Qualitative Ion Tests – Group 2, Halides, Sulfate, Carbonate, Ammonium, Hydroxide

17 question-and-answer flashcards summarising reagents, methods, and observations for common qualitative ion tests covered in the lecture.

Which five categories of ions are covered by qualitative tests in this topic?

Group 2 metal ions, halide ions, sulfate ions, carbonate ions, and ammonium ions.

What laboratory technique is used to identify Group 2 metal ions?

The flame test.

During a flame test, what colour (or lack thereof) indicates the presence of Mg²⁺?

No distinct colour – the flame is essentially colourless/faint.

What flame colour indicates Ca²⁺ in a flame test?

Brick-red.

Which flame colour corresponds to Sr²⁺?

Crimson red.

What flame colour is produced by Ba²⁺ in a flame test?

Apple green.

When testing for sulfate ions (SO₄²⁻), which reagent is added and what is the positive observation?

Add BaCl₂ solution in dilute HCl; a white precipitate of BaSO₄ forms.

Why is dilute hydrochloric acid added before BaCl₂ in a sulfate test?

To remove carbonate ions that could also produce a white precipitate with Ba²⁺, preventing false positives.

Which reagent is used to test for carbonate ions (CO₃²⁻) and what gas is produced?

Dilute HCl; carbon dioxide gas (CO₂) is evolved.

How is the presence of CO₂ confirmed after a carbonate test?

Bubble the gas through limewater; it turns milky or cloudy.

Outline the two-step procedure for halide ion tests.

1) Add dilute nitric acid to remove interfering ions; 2) Add AgNO₃ solution and observe precipitate colour.

What colour precipitate forms with Ag⁺ for Cl⁻, Br⁻, and I⁻ respectively?

Cl⁻: white (AgCl); Br⁻: cream (AgBr); I⁻: yellow (AgI).

How do the AgX precipitates behave in ammonia? (dilute vs. concentrated)

AgCl dissolves in dilute NH₃; AgBr is insoluble in dilute but dissolves in concentrated NH₃; AgI is insoluble in both dilute and concentrated NH₃.

Why is nitric acid used before adding silver nitrate in halide tests?

To remove carbonate or hydroxide ions that would otherwise form unwanted silver precipitates, preventing false positives.

What reagent is added to an ammonium salt to test for NH₄⁺, and what gas is released?

Add NaOH; ammonia gas (NH₃) is released on warming.

Which two observations confirm ammonia gas in the NH₄⁺ test?

A pungent smell and damp red litmus paper turning blue.

How can you test for hydroxide ions (OH⁻) in solution?

Dip red litmus paper; it turns blue, indicating alkalinity.