Elements and compounds are differentiated by their composition. Elements cannot be decomposed into simpler substances. Compounds are composed of two or more elements, but the chemical identities and properties of these elements change when they form a compound. Mixtures on the other hand are composed of elements which retain their chemical identities.
So in this segment let's go ahead and discuss the differences between what it means for something to be an element and what it means for something to be a compound. So first let's kind of discuss how we classify and describe matter. So we can describe it based on its physical or its composition. So the physical state meaning whether it's a solid, a liquid or a gas and we have encountered those already. So let's kind of delve into what it means for matter to be composed or have this particular composition. So it can be composed of an element or it can be in compound or it can be a mixture. So we're going to discuss elements and compounds specifically in this section. So again what do we mean my composition? So most forms of matter we encounter are not chemically pure for instance the air we breathe is a mixture of oxygen and nitrogen and other things.
And gasoline that you put in your car is methane and all kind of other additive things in it. So not chemically pure, it's composed of quite a few different substances. So these non pure substances can be resolved though into pure substances, so there are different processes filtration and different things of that nature where you can separate out things so that you can get the pure substance. So a pure substance is matter that has distinct properties and a composition that does not vary from sample to sample. So two good examples of those are water and sodium chloride, so water is always composed of two hydrogen atoms and one oxygen atom. And sodium chloride it's always one sodium for every chloride. So all substances then are either elements or compounds, so elements sounds like a familiar word because we have the periodic table of the elements. So if those are all represented on the periodic table. So elements cannot be decomposed into simpler substances, the way that we encounter them is that's what they are. So for instance water is composed of the elements hydrogen and oxygen.
A compound on the other hand they're composed of two or more elements and so they contain 2 or more kinds of atoms, two or more different types of atoms. So for instance again in water hydrogen and oxygen are the elements and the compound is water. And those are pretty much the basic differences between elements and compounds.