Like iron and sulphur. when you mix them together and you can take the iron out by using a magnet. But if you heat them both together, it will be a new substance; iron-sulphur, which you cannot take iron out by a magnet
Chemically or physically. Chemically, elements can not be broken down. Chemically, a mixture of solids can be seperated in most cases through solubility, precipitation, and by using chemicals.