Task: Paraphrasing
Input (English): Question: A sealed, vacuum-insulated reaction flask approximates what condition, in which neither matter nor heat can be exchanged between system and surroundings? 
Answer: isolated system. 
Sentence: A sealed, vacuum-insulated reaction flask approximates the condition of a isolated system, in which neither matter nor heat can be exchanged between system and surroundings.
Output (English): A reaction flask that is sealed and vacuum insulated, preventing matter and heat exchange between system and surroundings, is what is considered an isolated system.