System – It is the part of universe under study in definite conditions of pressure, volume and temperature or in other words, we can define as that portion of matter under study which is separated from rest of universe with the bounding surface if system consist one or more substances.
Surrounding – The remaining portion of universe which is not a part of system is called surrounding. Surrounding includes everything except system. System and surrounding together constituted universe.
Types of system – System may be classified into three types depending upon movement of matter and energy in or out of system.
1. Open system – A system in which can exchange both matter and energy with surrounding is called open system. For example reaction of zinc and HCL in an open flask.
2. Close system – A system which can exchange energy but not matter with its surrounding is called a closed system. For example decomposition of hydrogen iodide in a sealed glass bulb.
3. Isolated system – A system which can neither exchange matter nor energy with its surrounding is called an isolated system.