Rashi: Something that melts in front of a fire. When you put a coal or [red-hot] metal in water, the water bubbles. All this You did in Egypt - "va'Yhi Varad v'Esh Mislakachas b'Soch ha'Barad" (Shemos 9:24).

Rashi citing Targum Yonasan: This refers to Eliyahu on Har ha'Karmel - "v'Es ha'Mayim Asher ba'Te'alah Lichechah" (Melachim I, 18:38).

Radak: The root of ha'Masim is Masas. There should be a Dagesh in the Samech to compensate for the missing Samech; the Nach ¹ is in place of the Dagesh. A great fire is called Esh ha'Masim, for it melts everything. So was the fire on Har Sinai. Kedo'ach is burning, like "Ki Esh Kadchah v'Api" (Devarim 32:22). The fire was on the mountain, and there was water on the mountain. The water boiled, and the entire mountain smoked - both where the fire was, and where the water was.

Malbim: The previous verse said "did You tear Shamayim... did mountains drip"? Now, Ru'ach Eloki has a vision how fire consumed fire - an awesome fire descended from Shamayim. Mountains flowed, rocks melted and valleys split, like wax melts in front of fire. He sees mountains spewing fire, and snow covers their tops. The snow melts, and the resulting water boils The mountains dripped when they burned.

Radak: Esh is the subject, even though it is last. Mayim is the subject, even though it is first. Fire boils the water. One cannot err, therefore the verse was not adamant about the order. Tiv'eh is boiling. This expression is used, because the water bubbles.

Rashi: This is like it says in that Makah (hail) - "v'Ulam Ba'avur Zos He'emadticha..." (Shemos 9:16).

Radak: All this (Har Sinai smoking) was to inform Your name to Your afflicters, who do not know Your name and did not recognize You. Via the wonders, You made them know that there is a Master in the world, and He conducts as He desires. He burns mountains, unlike nature.

Rashi: If You would do so now, Goyim would tremble in front of You.

Radak: Goyim were agitated when the mountains were agitated. That day You were heard and known in the world.