Question

${{\left[ Cu{{\left( {{H}_{2}}O \right)}_{4}} \right]}^{2+}}$ absorbs orange light and the transmitted complementary colour will be:
(A) Green.
(B) Yellow.
(C) Blue.
(D) Violet.

Answer 1

We know that to answer this question you should recall the complementary colour scheme and apply it for the colour of ${{\left[ Cu{{\left( {{H}_{2}}O \right)}_{4}} \right]}^{2+}}$. The colour ${{\left[ Cu{{\left( {{H}_{2}}O \right)}_{4}} \right]}^{2+}}$ of is blue. So, find out the colour complementary to blue using the colour wheel.

Complete answer:
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 is blue. This is due to the d-d transition and means that the colour absorbed by the solution is complementary to blue colour. Applying the above colour scheme, we can conclude that the absorbed radiation is orange-red.
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 is blue. This is due to the d-d transition and means that the colour absorbed by the solution is complementary to blue colour. Applying the above colour scheme, we can conclude that the absorbed radiation is orange-red.
Orange and blue are complementary colors. Thus, an object has a particular color for one of two reasons:
It reflects or transmits light of that color;
It absorbs light of the complementary color.
Therefore, correct answer is option (C) i.e. ${{\left[ Cu{{\left( {{H}_{2}}O \right)}_{4}} \right]}^{2+}}$ absorbs orange light and the transmitted complementary colour will be blue.

Note:
Remember that the complementary colour scheme, especially for the colors 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.