Learn Russian: Что происходит в колбе, пока мы нагреваем раствор? - What is happening in the flask while we heat the solution?

Here, пока means "while". It is used to show two actions happening at the exact same time: something is happening in the flask while we are heating it. You would use пока не to mean "until", which implies one action continues up to the point when another action happens.

Both verbs in this sentence (происходит and нагреваем) are imperfective. Is this required when using пока?

Yes! When пока means "while" (describing simultaneous actions), both verbs almost always need to be in the imperfective aspect. This emphasizes that both actions are ongoing continuous processes happening parallel to each other.

I recognize the root in нагреваем from греть (to warm). What does the prefix на- change?

While греть just means to emit heat or to warm something up generally, нагревать focuses on the specific process of raising the temperature of a substance or object. You will often see нагревать in technical contexts, science experiments, or cooking.

Why doesn't the word раствор change its ending, even though it's the direct object we are heating?

Раствор (solution) is indeed in the accusative case here because it is the direct object of нагреваем. However, because it is a masculine inanimate noun, its accusative form is exactly the same as its dictionary (nominative) form.