Learn Spanish (Latin America): El sol iba a derretir las bolas casi inmediatamente. - The sun was going to melt the scoops almost immediately.

Yes, it often implies an unfulfilled action. Using the imperfect of ir + a + infinitive expresses what was going to happen at a past moment. It sets up the expectation that something interrupted the process before the ice cream could melt, like someone eating it quickly or moving it to the shade.

Could we use the conditional derretiría instead of iba a derretir?

They are close, but derretiría (would melt) is usually used for hypotheticals, like saying the sun would melt them if you left them outside. Iba a derretir (was going to melt) is used for a past impending action that was actively about to happen on the timeline of the story.

Why can't we use the preterite fue a derretir here?

The preterite fue a derretir means the sun literally went somewhere in order to melt the scoops, and completed the action. To say a subject "was going to" do something (an intention or impending event), you must use the imperfect form iba a.

Wait, bolas usually means balls. Is this the normal word for ice cream scoops?

Yes! While bolas literally means balls, it is the standard word for scoops when talking about ice cream in Spanish. For example, a two-scoop ice cream cone is un cono de dos bolas.