To dissolve (a solid into a liquid or make something dissolve into another substance), e.g. dissolving sugar in water.
ティーカップに砂糖を入れてスプーンで溶かした。
I put sugar in the teacup and stirred it until it dissolved.
錠剤を水に溶かしてから飲んでください。
Please dissolve the tablet in water before drinking it.
塩を海水で溶かす実験を行った。
We conducted an experiment dissolving salt in seawater.
To melt (to make something liquid by heat). For metals, special kanji such as 熔かす or 鎔かす are often used for melting.
バターを小鍋で溶かしてパンに塗った。
I melted butter in a small pan and spread it on the bread.
職人は古い金属を鎔かして新しい型に流し込んだ。
The craftsman melted the old metal and poured it into a new mold.
雪を溶かすために道路に塩をまいた。
They spread salt on the road to melt the snow.