Chemistry · Thermodynamics · NEET
The system is the small part of the world you have chosen to study. For example, the chemicals reacting inside a test tube. The surroundings are everything else outside the system that can exchange heat or matter with it, like the air and the water bath around the test tube. Simple rule: what you focus on = system; everything else = surroundings.
The reacting chemicals ARE the system. In NEET, when a question says heat is absorbed or released 'by the system,' it means the reaction mixture. The heat then flows to or from the surroundings. This matters because signs of heat (q) and work (w) are always written from the system's point of view.
The boundary is the real or imaginary surface that separates the system from the surroundings. The walls of a beaker are a real boundary. The boundary can be rigid or movable, and it can allow heat or matter to pass or block them. The type of boundary decides whether a system is open, closed or isolated.
Together they make the 'universe' in thermodynamics. So Universe = System + Surroundings. This is important for the Second Law, where the total entropy of the universe must increase for a spontaneous process. In problems, energy lost by the system is gained by the surroundings, so the total stays constant (First Law).
Yes. You decide the boundary based on what you want to study. If you study a gas in a cylinder, the gas is the system and the cylinder plus outside air is the surroundings. Once you fix the system, keep the same choice for the whole problem so your signs for heat and work stay correct.
At a certain temperature T (K), during a process, 500 J is absorbed by the system and work of 200 J is done by the system. Then the change in internal energy of the system is:
In which case is the change in entropy negative?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Thermodynamics is the branch of science that studies heat, work and energy changes in physical and chemical processes. For NEET, it explains why reactions release or absorb heat and whether a process happens on its own.
A thermodynamic system is the specific part of matter or space chosen for study, such as the gas inside a cylinder or the chemicals inside a flask. Everything outside it is the surroundings.
Every sign for heat (q) and work (w) is written from the system's point of view. If you do not fix the system first, you will get the wrong sign and the wrong answer in First Law and entropy problems.
The universe is the system plus its surroundings. Energy and matter move between the two, but for an isolated universe the total energy stays constant, which is the basis of the First Law.
No. Heat and work are not stored inside the system. They are ways energy is transferred across the boundary between the system and the surroundings. Internal energy (U) is the stored property.
