Explain why we did not see distinct lines (like on an emission spectrum) when the metal salts were burned.

Actually when we burn metal salts, we cannot actually really see distinct lines to appear because in reality, they are not really visible to the human eye. There is only a certain range of wavelength of light that our eyes can see.

To state in other way, some metal salts will give off light which has wavelengths that are outside of the visible region of the electromagnetic spectrum. 

The distinct lines of metal salts that cannot be seen give off lights with wavelengths outside the visible region of an electromagnetic spectrum

When we burn metal salts they produce lights with wavelengths outside the visible region wavelength contained in the electromagnetic spectrum, therefore the distinct lines when  metal salt is burnt, cannot be seen with our ordinary eyes.

Metal salts are used for industrial purposes like in photo voltaic cells and transparent electrodes. they are formed by the reaction between a metal cation and an anion. a common example of metal salts is ; sulfur dioxide.

Hence the distinct lines when metal salts are burnt cannot be seen within the visible region in the electromagnetic spectrum.

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explanation:

metals have excess of electrons whereas non-metals have deficiency of electrons. thus, in order to gain stability metals easily donate their electrons to the non-metals.

for example, electronic configuration of sodium is and electronic configuration of phosphorous is .

when a phosphorous atom and sodium atom combine together then sodium will donate is extra one electron to the phosphorous atom. as a result there will be formation of ionic bond between sodium and phosphorous.

the reaction will be as follows.

there, are three sodium sodium atoms and each of them will donate its one extra electron to phosphorous atom. thus, the sub-shell of phosphorous gets completely filled.  

thus, we can conclude that electrons are transferred from atoms of sodium to atoms of phosphorous. this transfer makes the sodium atoms donate one electron and the phosphorous atoms to gain one electron. as a result, the sodium and phosphorus atoms strongly attract each other.

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).