2.1 Class A

  Salt Analysis

• Dilute $HCl$: It gives effervescence due to the evolution of carbon dioxide ($CO_2$) $$CO_3^{2 - } + 2{H^ + } \to C{O_2} \uparrow + {H_2}O$$ The gas gives white turbidity with lime water and baryta water $$\begin{equation} \begin{aligned} C{O_2} + C{a^{2 + }} + 2O{H^ - } \to CaC{O_3} \downarrow + {H_2}O \\ C{O_2} + B{a^{2 + }} + 2O{H^ - } \to BaC{o_3} \downarrow + {H_2}O \\\end{aligned} \end{equation} $$ On prolonged passage of carbon dioxide in lime water , the turbidity slowly disappears due to the formation of soluble hydrogen carbonate of calcium $Ca{\left( {HC{O_3}} \right)_2}$.$$CaC{O_3} \downarrow + C{O_2} + {H_2}O \to Ca{(HC{O_3})_2}$$

• Barium chloride or Calcium chloride solution: White precipitate of barium or calcium carbonate is obtained which is soluble in mineral acid. $$CO_3^{2 - } + 2{H^ + } \to C{O_2} \uparrow + {H_2}O$$$$CO_3^{2 - } + C{a^{2 + }} \to CaC{O_3} \downarrow $$

• Silver nitrate solution: White precipitate of silver carbonate is obtained. $$\begin{equation} \begin{aligned} CO_3^{2 - } + 2A{g^ + } \to A{g_2}C{O_3} \downarrow \\ \\\end{aligned} \end{equation} $$ The precipitate so obtained is soluble in nitric acid ($HNO_3$) and in ammonia ($NH_3$). The precipitate becomes yellow or brown on addition of excess reagent and same may also happen if the mixture is boiled due to the formation of silver oxide. $$A{g_2}C{O_3} \downarrow \; \to A{g_2}O \downarrow + C{O_2} \uparrow $$

• Dil. $HCl$ or Dil. ${H_2}S{O_4}$: It decomposes with the evolution of sulphur dioxide. $$\begin{equation} \begin{aligned} SO_3^{2 - } + 2{H^ + } \to S{O_2} + {H_2}O \\ \\\end{aligned} \end{equation} $$

• Acidified potassium dichromate solution $\left( {{K_2}C{r_2}{O_7}} \right)$: The gas turns filter paper moistened with acidified potassium dichromate solution, green due to the formation of $C{r^{3 + }}$ ions. $$\begin{equation} \begin{aligned} 3S{O_2} + {K_2}C{r_2}{O_7} + {H_2}S{O_4} \to {K_2}S{O_4} + C{r_2}{(S{O_4})_3}\ (green) + {H_2}O \\ \\\end{aligned} \end{equation} $$

• Lime water: On passing the gas through lime water, a white milky precipitate is formed. $$S{O_2} + Ca{(OH)_2} \to CaS{O_3} \downarrow (milky) + {H_2}O$$ Percipitate dissolves on prolonged passage of the gas, due to the formation of soluble hydrogen sulphite ions. $$CaS{O_3} \downarrow + S{O_2} + {H_2}O \to Ca{(HS{O_3})_2}$$

• Barium chloride or Strontium chloride solution: Salt solutions gives white ppt of barium or strontium sulphite.$$SO_3^{2 - } + B{a^{2 + }} \to BaS{O_3} \downarrow $$$$SO_3^{2 - } + S{r^{2 + }} \to SrS{O_3} \downarrow $$

• Dil. $HCl$ or Dil. ${H_2}S{O_4}$: A colourless gas with a smell of rotten egg (${H_2}S$) is evolved.$${S^{2 - }} + 2{H^ + } \to {H_2}S \uparrow $$

• The gas turns lead acetate paper black. $${(C{H_3}COO)_2}Pb + {H_2}S \to PbS \downarrow (black) + 2C{H_3}COOH$$

• Salt solution gives yellow ppt with $CdC{l_2}$. $$N{a_2}S + CdC{l_2} \to CdS \downarrow (yellow) + 2NaCl$$

• Silver nitrate solution: Black ppt of silver sulphide insoluble in cold but soluble in hot dil. nitric acid. $${S^{2 - }} + 2A{g^ + } \to A{g_2}S \downarrow (black)$$

• Sodium nitroprusside solution: Turns sodium nitroprusside solution purple. $$N{a_2}S + N{a_2}[Fe{(CN)_5}NO] \to N{a_4}[Fe{(CN)_5}NOS]\ (purple)$$

• Dil. $HCl$ or Dil. ${H_2}S{O_4}$: Adding to solid nitrite in cold, yield pale blue liquid (due to the presence of free nitrous acid$HN{O_2}$ or its anhydride ${N_2}{O_3}$ ) & the evolution of brown fumes of nitrogen dioxide, the latter being largely produced by combination of nitric oxide with the oxygen of air. $$NO_2^ - + {H^ + } \to HN{O_2}$$ $$3HN{O_2} \to HN{O_3} + 2NO \uparrow + {H_2}O$$ $$2NO \uparrow + {O_2} \uparrow \to 2N{O_2} \uparrow (Brown\ colour\ gas)$$

• Following tests are performed with an aqueous salt solution.
  

• Dilute Sulphuric Acid: It smells like vinegar when dil. sulphuric acid is added to it. $$C{H_3}CO{O^ - } + {H^ + } \to C{H_3}COOH \uparrow $$

• Iron(III) Chloride Solution: It gives Brick red colouration. $$3C{H_3}COONa + FeC{l_3} \to {(C{H_3}COO)_3}Fe\ (reddish\ brown) + 3NaCl$$

• Dil. Hydrochloric Acid: It gives sulphur & sulphur dioxide. $${S_2}O_3^{2 - } + 2{H^ + } \to S \downarrow + S{O_2} \uparrow + {H_2}O$$

• Iodine Solution: It decolourises due to the formation of tetrathionate ion. $${I_2} + 2{S_2}O_3^{2 - } \to 2{I^ - } + {S_4}O_6^{2 - }$$

• Barium Chloride Solution: White ppt of barium thiosulphate is formed.$${S_2}O_3^{2 - } + B{a^{2 + }} \to Ba{S_2}{O_3} \downarrow $$ But no ppt is obtained with $CaC{l_2}$ solution.

• Silver Nitrate Solution: It gives white ppt of silver thiosuphate. $${S_2}O_3^{2 - } + 2A{g^ + } \to A{g_2}{S_2}{O_3} \downarrow $$ The precipitate is unstable, turning dark on standing, due to the formation of silver sulphide. $$A{g_2}{S_2}{O_3} \downarrow + {H_2}O \to A{g_2}S + {H_2}S{O_4}$$

• Lead acetate or Lead nitrate solution: It gives white precipitate $${S_2}O_3^{2 - } + P{b^{2 + }} \to Pb{S_2}{O_3} \downarrow $$ On boiling it turns black due to the formation of $PbS$. $$Pb{S_2}{O_3} \downarrow + {H_2}O \to PbS \downarrow + 2{H^ + } + SO_4^{2 - }$$

(ii) Gases or acid vapours evolved with conc. Sulphuric acid:

$$C{l^ - },B{r^ - },{I^ - },NO_3^ - $$

1. Chloride($C{l^ - }$)

• Conc.${H_2}S{O_4}$: It decomposes with the evolution of $HCl$. $$C{l^ - } + {H_2}S{O_4} \to HCl + HSO_4^ - $$ Gas so produced

(1) Turns blue litmus paper red.

(2) Gives white fumes of $N{H_4}Cl$ when a glass rod moistened with ammonia solution is brought to the mouth of test tube. $$N{H_4}OH + HCl \to N{H_4}Cl(white\ fumes) + {H_2}O$$

• Manganese dioxide & conc. sulphuric acid: When a solid chloride is treated with $Mn{O_2}$ & conc.${H_2}S{O_4}$, yellowish green colour is obtained. $$Mn{O_2} + 2{H_2}S{O_4} + 2C{l^ - } \to M{n^{2 + }} + C{l_2} \uparrow + 2SO_4^{2 - } + 2{H_2}O$$

The following tests are performed with the salt solution.

• Silver Nitrate Solution: White, curdy precipitate of AgCl insoluble in water & dil.nitric acid, but soluble in dilute ammonia solution. $$\begin{equation} \begin{aligned} C{l^ - } + A{g^ + } \to AgCl \downarrow \\ AgCl \downarrow + 2N{H_3} \to [Ag{(N{H_3})_2}]Cl \\ [Ag{(N{H_3})_2}]Cl + 2{H^ + } \to AgCl + 2NH_4^ + \\\end{aligned} \end{equation} $$

• Lead Acetate Solution: White precipitate of lead chloride is formed. $$2C{l^ - } + P{b^{2 + }} \to PbC{l_2} \downarrow $$

• Chromyl Chloride Test: When a mixture containing chloride ion is heated with ${K_2}C{r_2}{O_7}$ & conc. ${H_2}S{O_4}$ orange red fumes of Chromyl Chloride ($Cr{O_2}C{l_2}$) are formed. $${K_2}C{r_2}{O_7} + 4NaCl + 6{H_2}S{O_4} \to 2KHS{O_4} + 4NaHS{O_4} + 2Cr{O_2}C{l_2} \uparrow(orange-red\ fumes) + 3{H_2}O$$ When chromyl chloride vapours are passed into sodium hydroxide a yellow solution of sodium chromate is formed which when treated with lead acetate gives yellow precipitate of lead chromate. $$\begin{equation} \begin{aligned} Cr{O_2}C{l_2} + 2NaOH \to N{a_2}Cr{O_4}(yellow\ solution) + 2HCl \\ \\\end{aligned} \end{equation} $$ $$N{a_2}Cr{O_4} + {(C{H_3}COO)_2}Pb \to 2C{H_3}COONa + PbCr{O_4} \downarrow (yellow\ precipitate)$$

2. Bromide ($B{r^ - }$)

• Conc.${H_2}S{O_4}$: It gives reddish brown vapours of bromine. $$2KBr + {H_2}S{O_4} \to {K_2}S{O_4} + 2HBr$$ $$2HBr + {H_2}S{O_4} \to 2{H_2}O + S{O_2} \uparrow + B{r_2} \uparrow (reddish\ brown)$$

• Manganese Dioxide & Conc. Sulphuric Acid: When a mixture of solid bromide, $Mn{O_2}$ & Conc.${H_2}S{O_4}$ is heated reddish brown vapours of Bromine are evovled.

The following tests are performed with the salt solution.

• Silver Nitrate Solution: A pale yellow precipitate of silver bromide is obtained. This precipitate is sparingly soluble in dilute ammonia solution but readily soluble in concentrated ammonia solution & insoluble in dil. $HN{O_3}$.$$\begin{equation} \begin{aligned} B{r^ - } + A{g^ + } \to AgBr \downarrow (pale\ yellow\ ppt) \\ AgBr + 2N{H_4}OH \to Ag{(N{H_3})_2}Br + 2{H_2}O \\\end{aligned} \end{equation} $$

• Lead Acetate Solution: White crystalline precipitate of lead bromide which is soluble in boiling water. $$2B{r^ - } + P{b^{2 + }} \to PbB{r_2} \downarrow $$

• Chlorine Water: When this solution is added to a solution of bromide and chloroform free bromine is liberated, which colours the organic layer orange- red. $$\begin{equation} \begin{aligned} 2KBr + C{l_2}(water) \to 2KCl + B{r_2} \\ B{r_2} + Chloroform \to Orange\ red\ colour \\\end{aligned} \end{equation} $$

• Potassium dichromate & Conc.${H_2}S{O_4}$: When a mixture of solid bromide ,${K_2}C{r_2}{O_7}$ and conc.${H_2}S{O_4}$ is heated & passing the evolved vapours into water , a yellowish brown solution is obtained. $$2KBr + {K_2}C{r_2}{O_7} + 7{H_2}S{O_4} \to 3B{r_2} \uparrow + C{r_2}{(S{O_4})_3} + 4{K_2}S{O_4} + 7{H_2}O$$

3. Iodide(${I^ - }$)

• Conc.${H_2}S{O_4}$: It gives violet vapours of iodine. $$2{I^ - } + 2{H_2}S{O_4} \to {I_2}(violet\ vapours) + SO_4^{2 - } + 2{H_2}O + S{O_2} \uparrow $$

The following tests are performed with the salt solution.

• Silver Nitrate Solution: Yellow, curdy precipitate of silver iodide $AgI$, very slightly soluble in conc. ammonia solution and insoluble in dil. nitric acid. $${I^ -} +{ Ag^+} \to AgI$$

• Lead Acetate Solution: Yellow, curdy precipitate of lead iodide soluble in very hot water forming a colourless solution & yielding golden yelllow plates (spangles) on cooling. $$2{I^ - } + P{b^{2 + }} \to Pb{I_2}$$
  

• Potassium Dichromate & Conc. Sulphuric Acid: Iodine is liberated. $$6{I^ - } + C{r_2}O_7^{2 - } + 7{H_2}S{O_4} \to 3{I_2} \uparrow + 7SO_4^{2 - } + 7{H_2}O + 2C{r^{3 + }}$$

• Chlorine Water: Iodine is liberated, by the dropwise addition of chlorine water to iodide, & on addition of $CHC{l_3}$ violet coloured organic layer is obtained. $$\begin{equation} \begin{aligned} 2{I^ - } + C{l_2} \to {I_2} + 2C{l^ - } \\ {I_2} + Chloroform \to Violet\ coloured\ layer \\\end{aligned} \end{equation} $$

• Copper Sulphate Solution: It gives brown precipitate consisting of a mixture of Copper(I) Iodide & Iodine & on addition of Hypo solution brown ppt changes to white precipitate $$\begin{equation} \begin{aligned} 4{I^ - } + 2C{u^{2 + }} \to {I_2} + 2CuI \\ {I_2} + 2{S_2}O_3^{2 - } \to 2{I^ - } + {S_4}O_6^{2 - } \\\end{aligned} \end{equation} $$

• Mercury (II) Chloride Solution: It forms scarlet red ppt of $H{I_2}g$. $$2{I^ - } + HgC{l_2} \to Hg{I_2} \downarrow + 2C{l^ - }$$ This precipitate dissolves in excess of $KI$, forming Tetraiodo Mercurate (II) complex. $$Hg{I_2} + 2{I^ - } \to {[Hg{I_4}]^{2 - }}$$.

4. Nitrate($NO_3^ - $)

• Conc.${H_2}S{O_4}$: It gives reddish brown vapours of nitrogen dioxide. $$4NO_3^ - + 2{H_2}S{O_4} \to 4N{O_2} \uparrow + 2SO_4^{2 - } + 2{H_2}O + {O_2} \uparrow $$

The following tests are performed with the salt solution.

• Brown Ring Test: When a freshly prepared solution of Iron(II) Sulphate is added to Nitrate solution & Conc. ${H_2}S{O_4}$ is poured slowly down the side of the test tube, a brown ring is obtained. $$2NO_3^ - + 4{H_2}S{O_4} + 6F{e^{2 + }} \to 2NO + 4SO_4^{2 - } + 4{H_2}O + 6F{e^{3 + }}$$ $$F{e^{2 + }} + NO \uparrow \to {[Fe(NO)]^{2 + }}$$ On shaking and warming the mixture, the brown colour disappears, nitric oxide is evolved and a yellow solution of Iron(III) ions remains.

• Action of Heat: The result varies with the metal.
  

1. Nitrates of Sodium and Potassium evolve oxygen (test with glowing splint) & leave solid nitrites (brown fumes with dilute acid). $$2NaN{O_3} \to 2NaN{O_2} + {O_2} \uparrow $$
2. Ammonium nitrate yields Dinitrogen oxide & Steam. $$N{H_4}N{O_3} \to {N_2}O \uparrow + 2{H_2}O$$
3. Nitrates of the noble metals leave a residue of the metal and a mixture of Nitrogen Dioxide and Oxygen is evolved. $$2AgN{O_3} \to 2Ag + 2N{O_2} \uparrow + {O_2} \uparrow $$
4. Nitrates of other metals, such as those of Lead & Copper, evolve Oxygen & Nitrogen Dioxide & leave a residue of the oxide. $$2Pb{(N{O_3})_2} \to 2PbO + 4N{O_2} \uparrow + {O_2} \uparrow $$