Practical Work

Detection of Some Gases

Test for hydrogen (H₂):

Hydrogen gas is liberated when active metals such as Na, K, Mg react with dilute acids.

$$\begin{gathered} M + {\mathrm{H}}_2{\mathrm{SO}}_4 \stackrel{ }{\to } M{\mathrm{SO}}_4 + {\mathrm{H}}_2 \\ M + \mathrm{HCl} \stackrel{ }{\to } M\mathrm{Cl} + {\mathrm{H}}_2 \\ \left[M=\mathrm{Ca}, \mathrm{Mg}, \mathrm{Zn}, \mathrm{etc}.\right] \end{gathered}$$

Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that hydrogen gas is neutral to litmus.

iii. A burning wooden splint, when brought near this gas, gets off and burns with a pale blue flame producing a pop sound.

$2 {\mathrm{H}}_2 + {\mathrm{O}}_2 \stackrel{ }{\to } 2 {\mathrm{H}}_2\mathrm{O}$

 

Test for oxygen (O₂):

Oxygen gas is liberated on heating metal nitrates, potassium chlorate, potassium dichromate, potassium permanganate, hydrogen peroxide, barium peroxide and oxides such as HgO, PbO₂, Pb₃O₄.

$$\begin{gathered} 2\hspace{0.17em}{\mathrm{PbO}}_2 \stackrel{ \mathrm{\Delta } }{\to } 2 \mathrm{PbO} + {\mathrm{O}}_2 \\ 2 \mathrm{HgO} \stackrel{ \mathrm{\Delta } }{\to } 2 \mathrm{Hg} + {\mathrm{O}}_2 \end{gathered}$$

Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that oxygen gas is neutral to litmus.

iii. A burning wooden splint when brought near this gas re-lights brightly which shows that it is a supporter of combustion.

 

Test for water vapour (H₂O):

Water vapour is liberated on heating salts containing water of crystallisation, metallic hydroxides and metallic hydrogen carbonates.

$$\begin{gathered} {\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O} \stackrel{ \mathrm{\Delta } }{\to } {\mathrm{Na}}_2{\mathrm{CO}}_3 + 10 {\mathrm{H}}_2\mathrm{O} \\ {\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O} \stackrel{ \mathrm{\Delta } }{\to } {\mathrm{CuSO}}_4 + 5 {\mathrm{H}}_2\mathrm{O} \end{gathered}$$

Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that water vapour is neutral to litmus.

iii. It turns anhydrous copper sulphate to blue.

${\mathrm{CuSO}}_4 + 5 {\mathrm{H}}_2\mathrm{O} \stackrel{ }{\to } {\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$

iv. It turns blue copper chloride to pink.

${\mathrm{CoCl}}_2 + 2 {\mathrm{H}}_2\mathrm{O} \stackrel{ }{\to } {\mathrm{CoCl}}_2.2{\mathrm{H}}_2\mathrm{O}$

 

Test for ammonia (NH₃):

Ammonia is liberated by heating ammonium salts with alkalies and treating metallic nitrides with warm water.

$2 {\mathrm{NH}}_4\mathrm{Cl} + \mathrm{Ca}{\left(\mathrm{OH}\right)}_2 \stackrel{ }{\to } {\mathrm{CaCl}}_2 + 2 {\mathrm{H}}_2\mathrm{O} + 2 {\mathrm{NH}}_3$

Few characteristics of this gas are as follows.

i. It is a colourless gas with a strong biting odour that brings tears to eyes.

ii. When this gas is allowed to pass through a moist red litmus paper, the colour of the paper changes to blue. This shows that ammonia is basic in nature.

iii. Dense white fumes are formed when a rod dipped in HCl is brought near this gas.

${\mathrm{NH}}_3 + \mathrm{HCl} \stackrel{ }{\to } {\mathrm{NH}}_4\mathrm{Cl}$

iv. It turns Nessler's reagent (K₂HgI₄) brown.

v. It forms a pale blue precipitate when passed through copper sulphate solution. This precipitate is soluble in excess of the gas and the solution turns dark blue in colour.

 

Test for carbon dioxide (CO₂):

Carbon dioxide is liberated by strong heating of metallic carbonates and hydrogen carbonates. It is also liberated when dilute mineral acids are treated with carbonates and hydrogen carbonates of metals.

$$\begin{gathered} {\mathrm{ZnCO}}_3 \stackrel{ \mathrm{\Delta } }{\to } \mathrm{ZnO} + {\mathrm{CO}}_2 \\ {\mathrm{CaCO}}_3 + 2 \mathrm{HCl} \stackrel{ }{\to } {\mathrm{CaCl}}_2 + {\mathrm{H}}_2\mathrm{O} + {\mathrm{CO}}_2 \\ {\mathrm{NaHCO}}_3 + \mathrm{HCl} \stackrel{ }{\to } \mathrm{NaCl} + {\mathrm{H}}_2\mathrm{O} + {\mathrm{CO}}_2 \end{gathered}$$

Few characteristics of this gas are as follows.

i. It is a colourless and odourless gas.

ii. It turns lime water milky.

$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2 + {\mathrm{CO}}_2 \stackrel{ }{\to } {\mathrm{CaCO}}_3 + {\mathrm{H}}_2\mathrm{O}$

ii. When this gas is allowed to pass through a moist blue litmus paper, the colour of the paper changes to light red. This shows that carbon dioxide is acidic …