Question

Calcium sulphate is found in nature in two forms, anhydrous calcium sulphate and hydrated calcium sulphate is heated with coke, Sulphur dioxide gas is obtained. When hydrated calcium sulphate is heated at ${200^{\text{o}}}C$, it forms anhydrous salt. When calcium sulphate is mixed with conc. $HCl$ and the paste is formed. What colour is obtained when a pinch of this paste is brought near the flame?
A.Golden yellow
B.Brick red
C.Crimson red
D.Apple green

Answer 1

Hint:To answer this question, you must recall the flame test of metal cations. Flame test is performed to identify the cation present in a given salt during qualitative analysis. It is generally shown by alkali metals and alkaline earth metals.

Complete answer:
We know that alkaline earth metals and their salts impart some colors to flame and each metal has a characteristic colour of its own. This is because when an alkaline earth metal or its corresponding salt is brought in a flame, their valence electrons are excited to a higher energy level on absorbing energy from the flame. These excited electrons when returning to their original energy level release radiations lying in the visible region of the electromagnetic spectrum and thus impart colour to the flame.
If the metal has a higher ionization enthalpy, the electron is excited to a comparatively lower energy level than in the metal with a lower ionization enthalpy and thus the radiations emitted are of low frequencies. This is why calcium shows brick red colour in the flame test.
Golden yellow colour is shown by sodium in the flame test
Crimson red colour is shown by strontium in the flame test
And Apple green colour is shown by barium in the flame test.

Thus, the correct answer is B.

Note:
Flame test is shown by alkali metals and alkaline earth metals because they have comparatively lower ionization enthalpies and the energy of the flame is sufficient to overcome this enthalpy. However, beryllium and magnesium do not impart colour to the flame because of small sizes and thus high ionization enthalpies.