1. Carbonate \(( CO_{3} ^ {2-} )\) and Bicarbonate \((HCO_{3} ^ {-})\) :
(i) Solubility. Normal carbonates of only \(\mathrm{Na}^{+}, \mathrm{K}^{+}\) and \(\mathrm{NH}_4{ }^{+}\) are soluble in water. All bicarbonates are soluble in water.
(ii) Action of heat. All carbonates, except those of \(\mathrm{Na}^{+}, \mathrm{K}^{+}, \mathrm{Rb}^{+}\) and \(\mathrm{Cs}^{+},\) decompose on heating to give \(\mathrm{CO}_2,\) a colourless gas, e.g.
\(\mathrm{PbCO}_3 \xrightarrow{\text { heat }} \mathrm{PbO}+\mathrm{CO}_2\)
Bicarbonates decompose on heating to give normal carbonates and \(\mathrm{CO}_2\). Only \(\mathrm{NaHCO}_3\) and \(\mathrm{KHCO}_3\) are known in the solid state; \(\mathrm{Mg}\left(\mathrm{HCO}_3\right)_2\) and \(\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2\) are known only in solution.
\(2 \mathrm{NaHCO}_3 \xrightarrow{\text { heat }} \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2\)
(iii) Action of acids. Almost all carbonates are decomposed with dil. HCl or dil. \(\mathrm{H}_2 \mathrm{SO}_4\) with the evolution of \(\mathrm{CO}_2\) gas. When this gas is passed in lime water**, the latter turns milky due to the formation of insoluble calcium carbonate, \(\mathrm{CaCO}_3\).
\(\mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{SO}_4 \longrightarrow \mathrm{Na}_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2 ; \mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \longrightarrow \mathrm{CaCO}_3 \downarrow+\mathrm{H}_2 \mathrm{O}\)
If more gas is passed, the solution again becomes colourless due to the formation of soluble calcium bicarbonate
\(\underset {Cal. carbonate (insoluble)}{\mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}}+\mathrm{CO}_2 \longrightarrow \underset {Cal. bicarbonate (soluble)}{\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2}\)
Note. Lead carbonate reacts with dil. \(\mathrm{HCl}{\text { or }} \mathrm{H}_2 \mathrm{SO}_4\) to give \(\mathrm{CO}_2\) for a moment and then the reaction slows down. It is because of the formation of a protective layer of \(\mathrm{PbCl}_2\) or \(\mathrm{PbSO}_4\) on the surface of \(\mathrm{PbCO}_3\) crystals.
2. Sulphite :
(i) A sulphite when treated with dil. \(\mathrm{H}_2 \mathrm{SO}_4\) gives out \(\mathrm{SO}_2\) having suffocating smell, like that of burning sulphur.
(ii) When acidified potassium dichromate paper is exposed to this gas, the paper acquires green colour due to formation of chromium sulphate, \(\mathrm{Cr}_2\left(\mathrm{SO}_4\right)_3\).
(iii) Like \(\mathrm{CO}_2,\) sulphur dioxide also turns lime water milkly due to formation of calcium sulphite, \(\mathrm{CaSO}_3\).
(iv) It forms white precipitate of \(\mathrm{BaSO}_3\) with \(\mathrm{BaCl}_2\)
3. Sulphide :
A sulphide when treated with dil. \(\mathrm{H}_2 \mathrm{SO}_4\) gives out \(\mathrm{H}_2 \mathrm{S}\) gas having smell of rotten eggs. Hydrogen sulphide gives
(i) black precipitate with lead acetate or lead carbonate.
(ii) yellow precipitate with cadmium carbonate.
(iii) violet colour with sodium nitroprusside.
4. Nitrite :
A nitrite, when treated with dil. \(\mathrm{H}_2 \mathrm{SO}_4\) gives light brown fumes of \(\mathrm{NO}_2\). The light brown gas \(\left(\mathrm{NO}_2\right)\) decomposes iodides to free iodine which when treated with starch solution gives blue colour.
\(2 \mathrm{KI}+2 \mathrm{NO}_2 \longrightarrow 2 \mathrm{KNO}_2+\mathrm{I}_2\)
\(\mathrm{I}_2+\text { starch } \longrightarrow \text { Starch iodide (blue) }\)
When ferrous sulphate is added to a mixture of \(\mathrm{NO}_2^{-}\) and dil. \(\mathrm{H}_2 \mathrm{SO}_4,\) brown ring due to the formation of nitroso ferrous sulphate is formed
\(\mathrm{FeSO}_4+\mathrm{NO} \longrightarrow \mathrm{Fe}(\mathrm{NO}) \mathrm{SO}_4 \text { (Brown ring) }\)
5. Acetate :
(a) Acetates when treated with dil. \(\mathrm{H}_2 \mathrm{SO}_4\) or oxalic acid decompose to give acetic acid, having smell of vinegar.
(b) Acetates are soluble in water; the aqueous solution when treated with neutral ferric chloride gives blood red colour of ferric acetate.
6. Chloride, Bromide and Iodide :
(i) When heated with \(\mathrm{MnO}_2\) in presence of conc. \(\mathrm{H}_2 \mathrm{SO}_4\)
(a) chlorides give greenish yellow pungent gas \(\left(\mathrm{Cl}_2\right),\)
(b) bromides give red brown vapours of \(\mathrm{Br}_2,\)
(c) iodides give violet vapours of iodine.
(ii) Halides when treated with \(\mathrm{AgNO}_3\) solution in presence of dil.
\(\mathrm{HNO}_3\) yield ppt. of the corresponding silver halide.
(a) AgCl (white), insoluble in \(\mathrm{HNO}_3\) but dissolves is \(\mathrm{NH}_4 \mathrm{OH}, \mathrm{KCN}, \mathrm{NaCN},\) hypo, etc. due to complex formation.
(b) AgBr (light yellow), insoluble in \(\mathrm{HNO}_3,\) sparingly soluble is \(\mathrm{NH}_4 \mathrm{OH}\) but completely soluble in \(\mathrm{NaCN}, \mathrm{KCN},\) hypo etc.
(c) AgI (pale yellow), insoluble in \(\mathrm{HNO}_3\) and \(\mathrm{NH}_4 \mathrm{OH},\) but soluble in hypo. It has minimum lattice energy.
(d) Remember that among silver halide, AgF has maximum lattice energy and soluble in water.
(iii) Chlorides when heated with solid \(\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7\) and conc. \(\mathrm{H}_2 \mathrm{SO}_4\) evolve orange yellow vapours of chromyl chloride \(\left(\mathrm{CrO}_2 \mathrm{Cl}_2\right)\). The vapours when passed through NaOH solution and the solution acidified with acetic acid and then treated with lead acetate gives yellow ppt. of lead chromate.
(iv) Bromides (and iodides) impart orange (and violet) colour to the \(\mathrm{CS}_2, \mathrm{CCl}_4\) or \(\mathrm{CHCl}_3\) layer in presence of chlorine water.
7. Fluoride and Oxalate :
(i) Fluorides and oxalates give white precipitate with \(\mathrm{CaCl}_2\) solution due to the formation of insoluble \({ }^* \mathrm{CaF}_2\). However, \(\mathrm{CaF}_2\) is insoluble in dil. \(\mathrm{H}_2 \mathrm{SO}_4\) while calcium oxalate is soluble in dil. \(\mathrm{H}_2 \mathrm{SO}_4\) and decolourize acidified \(\mathrm{KMnO}_4\) solution.
(ii) Oxalates evolve CO and \(\mathrm{CO}_2\) gas when heated with conc. \(\mathrm{H}_2 \mathrm{SO}_4\). CO burns with blue flame while \(\mathrm{CO}_2\) turns lime water milky.
8. Sulphate :
All sulphates, except those of \(\mathrm{Ba}, \mathrm{Pb}\) and Sr are soluble in water.
(i) With \(\mathrm{BaCl}_2\) they give white precipitate of \(\mathrm{BaSO}_4\) insoluble in hot water and conc. \(\mathrm{HNO}_3\).
(ii) With lead acetate, they give white ppt. of \(\mathrm{PbSO}_4\).
9. Phosphate, \(\mathrm{PO}_4{ }^{3-}\) :
Phosphates give yellow precipitate with ammonium molybdate in presence of conc. \(\mathrm{HNO}_3\).
