless gas, and it re-lights a glowing splinter. This matches a solid changing into a red-brown solid and releasing a colourless gas.
The other compounds behave differently: potassium permanganate is a purple solid that gives green \(K_2MnO_4\), black \(MnO_2\) and oxygen; silver bromide decomposes in light (not mainly by heating) into grey silver and bromine; zinc carbonate is white and gives white zinc oxide (yellow only while hot) and carbon dioxide – none of these leaves a red-brown solid.
Hence, the compound used was lead nitrate.
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Important Questions from Chemical Reaction
Which of the following statement is/are true?
I. In physical change new substance is formed. substance is formed.
II. In chemical change no new substance is formed.
Which of the following cells involves similar chemical reactions?
The information not obtained from balanced chemical equation is
Oxygen cannot be obtained by which one of the following methods?
Which of the following statement is correct?
I. Insoluble salt is obtained by precipitation reaction.
II. Addition of oxygen or loss of hydrogen is called oxidation.
