Alkali metals give colour in Bunsen flame due to:

Consider the following statements;

${\mathrm{S}}_1:$ Among alkali metal halides' lithium iodide is the most covalent in nature.

${\mathrm{S}}_2:$ Potassium has greater photoelectric work function than sodium. 

${\mathrm{S}}_3$: The blue solution of alkali metals in liquid ammonia is stable at room temperature, where ammonia is still a liquid, in the presence of $\mathrm{Fe}$.

${\mathrm{S}}_4$: The melting and boiling points of alkali metal halides always follow the trend : chloride$>$fluoride$>$bromide $>$iodide, 

and arrange in the order of true/false.