Classifying matter
Matter is anything that has mass and takes up space. Chemists sort it by what it is made of.
A pure substance has the same composition throughout.
- An element is made of one kind of atom and cannot be broken down chemically. Examples: copper (Cu) and oxygen (O2).
- A compound is made of two or more elements chemically bonded in a fixed ratio. Examples: water (H2O) and table salt (NaCl).
A mixture is two or more substances physically combined. Each keeps its own properties, and the proportions can vary.
- A homogeneous mixture looks the same throughout. Salt water and air are examples. These are also called solutions.
- A heterogeneous mixture has visibly different parts, like sand in water or a salad.
Worked example: classify air, CO2 and gold
- Air is several gases mixed evenly, so it is a homogeneous mixture.
- CO2 is carbon and oxygen bonded in a fixed 1 to 2 ratio, so it is a compound.
- Gold is one kind of atom, so it is an element.
Common mistake
Calling a compound a mixture because it contains more than one element. In a compound the elements are bonded and can only be separated by a chemical reaction. A mixture can be separated physically, by filtering, evaporating or distilling.