Cation tests (flame test), gas tests, purity tests：練習問題

A student collected an unknown gas. When a piece of moist blue litmus paper was inserted into the gas, it turned red. The gas was then bubbled through a solution of acidified potassium permanganate, which changed from purple to colourless. Which of the following gases is most likely the unknown sample?

An unknown gaseous mixture is suspected to contain carbon dioxide (\(CO_2\)), sulfur dioxide (\(SO_2\)), and hydrogen chloride (\(HCl\)). A student passes the mixture sequentially through three specific reagents to identify the components:

I. Aqueous potassium permanganate solution (\(KMnO_4(aq)\)).

II. Concentrated sulfuric acid (\(H_2SO_4(l)\)).

III. Limewater (\(Ca(OH)_2(aq)\)).

The observations recorded were:

• In step I, the purple colour of the \(KMnO_4(aq)\) solution immediately disappeared.
• In step II, there was no measurable change in the mass of the concentrated \(H_2SO_4(l)\) absorption tube.
• In step III, a dense white precipitate formed.

Based on these specific observations and the sequential testing procedure, which conclusion is the most scientifically sound?

When a compound containing sodium ions ( \(Na^+\) ) is subjected to a flame test, what characteristic colour is typically observed?

An unknown gas turns damp blue litmus paper red and produces white fumes when a glass rod dipped in aqueous ammonia is brought near it. Identify the gas and write the chemical equation for the formation of white fumes.

A sample of magnesium carbonate is heated strongly in a test tube. Describe the chemical test to confirm the identity of the gas evolved and explain why the solid residue remaining in the tube would produce no characteristic color in a flame test despite containing a metal cation.

A forensic scientist is investigating a white powder found in a suspicious container. The following tests were performed to identify the substance and determine its quality.

(a) A flame test was conducted on the solid, resulting in a brick-red flame. Identify the cation present in the solid.
(b) A portion of the solid was dissolved in deionised water. Upon adding dilute nitric acid followed by aqueous silver nitrate, a white precipitate was formed. Identify the anion and write the ionic equation for the formation of this precipitate.
(c) The substance is suspected to be calcium chloride. The literature value for the melting point of pure calcium chloride is \(772 \text{ } ^\circ\text{C}\). The sample was found to melt over a temperature range of \(748 \text{ } ^\circ\text{C}\) to \(762 \text{ } ^\circ\text{C}\). State two conclusions that can be drawn about the purity of the sample based on these observations.

An unknown white crystalline solid, X, is suspected to be an impure sample of an alkali metal carbonate.

(a) Purity Assessment
A small amount of solid X is subjected to melting point determination. It is observed that the solid starts to melt at \(850\,\circ\text{C}\) and finishes melting at \(870\,\circ\text{C}\). The literature value for the pure alkali metal carbonate is \(891\,\circ\text{C}\).

(i) Based on the observation, what can be concluded about the purity of solid X? Explain the relationship between the observed melting behaviour and the purity of a solid substance.

(b) Cation and Carbonate Anion Identification
(i) A flame test is performed on a sample of solid X using a clean platinum wire. A persistent lilac flame is observed. Identify the alkali metal cation present in X.

(ii) A sample of solid X is reacted with excess dilute nitric acid, resulting in vigorous effervescence of a colourless gas, G. Gas G is then passed through limewater (aqueous calcium hydroxide), causing the limewater to turn milky. Identify gas G and write the overall ionic equation for the reaction occurring when the carbonate ions in X react with the acid.

(c) Identifying the Impurity
The remaining solution from part (b)(ii) contains the cation identified in (b)(i) and an impurity anion. When aqueous silver nitrate solution is added to this solution, a white precipitate is formed, which is insoluble in excess dilute nitric acid.
Identify the anion of the impurity and explain the crucial role of adding dilute nitric acid (before or during this testing procedure) in confirming the presence of this specific impurity anion.