Question

A colourless gas which burns with blue flame and reduces CuO to Cu is:
(A) ${{N}_{2}}$
(B) CO
(C) $C{{O}_{2}}$
(D) $N{{O}_{2}}$

Answer 1

When gas is burnt in presence of oxygen the flame is blue, this is not true for all gases, but LPG gases burn with a blue flame in presence of oxygen however, in absence of oxygen, or if there is less oxygen present, then it burns with a yellow flame.

Complete step by step answer: Copper(II) oxide or cupric oxide is the inorganic compound with the formula CuO. A stable black solid with 2 oxides, the other being $C{{u}_{2}}O$ or copper(I) oxide.As a mineral it is known as tenorite.It is produced on a large scale by pyrometallurgy, as one stage in extracting copper from its ores. It can be formed by heating copper in air at around $300-{{800}^{{}^\circ }}C$.
It can be reduced to copper using metal using hydrogen, carbon monoxide, or carbon:

& CuO+{{H}_{2}}\to Cu+{{H}_{2}}O \\\ & CuO+CO\to Cu+C{{O}_{2}} \\\ & 2CuO+C\to 2Cu+C{{O}_{2}} \\\ \end{aligned}$$ **Therefore, carbon monoxide reduces CuO to Cu.** Carbon monoxide burns with blue flame. In the presence of oxygen including atmospheric concentrations carbon monoxide burns with a blue flame producing carbon dioxide. When nitrogen gas burns, It has no color, odor or taste, and produces water when it burns in air. Carbon dioxide is released when clean fuels like propane,alcohol or natural gas burn, with water vapor as a by-product, both of which are invisible. Nitric oxide is a colorless, flammable gas with a slight odor. Nitrogen dioxide is a deep red-orange gas that is poisonous but not flammable. It, along with aerosols, is responsible for the reddish-brown color of smog. **Note:** The inner core of the candle flame is light blue, with a temperature of around 1800 K ($1500\text{ }{}^\circ C$). That is the hottest part of the flame. The color inside the flame becomes yellow, orange, and finally red. The further you get from the center of the flame, the lower the temperature will be.