Final answer:
The correct answer is option a. Copper salts generally have a blue color, particularly when in the form of copper(II) complexes, which interact with light to absorb certain wavelengths, leading to the characteristic blue color observed in solutions or in hydrated forms.
Step-by-step explanation:
Copper salts generally have a blue color. This is attributed to the electronic transitions in copper(II) complexes, which can absorb certain wavelengths of light from the visible spectrum. For example, copper(II) sulfate, when hydrated with water molecules, appears blue due to the interactions between the water molecules and the d electrons in the copper ion. In contrast, anhydrous copper sulfate, which lacks water, is white because removing the water changes the electron configuration and thus the color.
When dealing with copper(I) salts, such as CuI, they tend to be colorless due to a filled d^10 electron configuration, which does not allow for the same electron transitions that give color to copper(II) salts. However, copper(II) salts, which are d^9 complexes, like Cu(NO3)2.5H2O, are notable for their bright colors. In solutions, the color is also affected by the type of ligands attached to the copper ion. Copper phthalocyanine blue, a square planar copper complex, is used in some blue dyes and provides a vivid example of the impact of copper compounds on coloration.
This color change mechanism is evident in a reaction where a copper wire is placed in a solution with dissolved silver salts. The silver ions are reduced to a gray-color metallic silver, and the Cu^2+ ions dissolve, giving the solution a characteristic blue color. When visible light, such as sunlight, passes through a copper(II) sulfate solution, the copper(II) ions absorb light in the red region of the spectrum, which results in the pale blue color we perceive.