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Module 8: Applying Chemical Ideas

Quick questions on Cation and anion identification tests explained: HSC Chemistry Module 8

12short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is flame tests (group 1 and 2 cations mostly)?
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Heat a clean platinum or nichrome wire in a Bunsen flame until it glows colourless. Dip it in concentrated HCl to clean residual ions, then in the unknown, and observe the flame colour.
What is precipitation tests for cations?
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Add a named reagent and observe the precipitate (colour, texture, solubility in excess).
What is precipitation tests for anions?
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The order halide colours (white, cream, yellow) and ammonia solubility (yes, partial, no) is the standard halide differentiation.
What is complexation tests?
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Complexation distinguishes ions that give similar precipitates by re-dissolving one in excess reagent through formation of a soluble complex ion.
What is a systematic procedure?
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When you do not know what is in the sample:
What is copper with ammonia?
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Add dilute $NH_3$ to a $Cu^{2+}$ solution; pale blue $Cu(OH)_2$ forms, then with excess ammonia it dissolves to give the deep blue tetraammine complex:
What is iron with thiocyanate?
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Add $KSCN$ to a $Fe^{3+}$ solution; a deep blood-red complex forms:
What is iron with hydroxide vs iron with hydroxide?
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$Fe^{3+}$ gives rust-brown $Fe(OH)_3$; $Fe^{2+}$ gives dirty green $Fe(OH)_2$ that browns on standing. Adding $KSCN$ confirms which is present, since only $Fe^{3+}$ gives the red colour.
What is confusing $Fe^{2+}$ and $Fe^{3+}$ hydroxides?
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Dirty green = $Fe^{2+}$, rust brown = $Fe^{3+}$. The $Fe^{2+}$ precipitate goes brown on standing.
What is forgetting to acidify before $AgNO_3$ or $BaCl_2$ tests?
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Carbonate gives a false positive with both silver and barium. A drop of dilute acid before the reagent removes it.
What is treating flame tests as quantitative?
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They are not. Two ions in the same flame mask each other (sodium is so bright it hides almost everything; use cobalt-blue glass).
What is mixing test reagents into one tube?
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Each test gets a fresh sample portion. Sequential addition produces unreadable mixtures.

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