Learn Spanish (Latin America): Sopla la sopa, que está caliente.

In colloquial Spanish, an unstressed que is often used as a shortcut for porque (because) to explain or justify a command you just gave. You will hear this constantly in spoken language, especially after an imperative like sopla (blow).

In English we say 'blow on the soup'. Why is there no preposition like en after sopla?

In Spanish, the verb soplar takes a direct object when you are blowing something to cool it down. You do not need a preposition to translate 'on'. You just directly 'blow the soup' (sopla la sopa).

Could I say que es caliente instead of que está caliente?

Not for a bowl of soup. You use estar here because being hot is a temporary physical state of this specific soup right now, and it will eventually cool down. You would use ser to describe something that is permanently hot by nature, like fire.

Why is está in the indicative mood rather than the subjunctive?

While the word que often triggers the subjunctive (like when expressing desires or doubts), here it functions as a causal conjunction meaning 'because'. When you are stating a concrete fact as the reason for your command, you use the indicative mood.