Q.What is meant by an isolated system in thermodynamics? OR Write the SI unit of entropy.

In thermodynamics, before any quantitative analysis can begin, the small portion of the universe under study must be clearly separated, by a real or imaginary boundary, from everything else. The part under study is the system; everything outside the boundary is the surroundings; the dividing surface itself is the boundary, which may be rigid or movable, and permeable or impermeable to matter and heat.

Systems are classified into three types based on what is permitted to cross the boundary:

• An open system exchanges both matter and energy with its surroundings — for example, an open beaker of hot liquid, which loses both vapour and heat to the surrounding air.
• A closed system exchanges energy (heat and/or work) but not matter with its surroundings — for example, a fixed quantity of gas sealed inside a cylinder fitted with a movable, airtight piston: no gas enters or leaves, but heat can flow through the cylinder walls and work can be done as the piston moves.
• An isolated system exchanges neither matter nor energy with its surroundings — the idealized case, approximated in practice by a well-insulated, sealed vacuum flask, though no real system is ever perfectly isolated over an indefinitely long time. …