NO2 is obtained by heating:

NO2, also known as nitrogen dioxide, is a reddish-brown gas that is formed when certain nitrogen compounds are heated. It is an important atmospheric pollutant and plays a role in the formation of photochemical smog. Let's explore the compounds that can produce NO2 when heated.

A) Lead Nitrate (Pb(NO3)2):
- When lead nitrate is heated, it undergoes decomposition to produce lead oxide, nitrogen dioxide, and oxygen gas.
- The balanced chemical equation for this reaction is:
2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
- Therefore, lead nitrate can be used to obtain NO2 by heating.

B) Potassium Nitrate (KNO3):
- Potassium nitrate does not directly produce NO2 when heated.
- Instead, it undergoes decomposition to produce potassium nitrite (KNO2), oxygen gas (O2), and nitrogen gas (N2).
- The balanced chemical equation for this reaction is:
2KNO3(s) → 2KNO2(s) + O2(g) + N2(g)
- Therefore, potassium nitrate cannot be used to obtain NO2 by heating.

C) Lithium Nitrate (LiNO3):
- Lithium nitrate does not directly produce NO2 when heated.
- It undergoes decomposition to produce lithium oxide (Li2O), nitrogen dioxide (NO2), and oxygen gas (O2).
- The balanced chemical equation for this reaction is:
2LiNO3(s) → Li2O(s) + 2NO2(g) + O2(g)
- Therefore, lithium nitrate can be used to obtain NO2 by heating.

D) Silver Nitrate (AgNO3):
- Silver nitrate does not directly produce NO2 when heated.
- It undergoes decomposition to produce silver oxide (Ag2O), nitrogen dioxide (NO2), and oxygen gas (O2).
- The balanced chemical equation for this reaction is:
2AgNO3(s) → Ag2O(s) + 2NO2(g) + O2(g)
- Therefore, silver nitrate can be used to obtain NO2 by heating.

In summary, NO2 can be obtained by heating lead nitrate, lithium nitrate, and silver nitrate. These compounds undergo decomposition reactions to produce NO2 along with other products. Potassium nitrate, on the other hand, does not directly produce NO2 when heated.