In which example are both gas evolution and a colour change observed as signs of a chemical reaction?
Answer and explanation
Correct answer: Heating lead nitrate
On heating, lead nitrate undergoes thermal decomposition. The white lead nitrate changes into yellow lead oxide, and brown nitrogen dioxide gas is evolved. Thus, both a colour change and gas evolution are observed. The reaction is: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). In contrast, zinc reacting with dilute hydrochloric acid mainly shows evolution of hydrogen gas, not a clear colour change.
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What is the correct answer to this question?
Heating lead nitrate
Why is this the correct answer?
On heating, lead nitrate undergoes thermal decomposition. The white lead nitrate changes into yellow lead oxide, and brown nitrogen dioxide gas is evolved. Thus, both a colour change and gas evolution are observed. The reaction is: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). In contrast, zinc reacting with dilute hydrochloric acid mainly shows evolution of hydrogen gas, not a clear colour change.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Chemical Reactions and Equations.
