1For a solid to turn into liquid (or soften) due to heat. The kanji used can signal what's melting — 熔ける/鎔ける are used only for metals, and 解ける only for ice or snow.
夏の日差しでチョコレートがすぐに溶けた。
The chocolate melted quickly in the summer sun.
春になると雪が解ける。
When spring comes, the snow thaws.
工場では高温で金属を溶かして加工するが、この炉なら鉄も自然に溶ける。
At the factory they melt metal at high temperatures to work it, and this furnace is hot enough that even iron melts on its own.
アイスクリームが溶けないうちに食べよう。
Let's eat the ice cream before it melts.
2For a substance to mix into a liquid and disappear, i.e. to dissolve — like sugar or salt dissolving in water.
砂糖を入れるとすぐにコーヒーに溶けた。
When I added sugar, it dissolved immediately in the coffee.
塩は水に溶けやすい物質だ。
Salt is a substance that dissolves easily in water.
この薬は水に溶けにくいので、よくかき混ぜてください。
This medicine doesn't dissolve easily in water, so please stir it well.