Eukaryotic cells have a nucleus and contains organelles.
A chemical change is a change in which the composition of the matter is changed at the molecular level, most chemical changes are not easily irreversible, for example, an egg being cooked, the egg changes from its liquid like state to a solid, and it is not possible to revert it back to its original change. The egg itself is changed in this case.
A physical change on the other hand however, is only changing the substance of a matter, not its composition, this is usually easily reversible. For example, water turning into ice, the water's substance changed from its liquid state to its solid state, it is still water, and it can easily be turned back into water by heating it.
Sorry for the long explanation,
Hope this helps!
The answer to the question mentioned above is "Synaptic Cleft".
The region composed of a presynaptic cell, its target cell and the space between them is a Synaptic Cell. This cell exists between the neurons and this is a microscopic gap.
