Question

Which of the following colour of light is absorbed by an aqueous solution of\[{\text{CuS}}{{\text{O}}_{\text{4}}}\]?
A.orange-red
B.blue-green
C.yellow
D.violet

Answer 1

Hint: To answer this question you should recall the complementary colour scheme and apply it for the colour of \[{\text{CuS}}{{\text{O}}_{\text{4}}}\]. The colour of \[{\text{CuS}}{{\text{O}}_{\text{4}}}\] is blue. So, find out the colour complementary to blue using the colour wheel.

Complete step by step answer:
You can recall from the complementary colour scheme that major complementary colour scheme combinations are:
Red and green
Blue and orange-red
Yellow and purple
Yellow-green and red-purple
The basic concept behind the colour of any solution is the absorbance of a part of the spectrum of radiation which falls on the solution and emission of the complementary spectrum of the absorbed radiation. We know that the colour \[{\text{CuS}}{{\text{O}}_{\text{4}}}\] is blue. This is due to the d-d transition and means that the colour absorbed by \[{\text{CuS}}{{\text{O}}_{\text{4}}}\] the solution is complementary to blue colour.
Applying the above colour scheme, we can conclude that the absorbed radiation is orange-red.

Hence, the correct answer is option A.

Additional information:
 The reason why transition metals, in particular, are colourful is that they have unfilled or either half-filled d orbitals resulting in d-d transition. d–d transition involves excitation of an electron in a d orbital to another high energy d orbital. Complementary colours can be defined as a pair of colours which on mixing, produce a grayscale colour. They may also be termed as opposite colours as they create the strongest contrast specific to those two colours only.

Note:
The complementary colour scheme, especially for the colours of solutions (p-block and s-block), to avoid confusion and negative marking. Even if you don’t remember the colour of salts, make sure you remember the colour of cation as usually, it is the cation which imparts the colour to the salt.