A mixture, such a pile of dirt, consists a number of different substances, some of which are compounds or chemical elements. When the mixture is homogeneous throughout the entire body of the material, it is called a solution.
An aggregate is a solid chunk of material that are bonded together due via weak chemical bonds. Typical agregates are concret, ceramics, and metal composites. Since the properties of aggregates depends on the composition, finding the right combination of substances to give aggregates of the desirable properties is particularly interesting from engineering point of view.
A substance is a single pure material, be it a chemical element or a compound. Properties of a substance are usually well defined, and deviation from reported properties is an indication of presence of impurity.
A compound is a substance that is made up of two or more elements. Compounds usually can be represented by a chemical formula. For example, water, H2O, ethanol, C2H5OH, ammonia NH3, (table) salt, NaCl, and suggar CH22O11O are all chemicals.
A chemical element or element is a substance that cannot be decomposed into other substances by chemical means. Today, we all know that there are about 111 known chemical elements. In reality, only about 92 elements occur naturally on the planet Earth. The names and symbols of elements are usually arranged in a periodic table, and tables of elements with some of their properties are given in handbooks.