Which pair of the following chlorides do not impart colour to the flame?

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BeCl₂ and SrCl₂

BeCl₂ and MgCl₂

CaCl₂ and BaCl₂

BaCl₂ and SrCl₂

answer is B.

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Detailed Solution

large begin{array}{*{20}{c}} {Li; - ;CRIMSON;RED;;;;;;} \ {Na; - ;GOLDEN;YELLOW} \ {K; - ;LILAC;;;;;;;;;;;;;;;;;;;;;} \ {Rb; - ;VIOLET;RED;;;;;;;;;} \ {Cs; - ;VIOLET;BLUE;;;;;;;;} end{array}

large begin{array}{*{20}{c}} {BC; - ;COLOUR;LESS} \ {Mg; - ;COLOUR;LESS} \ {Ca; - ;BRICK;RED;;;;;} \ {Sr; - ;CRIMSON;RED} \ {Ba; - ;APPLE;GREEN} end{array}

When alkaline earth metals and their compounds are exposed to the bunsen flame, the electron absorbs energy and are excited to higher energy levels. when the excited electron returns back to their general state, the absorbed energy is emitted in the form of radiation ,which falls in visible region. Therefore those metals imparts colour to the flame
Exception:
In case of Be and Mg due to their small size, their valency electrons are strongly held by the nucleus. The energy which is available in the flame is not sufficient to excite the electron to the higher energy level.so they can't emit radiation and hence they can't impart colour to the flame.

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