The portion of the universe that we single out to study is called the (here, the hydrogen and oxygen molecules). Theare everything else (here, the cylinder, piston and everything beyond).

The portion of the universe that we single out to study is called the SYSTEM

The SURROUNDINGS are everything else

Explanation:

The universe is composed of system and surrounding.

The system is the portion of the universe that is under the observation. It consists of molecules that are undergoing a chemical reaction.

Whereas, the surrounding or environment is the portion of the remaining universe that is not under observation, but can interact with the system.

For example: A cylinder containing hydrogen and oxygen gas.

In this example, the molecules inside the cylinder is the system. Whereas, the cylinder, piston and the surrounding space is the surrounding.