Chemistry · Thermodynamics · NEET
The system is the part of the universe you choose to observe and study, for example the reactants inside a beaker. The surroundings are everything else outside the system. NCERT states it directly: a system is that part of the universe in which observations are made, and the remaining universe is the surroundings. You get to choose the system yourself.
It can be either. The boundary is the wall that separates the system from the surroundings. Sometimes it is a real wall, like the metal wall of a sealed steel vessel. Sometimes it is imaginary, like the invisible surface you draw around reactants in an open beaker. NCERT says this wall may be real or imaginary. Its job is to let us keep track of matter and energy going in or out.
In thermodynamics, universe does not mean outer space or stars. It simply means the system plus the surroundings together. The key relation to remember is: Universe = System + Surroundings. So if you add the part you study and everything outside it, you get the whole universe of the problem.
For practical work, yes. In theory the surroundings are the entire universe outside the system. But NCERT notes that only the part near the system is really affected during a change, so for real problems we treat the surroundings as the region close to the boundary (like the water bath or air around the beaker). The far-away universe does not change.
They differ by what can cross the boundary. Open system: both matter and energy can cross (reactants in an open beaker). Closed system: energy can cross but matter cannot (reactants in a sealed steel vessel). Isolated system: neither matter nor energy can cross (reactants in a thermos flask). NEET often gives an example and asks you to name the type.
The boundary is an imaginary surface enclosing the beaker and the reactants. Since the beaker is open at the top, matter (like escaping gas or water vapour) and energy can both leave, so it is an open system with an imaginary boundary. This is exactly the example NCERT uses.
Two moles of an ideal gas undergo free expansion from 10 L to 100 L at 300 K. The values of ΔS_system and ΔS_surroundings are (R = universal gas constant):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Temperature is a property, not a place. But the temperature of the system and the temperature of the surroundings are tracked separately. When they become equal and stop changing, the system is in thermal equilibrium with its surroundings.
Because energy changes in a chemical reaction only make sense when we say where the energy comes from and where it goes. Marking a boundary lets us count exactly how much heat and matter cross in or out, which is the whole basis of the first law of thermodynamics.
It depends on how you draw the boundary. If your system is only the reactants, then the beaker walls act as the boundary and the glass plus outside air are the surroundings. You are free to choose, as long as you stay consistent within the problem.
Yes, this is a definition, so it is always true in thermodynamics. Anything that is not the system, by definition, belongs to the surroundings. There is no third category.
