will acids/ alkalis attack a metal when the metal is molten? e.g will hydrochloric acid attack molten iron?

yes, explosively so.

how explosively and why?

Very, and why don`t You try and figure out why, trust me, it`s not that hard to work out

HOW explosively would depend upon the metal.

Just to be a PITA ;-)

Mercury, which is generally a molten metal in my experience, does not react all that vigorously with most acids- in fact, with a lot of acids the reaction is rather slow.

However, for the example given (iron and HCl) I wouldn't want to be there when someone tried it.

Every now and again there are explosions in metal foundries when the white hot metal comes into contact with water. The water flashes to steam and expands a couple of thousand times. In doing so it sprays molten metal everywhere.

In the circumstances, the reactivity of the metal with the water isn't the issue (and molten iron does react with steam BTW)

 

http://news.bbc.co.uk/1/hi/wales/south_west/4716918.stm

 

Adding acid would not make that any nicer.