Aqueous ammonia reacts with aqueous solution of metal ion X to produce a coloured
precipitate. The colour of the precipitate changes slowly on standing in air. What could be ton
X?
A Al 3+
B Cu2+
C Fe2+
D Fe3+
Pls explain how to get the answer, thks!
Al(OH)3 is not colored.
Cu(OH)2, Fe(OH)2 and Fe(OH)3 are colored. Fe(OH)2 is the only one that is colored and changes color on standing because it is being oxidized to Fe(OH)3 by air. So Fe(OH) is greenish and it changes to a red/brown.
Fe)OH)2 is green and gelatinous with white streaks and changes to a red/brn.
To determine the answer, we need to consider the reactivity of the metal ion X with aqueous ammonia.
The reaction between aqueous ammonia and metal ions typically forms coordination complexes, which can result in the formation of colored precipitates. The color of the precipitate often provides information about the metal ion present.
In this case, the fact that the color of the precipitate changes slowly on standing in air suggests the formation of a complex that can oxidize and undergo slow changes.
From the given options, both Cu2+ and Fe2+ ions are capable of forming colored precipitates with aqueous ammonia. However, the fact that the color changes slowly in the presence of air narrows down the options.
Cu2+ forms a blue precipitate with aqueous ammonia, but this precipitate does not change color upon standing in air. Therefore, option B (Cu2+) can be eliminated.
On the other hand, Fe2+ ions react with aqueous ammonia to form a greenish precipitate called iron(II) hydroxide, Fe(OH)2. This precipitate is known to slowly oxidize in air, changing color to a brownish-black precipitate, which is iron(III) hydroxide, Fe(OH)3. Hence, option C (Fe2+) is a possible answer.
However, before making a final decision, let's consider option D (Fe3+). Fe3+ ions react with aqueous ammonia to form a reddish-brown precipitate called iron(III) hydroxide, Fe(OH)3. This precipitate is also known to slowly change color on standing in air. Hence, option D (Fe3+) is another possible answer.
To conclude, both options C (Fe2+) and D (Fe3+) can potentially result in a colored precipitate that changes slowly in the presence of air. Without additional information or clues, it is challenging to determine the precise metal ion X.
To determine the answer, we need to understand the chemical reactions involved.
Aqueous ammonia (NH3) is a weak base that can react with metal ions to form insoluble metal hydroxides. This reaction is known as a precipitation reaction.
In this case, we are looking for the metal ion X that forms a colored precipitate with aqueous ammonia.
Let's consider each option:
A) Al3+: When aqueous ammonia reacts with aluminum ions (Al3+), it forms a white precipitate of aluminum hydroxide. This precipitate is generally colorless.
B) Cu2+: When aqueous ammonia reacts with copper ions (Cu2+), it forms a blue precipitate of copper hydroxide. This is a colored precipitate.
C) Fe2+: When aqueous ammonia reacts with iron(II) ions (Fe2+), it forms a green precipitate of iron(II) hydroxide. This is also a colored precipitate.
D) Fe3+: When aqueous ammonia reacts with iron(III) ions (Fe3+), it forms a reddish-brown precipitate of iron(III) hydroxide. This is a colored precipitate.
Based on the given information, we can conclude that options B, C, and D (Cu2+, Fe2+, and Fe3+) are all potential candidates that can produce a colored precipitate.
However, it is mentioned that the color of the precipitate changes slowly on standing in air. This behavior is typical for Cu2+. Copper(II) hydroxide is initially blue, but it slowly oxidizes in the presence of air to form a black precipitate of copper(II) oxide.
Therefore, the most likely answer is option B: Cu2+.
To arrive at this conclusion, it is important to understand the characteristic colors of precipitates formed by different metal ions and how they may change with exposure to air.