Pure and Impure

Section 1 of 6Pure substances and mixtures

Chemistry meaning of pure

• Chemistry meaning of pure In everyday life, 'pure' can mean natural or additive-free.
  • In chemistry, a pure substance is a single element or a single compound with nothing else mixed in.
• Pure water contains only H₂O molecules.

Mixtures

• Mixtures A mixture contains two or more elements or compounds that are physically mixed but not chemically joined.
  • Each keeps its own chemical properties.
• Mixtures can be separated by physical methods such as filtration, distillation or chromatography.
  • Air is a mixture of nitrogen, oxygen, argon and carbon dioxide.

Section 2 of 6Using melting and boiling points

Sample	Melting / boiling behaviour	Reason
Pure substance	Melts and boils at one sharp, fixed temperature. Pure water melts at 0 °C and boils at 100 °C.	All the particles are identical, so they need the same energy to change state.
Impure substance or mixture	Melts over a range of temperatures, usually lower than the pure value, and boils over a range.	Different particles are present, which disrupts the structure and gives a mix of energies.

Section 3 of 6Testing a sample

1

Heat a small amount of the sample slowly in a melting point apparatus so that the exact melting temperature can be observed.

2

Record the temperature at which melting starts and the temperature at which it finishes.

3

Compare the result with data tables of the known melting point.

4

The closer the measured value is to the data-book value, and the narrower the range, the purer the sample. This method is used routinely to check the purity of drugs.

Section 4 of 6Cooling curves and percentage change

Pure substance cooling

• Pure substance cooling When a pure liquid cools and freezes, the temperature falls,
  • then stays flat at the freezing point while the substance solidifies,
  • then falls again.
  • The flat section shows a sharp freezing (melting) point.

Impure substance cooling

• Impure substance cooling An impure sample shows no clear flat section.
  • The temperature drops gradually as it freezes, so the melting point is not sharp.

Percentage change calculation

• Percentage change calculation Percentage change = (final value − original value) ÷ original value × 100.
• Worked example
  • a liquid has a measured boiling point of 76 °C but the pure value is 65 °C.
  • The difference is 11 °C, so 11 ÷ 65 × 100 = 16.9%.

Section 5 of 6Key terms

Term	Meaning
Pure substance	A single element or compound containing no other substances.
Mixture	Two or more substances physically mixed, not chemically combined.
Impurity	A different substance present in a sample that should contain only one substance.
Melting point range	The span of temperatures over which a sample melts; narrow for pure substances.
Cooling curve	A graph of temperature against time as a substance cools.

Section 6 of 6Exam technique

1

Saying that a pure substance contains nothing at all, when it simply means one substance only (air-free water is not required, but only H₂O molecules).

2

Claiming impurities raise the melting point; they usually lower it and broaden the range.

3

Reading the flat part of a cooling curve as 'no cooling' instead of the substance freezing and releasing energy.

4

Dividing by the final value rather than the original value when calculating percentage change.