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Lv 7
Tieria 發問於 科學及數學化學 · 1 十年前

關於Detection of substances的簡單問題

1. A pale green substance X contains one cation and one anion. The following table shows the observations of tests on an aqueous solution of X.


Procedure of test



Adding dilute nitric acid to a solution of X, followed by silver nitrate solution

A white precipitate forms.


Adding dilute sodium hydroxide solution to a solution of X

A green precipitate forms.


Adding a solution of X to a purple solution

The colour of the purple solution changes to pale yellow.

A) Explain the observation in test II.

B) i) suggest what the purple solution might be.

ii) write an ionic equation for the reaction occurred in test III.

2 個解答

  • 1 十年前

    A) Since green precipitate is obtained in test II, we can prove that Fe2+ is present in solution X. Fe2+ will react with OH- in sodium hydroxide to give green precipitate.

    B) i) Acidified potassium permanganate solution. Since ther colour of this solution is purple, and it will be reduced to colourless due to formation of Mn2+.

    B) ii) Fe2+ (aq) -> Fe3+ + e- ----------(1)

    MnO4- (aq) + 5e- + 8H+(l) -> Mn2+ (aq) + 4H2O(l) --------(2)

    By combining equation 1 and 2, we can obtain the following equation:


    5Fe2+ (aq) + MnO4- (aq) + 8H+ (aq) -> 5Fe3+ (aq) + Mn2+(aq) + 4H2O(l)


    In CE level, you should remember the colour of the substances especially the common strong oxidizing agent (e.g. MnO4- / Cr2O7 2-)

    Hope my explanations help.

  • 1 十年前


    X contains iron(II) ions. The iron(II) ions reacts with the hydroxide ions in the sodium hydroxide solution to give a green precipitate of iron(II) hydroxide.



    The purple solution might be acidified potassium permanganate solution.


    MnO4^-(aq) + 8H^+(aq) + 5Fe^2+(aq) → Mn^2+(aq) + 4H2O(l) + 5Fe^3+(aq)