What pure means to a chemist, how melting and boiling points reveal impurities, and the percentages in a formulation.
What you will learn
- Explain the difference between pure in chemistry and pure on a label
- Use melting and boiling point data to spot a pure sample
- Calculate the percentage and mass of a component in a formulation
Pure means one substance
To a chemist a pure substance is a single element or a single compound with nothing else mixed in.
Distilled water and copper wire are pure. Orange juice sold as "100% pure" is a mixture of water, sugars, acids and much more: pure on the label only means nothing was added.
A pure substance has a fixed melting point and boiling point. Impurities change them:
- a pure solid melts sharply at its own melting point
- an impure solid melts lower, over a range of temperatures
- dissolved impurities raise the boiling point of a liquid, and it boils over a range: as the water boils away, what is left is more concentrated, so its boiling point keeps rising. Pure water boils at exactly 100 °C at normal air pressure; the more salt is dissolved in a sample, the higher above 100 °C it starts to boil.
- Pure stearic acid melts at 69 °C
- Sample A melts sharply at 69 °C: pure
- Sample B melts over 61 to 66 °C: impure (lower, and over a range)
A sample that melts sharply at the wrong temperature is not your substance: it may be a different pure substance.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Pure means one substance
- Formulations
- Try it: Which sample is pure?
10 practice questions, marked as you go; about 20 minutes in all.
Key terms
Pure substance, Sharply, Lower, Range, Raise, Formulation
In this unit: Chemical analysis
Purity and formulations, chromatography and Rf values, tests for gases, and for separate Chemistry the tests for ions and flame emission spectroscopy.
- Pure substances and formulations
- Required practical: chromatography
- Tests for gases
- Required practical: identifying ions
- Instrumental methods