Question

Alkaline earth metal(s) which does (do) not impart colour to the flame is (are):
(A) Be
(B) Mg
(C) Ca
(D) Sr

Answer 1

Hint: Recollect the Flame test used to detect the presence of cations in inorganic qualitative salt analysis. During flame test, some metals give a characteristic colour to the flame because of which we are able to infer about the cation present in the salt mixture. List down the observations and inferences of alkaline earth metals and see which element given in the options doesn’t give any colour to the flame.

Complete step by step solution:
- Alkaline earth metals are the metals present in the second group of the periodic table. They are s-block elements.
- Flame test is a dry test used in inorganic qualitative analysis of a salt mixture to identify or detect the presence of metallic cations in an unknown salt mixture. In flame test, a small amount of salt sample is mixed with few drops of conc. HCl and a paste is made which is then held in the flame using porcelain chip or match-stick or platinum loop.
- Flame test works on the principle of excitation of electrons present in the metals and therefore, during the de-excitation of electrons, a characteristic colour of the flame is observed due to the energy which is released in the form of visible light.
- If in an element excitation of electrons is difficult then, no colour change of flame is observed.
- In alkaline earth metals, calcium gives brick red flame, strontium gives crimson red flame, barium gives apple green flame. Now, beryllium and magnesium do not give any colour.
- The atomic size of beryllium and magnesium is smaller than the elements present in the second group. The nuclear force of attraction for the valence shell electrons is more in case of magnesium and beryllium due to less number of electrons present in them and absence of shielding effect. Therefore, the electrons won’t get excited to higher energy levels.
- Therefore, alkaline earth metals which do not impart colour to the flame are beryllium and magnesium.

Therefore, option (A) and (B) is the correct answer.

Note: Remember in flame test, electrons get excited by absorbing energy from the flame and then get de-excited and release energy in the form of visible light. This is the principle behind flame tests. Some more metals which give flame tests are sodium (yellow flame), potassium (lilac flame), copper (blue flame), etc.