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Pure substances and mixtures

Syllabus VMBO: Basic chemistry

Question 1 / 50 correct

Salt has dissolved completely in water. What do you have?

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More explanation and a worked example

Purity is about composition

A pure substance consists of one chemical substance. Water can therefore be pure even though a water molecule contains two types of atom. Salt water contains water and dissolved salt: it is a mixture.

What can you see?

A solution can be clear. A suspension contains undissolved solid particles dispersed in a liquid. Filtration can remove those particles but does not remove dissolved salt. Melting or boiling measurements at a specified pressure can provide further evidence; one flat graph alone does not prove purity.

Compare distilled water, salt solution and chalk in water.

  1. Here we model distilled water as pure H₂O.
  2. Salt solution is a clear mixture; chalk in water is a suspension.
Clear and pure do not mean the same thing.
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