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4.1.1Chemical Analysis

Pure Substances

5 min read 5 parts Lesson 2 of 21 1 quiz

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Section 1 of 5What Counts as Pure

4.1.1 Pure Substances

In chemistry the word pure has a stricter meaning than in everyday language.

Chemical purity
  • A pure substance contains a single element or compound, with no other substances present.
Everyday meaning
  • In everyday language pure usually means natural or clean, which is a different idea from chemical purity.
Pure water
  • Pure water contains only H₂O molecules, whereas tap water also contains dissolved ions and other substances.
Mixtures
  • A mixture contains two or more substances that are not chemically combined, so it is not pure.

Section 2 of 5Using Melting and Boiling Points

Sharp melting and boiling points are the key test for purity.

1
A pure substance melts and boils at a sharp, specific temperature, such as water at 0 °C and 100 °C at standard pressure.
2
An impure substance melts over a range of temperatures, because the different components do not all melt at the same point.
3
Impurities generally lower the melting point and raise the boiling point of a substance.
4
Comparing a measured melting or boiling point with data tables shows how pure a sample is.

Section 3 of 5Assessing Purity in Practice

Assessing Purity in Practice
Drugs
  • Melting point analysis is routinely used to check the purity of drugs.
Foodstuffs
  • Measuring purity is also important in foods, for safety and quality.
Method
  • A small sample is heated slowly in a melting point apparatus, which makes the melting point easier to observe precisely.
Closeness
  • The closer the measured value is to the data value, the purer the sample is.

Section 4 of 5Worked Example

Question

  • Sulfur has a melting point of 114 °C.
  • A sample begins to melt at 100 °C and finishes melting at 113 °C.
  • Is the sample pure?

  1. A pure substance melts at one sharp temperature, which for sulfur is 114 °C.
  2. This sample melts over a range from 100 °C to 113 °C, so it does not melt at one sharp temperature.
  3. A melting range indicates that the sample contains more than one substance.

Answer The sample is impure, because it melts over a range of temperatures below the pure melting point of 114 °C.

Section 5 of 5Exam Checklist & Tips

4.1.1 – Key Points for the Exam

  • Pure substance A single element or compound with no other substances mixed in.
  • Mixture Two or more substances that are not chemically joined and can be separated by physical means.
  • Melting point The temperature at which a solid changes into a liquid.
  • Impurity A substance present in a sample that is not the substance of interest.

Common Mistakes

  • Calling drinking water pure in the chemical sense; it contains dissolved substances.
  • Saying an impure substance has one sharp melting point.
  • Reversing the effect of impuritiesthey lower the melting point and raise the boiling point.
  • Deciding a substance is pure from its appearance alone, without melting or boiling point data.
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