Energy:

The first law of thermodynamics asserts that energy (but not necessarily thermodynamic free energy) is always conserved and that heat flow is a form of energy transfer. For homogeneous systems, with a well-defined temperature and pressure, a commonly used corollary of the first law is that, for a system subject only to pressure forces and heat transfer (e.g., a cylinder-full of gas) without chemical changes, the differential change in the internal energy of the system (with a gain in energy signified by a positive quantity) is given as

Please answer a question about this article. If the question is unanswerable, say "unanswerable". The first law of thermodynamics asserts that what is always conserved and that heat flow is a form of energy transfer?
energy