What is a thermodynamic system? Explain how open, closed and isolated systems differ, with an everyday example of each, and state the zeroth law of thermodynamics.

1. 1 · Define a system. A definite amount of matter chosen for study, enclosed by a boundary; outside it are the surroundings.
2. 2 · Open system. Exchanges both mass and energy with the surroundings, such as a water heater.
3. 3 · Closed system. Exchanges energy but not mass, such as gas in a cylinder with a piston.
4. 4 · Isolated system. Exchanges neither mass nor energy, such as a filled thermos flask (very nearly).
5. 5 · State the zeroth law. If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other.
6. 6 · Real life. A clinical thermometer reads your temperature once it is in thermal equilibrium with your body.
7. 7 · Close in one line. The zeroth law gives us temperature: the property shared by bodies in thermal equilibrium.

Scoring tip: Give a different everyday example for each kind of system. One correct example per system earns its own mark.