How can we test for the presence of hydrogen gas?

Write a chemical equation for the reaction taking place in the test tube. How does the colour of the solution change? What is the change in colour of iron nails? 

_____ reacts with metals and non-metals and forms their oxides.

(Sulphur, Nitrogen, Oxygen, Carbon dioxide, Lighter, Oxygen, Air pollution, CFCs)

Complete the following equation:
 $\mathrm{Mg}\left(\mathrm{s}\right)$ $+$ _____$\left(\mathrm{l}\right)$ $\to \mathrm{MgO}\left(\mathrm{s}\right) +{\mathrm{H}}_2\left(\mathrm{g}\right)$

$\mathrm{A}$: ${\mathrm{O}}_2$

$\mathrm{B}$: ${\mathrm{H}}_2{\mathrm{O}}_2$

$\mathrm{C}$: ${\mathrm{H}}_2\mathrm{O}$

$\mathrm{D}$: $\mathrm{NaCl}$

Enter your correct answer as $\mathrm{A}, \mathrm{B}, \mathrm{C}$ or $\mathrm{D}$.

Choose the right word and fill in the blank provided below.

Silver articles get tarnished after sometime when exposed to air because of the formation of _____. (silver sulphide, copper sulphide)

Fill in the blanks with the correct options from the box.

Metals react with acids to produce _____ gas.

Complete the following reaction sequence showing the rusting of iron.

 _{$3\mathrm{Fe}+{\mathrm{O}}_2+$_____$\to 3\mathrm{FeO}+{\mathrm{H}}_2$}