Question

Give reasons for the following:
(i) Alkali metals impart colour to the flame.
(ii) LiF is almost insoluble in water whereas LiCl is soluble not only in water but also in acetone.

Answer 1

(i) The electrons in the valence shell of the alkali metals easily absorb energy from the flame and consequently, they get excited to higher energy levels. When such excited electrons get back to the "ground state", an "energy" is released in the form of light.

(ii) There exists very high lattice enthalpy in LiF which makes it insoluble in water. However, it is the opposite in the case of LiCl. There exists comparatively low lattice enthalpy in LiCl and thus, there is a higher hydration enthalpy which makes it easily soluble in water. As acetone is polar in nature, LiCl is also soluble in acetone because of the effect of dipolar attraction.