Where Group V Sits in the Scheme
Systematic analysis of cations works by precipitating ions in groups, one stage at a time, using a characteristic group reagent for each stage. The original solution is treated sequentially: dilute HCl removes Group I (Pb²⁺); H₂S in acidic medium removes Group II; ammonia with NH₄Cl removes Group III hydroxides; H₂S in ammoniacal medium removes Group IV sulphides. Whatever survives all four stages is carried forward to Group V.
Group V — sometimes called the ammonium carbonate group — comprises the alkaline-earth cations Ba²⁺, Sr²⁺ and Ca²⁺. They are precipitated together as their insoluble white carbonates. Only the most soluble cation of the syllabus, Mg²⁺, lies beyond it in Group VI, where no group reagent is used. This ordering is exactly what NEET tested in 2024 and 2025 through its group-matching questions.
The filtrate that has escaped Groups I–IV is made ammoniacal and treated with ammonium carbonate; the three alkaline-earth ions drop out together as carbonates.
The Group Reagent and Its Conditions
The Group V reagent is ammonium carbonate, (NH₄)₂CO₃, added in the presence of NH₄Cl and an excess of NH₄OH. To the original solution one adds a small amount of solid NH₄Cl, then excess NH₄OH, and finally solid (NH₄)₂CO₃. A white precipitate signals Group V. The carbonate ion supplied by the reagent meets the alkaline-earth ions and their sparingly soluble carbonates separate out:
$$\ce{Ba^2+ + CO3^2- -> BaCO3 v}\qquad \ce{Sr^2+ + CO3^2- -> SrCO3 v}\qquad \ce{Ca^2+ + CO3^2- -> CaCO3 v}$$
All three carbonates are white, so the precipitate alone cannot tell the ions apart — separation comes later. The role of the two ammonium reagents is subtle but central, and is examined directly in viva and in NEET reasoning items.
| Reagent | Why it is present |
|---|---|
| Excess NH₄OH | Keeps the medium basic so the carbonate-ion concentration from (NH₄)₂CO₃ is high enough to precipitate the alkaline-earth carbonates. |
| NH₄Cl | Supplies the common ion NH₄⁺, suppressing ionisation of NH₄OH; this keeps the precipitation selective and prevents premature precipitation of higher-group ions. |
| (NH₄)₂CO₃ | The actual precipitant — its carbonate ions combine with Ba²⁺, Sr²⁺, Ca²⁺ to give white BaCO₃, SrCO₃, CaCO₃. |
Order of addition is not optional
A common slip is adding (NH₄)₂CO₃ first and NH₄OH later. The NCERT scheme requires NH₄Cl and excess NH₄OH before the carbonate, so the medium is correctly buffered when carbonate ions arrive.
Sequence: NH₄Cl → excess NH₄OH → (NH₄)₂CO₃. Then look for the white precipitate.
Why Group V Comes After Group IV
The order of groups is governed by solubility. Each group reagent is chosen so it precipitates only the ions of that stage and leaves the rest in solution. The alkaline-earth carbonates of Group V are more soluble than the chlorides, sulphides and hydroxides removed in Groups I–IV, so these cations are not caught earlier — they survive dilute HCl, both H₂S stages and the ammonia treatment.
They are precipitated only when carbonate ion is deliberately introduced in a basic medium. Placing Group V after Group IV also ensures that interfering cations — which would themselves form carbonates — have already been pulled out, so the white carbonate that now appears can be assigned cleanly to Ba²⁺, Sr²⁺ or Ca²⁺. If even Group V gives no precipitate, the analyst proceeds to Group VI to test for Mg²⁺, whose carbonate stays dissolved in the ammoniacal, ammonium-salt-rich medium.
Dissolving the Carbonate in Acetic Acid
The white Group V precipitate is a mixture of carbonates and cannot be tested as a solid. It is brought into solution by boiling with dilute acetic acid, which converts each carbonate into the corresponding soluble metal acetate with brisk effervescence of CO₂:
$$\ce{BaCO3 + 2CH3COOH -> (CH3COO)2Ba + H2O + CO2 ^}$$ $$\ce{SrCO3 + 2CH3COOH -> (CH3COO)2Sr + H2O + CO2 ^}$$ $$\ce{CaCO3 + 2CH3COOH -> (CH3COO)2Ca + H2O + CO2 ^}$$
The clear acetic-acid solution is then divided into three parts — one each for the barium, strontium and calcium tests — and a small portion of the original precipitate is preserved for flame tests. Working in acetic acid (a weak acid) rather than a strong mineral acid is deliberate: it is exactly the medium in which the chromate test for barium becomes specific.
Confirming Barium (Ba²⁺)
To the first part of the acetic-acid solution add potassium chromate, K₂CrO₄. A yellow precipitate of barium chromate confirms Ba²⁺:
$$\ce{(CH3COO)2Ba + K2CrO4 -> BaCrO4 v + 2CH3COOK}$$
The flame test on the preserved precipitate gives a grassy apple-green flame that appears bluish-green when viewed through blue glass. A second confirmation of barium relies on the extreme insolubility of barium sulphate: BaSO₄ is a dense white precipitate that does not dissolve in concentrated HCl or HNO₃, a property exploited separately when sulphate itself is being confirmed.
$$\ce{Ba^2+ + SO4^2- -> BaSO4 v}$$
Only barium gives the yellow chromate here
In acetic-acid medium, only Ba²⁺ produces a chromate precipitate. Strontium and calcium do not give a chromate precipitate under these conditions, so a yellow precipitate with K₂CrO₄ is decisive for barium and rules the other two out at the first part.
Yellow with chromate = Ba²⁺. No chromate precipitate = test for Sr²⁺ / Ca²⁺ next.
The yellow chromate here mirrors the lead-chromate confirmation you met earlier — revisit Cation Group I (Lead) to see how the same reagent serves two different groups.
Confirming Strontium (Sr²⁺)
If barium is absent, take the second part of the acetic-acid solution and add ammonium sulphate, (NH₄)₂SO₄. On heating and scratching the sides of the test tube with a glass rod, a white precipitate of strontium sulphate forms:
$$\ce{(CH3COO)2Sr + (NH4)2SO4 -> SrSO4 v + 2CH3COONH4}$$
The flame test on the preserved precipitate gives a crimson-red flame, which confirms Sr²⁺ (it appears purple through blue glass). Crucially, strontium gives no chromate precipitate in acetic acid, which is why barium must be eliminated before strontium can be claimed.
Confirming Calcium (Ca²⁺)
If both barium and strontium are absent, take the third part of the solution and add ammonium oxalate, (NH₄)₂C₂O₄, then shake well. A white precipitate of calcium oxalate confirms Ca²⁺:
$$\ce{(CH3COO)2Ca + (NH4)2C2O4 -> CaC2O4 v + 2CH3COONH4}$$
The flame test gives a brick-red flame that looks greenish-yellow through blue glass. Like strontium, calcium gives no chromate precipitate in acetic acid, so it can be confirmed only after the two heavier ions are excluded.
The Order Ba → Sr → Ca
The three ions are always tested in the fixed sequence barium first, strontium second, calcium last. The logic is one of decreasing test-specificity and decreasing sulphate insolubility:
| Step | Ion | Why tested at this point |
|---|---|---|
| 1 | Ba²⁺ | The chromate test is specific for barium in acetic acid; removing and confirming Ba²⁺ first stops it from interfering with later tests. |
| 2 | Sr²⁺ | Tested after barium is excluded. Its sulphate, SrSO₄, is far less soluble than calcium sulphate, so the sulphate test catches strontium before it would catch calcium. |
| 3 | Ca²⁺ | Confirmed last with ammonium oxalate, once the interfering heavier ions are gone, avoiding a false positive. |
Each part receives a different reagent; the precipitate colour plus the flame colour together confirm the ion.
Flame Tests Compared
For the flame test, a platinum wire is dipped in concentrated HCl and heated until it imparts no colour to the non-luminous flame; it is then dipped in a paste of the Group V precipitate in concentrated HCl and heated. The chlorides are volatile in the flame and emit characteristic colours. Viewing through blue (cobalt) glass filters the colour and helps separate ions whose naked-eye colours can look similar.
| Ion | Flame (naked eye) | Through blue glass |
|---|---|---|
| Ba²⁺ | Apple green (grassy green) | Bluish green |
| Sr²⁺ | Crimson red | Purple |
| Ca²⁺ | Brick red | Greenish-yellow (green) |
Strontium and calcium are both "red" to the naked eye
Sr²⁺ (crimson-red) and Ca²⁺ (brick-red) can be confused in a quick look. The blue glass resolves them: strontium turns purple, calcium turns green / greenish-yellow. Always confirm a flame colour with the wet test as well, never the flame alone.
Through blue glass: Sr²⁺ → purple, Ca²⁺ → green, Ba²⁺ → bluish-green.
Observation → Inference Table
The complete Group V workflow, from group precipitation to the three confirmatory branches, is summarised below. Each row is a single decision an analyst makes at the bench.
| Experiment | Observation | Inference |
|---|---|---|
| O.S. + NH₄Cl + excess NH₄OH + (NH₄)₂CO₃ | White precipitate | Group V present (Ba²⁺ / Sr²⁺ / Ca²⁺) |
| Precipitate boiled with dil. CH₃COOH | Dissolves, CO₂ effervescence | Carbonates → soluble acetates |
| Part 1 + K₂CrO₄ solution | Yellow precipitate (BaCrO₄) | Ba²⁺ confirmed |
| Flame test (Ba) | Apple-green flame | Ba²⁺ confirmed |
| Part 2 + (NH₄)₂SO₄, heat & scratch | White precipitate (SrSO₄) | Sr²⁺ confirmed (if Ba absent) |
| Flame test (Sr) | Crimson-red flame | Sr²⁺ confirmed |
| Part 3 + (NH₄)₂C₂O₄, shake | White precipitate (CaC₂O₄) | Ca²⁺ confirmed (if Ba, Sr absent) |
| Flame test (Ca) | Brick-red flame | Ca²⁺ confirmed |
A salt's filtrate gives no precipitate in Groups I–IV. On adding NH₄Cl, excess NH₄OH and (NH₄)₂CO₃ a white precipitate forms. It dissolves in acetic acid; the first part gives a yellow precipitate with K₂CrO₄ and the preserved solid gives an apple-green flame. Identify the cation.
Reasoning: Survival through Groups I–IV plus a white carbonate with the ammonium-carbonate reagent places the ion in Group V. The yellow chromate precipitate is specific to barium in acetic acid: $\ce{(CH3COO)2Ba + K2CrO4 -> BaCrO4 v + 2CH3COOK}$. The apple-green flame independently confirms it.
Answer: The cation is Ba²⁺. No need to run the strontium or calcium parts once barium is confirmed.
Group V in one screen
- Members: Ba²⁺, Sr²⁺, Ca²⁺ — the ammonium carbonate group.
- Group reagent: (NH₄)₂CO₃ with NH₄Cl + excess NH₄OH → white carbonates BaCO₃, SrCO₃, CaCO₃.
- Position: after Group IV, before Group VI (Mg²⁺); reached only because alkaline-earth carbonates are more soluble than earlier precipitates.
- Dissolve: boil with dilute acetic acid → soluble acetates; split into 3 parts; preserve solid for flame.
- Ba²⁺: yellow BaCrO₄ with K₂CrO₄; apple-green flame; BaSO₄ insoluble in conc. acids.
- Sr²⁺: white SrSO₄ with (NH₄)₂SO₄ on heating; crimson-red flame; no chromate precipitate.
- Ca²⁺: white CaC₂O₄ with (NH₄)₂C₂O₄; brick-red flame; no chromate precipitate.
- Order: Ba → Sr → Ca, because the chromate test is specific to barium and must be cleared first.