A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

Why do silver articles turn black when kept in the open for a few days?

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

Name the black substance formed and give its chemical formula.

On heating blue coloured powder of copper $\left(\mathrm{II}\right)$ nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas $\mathrm{X}$ is formed.

Write a balanced chemical equation of the reaction.

On heating blue coloured powder of copper $\left(\mathrm{II}\right)$ nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas $\mathrm{X}$ is formed.

Identify the brown gas $\mathrm{X}$ evolved.

On heating blue coloured powder of copper$\left(\mathrm{II}\right)$ nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas $\mathrm{X}$ is formed.

Identify the type of reaction.

On heating blue coloured powder of copper$\left(\mathrm{II}\right)$ nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas $\mathrm{X}$ is formed.

What could be the $\mathrm{pH}$ range of aqueous solution of the gas $\mathrm{X}$?

Give the characteristic test for the following gas.

${\mathrm{CO}}_2$

Give the characteristic test for the following gas.

${\mathrm{SO}}_2$

Give the characteristic test for the following gas.

 ${\mathrm{O}}_2$