Translate the following statement into a chemical equation and balance it. 

 Ammonia gas reacts with water to give ammonium hydroxide. 

Translate the following statement into a chemical equation and balance it. 

Copper$\left(\mathrm{II}\right)$ oxide on heating with hydrogen gas gives copper and water. 

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:

${\mathrm{O}}_2$

Give the characteristic test for the following gas:

${\mathrm{H}}_2$