A chemically pure substance...
o only contains a single element or compound (not mixed with any other substance)
o will only melt/boil at a specific temperature, impurities in a sample...
▪ decrease melting point & increase melting range of the substance
▪ increase boiling point & increasing boiling range of the substance
o the closer you measured value is to the actually melting/boiling point, the purer the sample
o e.g. oxygen gas, carbon dioxide gas