An element is defined as a substance which contains same type of atoms.
For example, a small block made up of only copper metal will contain only atoms of copper.
A compound is defined as a substance in which atoms chemically combine together in a fixed ratio by mass.
For example, HgO is a compound.
A mixture is defined as a substance in which two or more components are physically combined but none of the components are chemically combined to each other.
For example, a pan cake syrup is a mixture.
Components of a mixture can be separated by physical means.
Thus, we classify each of the given species as follows.
iron - element
oxygen - element
mercury oxide - compound
pancake syrup - mixture
carbon dioxide - compound
a substance composed of molecules each of which contains one hydrogen atom and one chlorine atom - compound
