Purity

Level 1

Pure substances and mixtures

Matter around us can belong to two main families:

  • A pure substance: it is made up of a single chemical species.
  • A mixture: it contains several different chemical species.

  Examples of pure substances:

  • Salt, sugar, copper or graphite (pencil lead) are solid pure substances.
  • Demineralised water or certain oils are liquid pure substances.

  Examples of mixtures:

  • Tap water contains water and dissolved minerals.
  • Fruit juice contains water, sugars, flavourings, vitamins...

Homogeneous mixtures

In a homogeneous mixture, the constituents cannot be distinguished with the naked eye.

   Example:

  • Water + mint syrup
  • Water + dissolved salt

Heterogeneous mixtures

In a heterogeneous mixture, the constituents can be distinguished with the naked eye.

   Example:

  • Water + oil
  • Water + plastic pieces

Filtration

Filtration allows a heterogeneous solid-liquid mixture to be separated. The mixture passes through a filter:

  • The largest solid particles are trapped.
  • The liquid (and very fine particles) passes through.

  Example: Filtering a sand + water mixture. The sand remains in the filter, the water passes through.

Decantation

Decantation uses the difference in density between two substances. 

  • The denser one settles at the bottom.
  • The less dense one stays on top.

  Example: In a water + oil mixture, the oil (less dense) stays on top and the water stays below.

Separating two immiscible liquids

When two liquids do not mix (immiscible), they form two superimposed layers. To separate them, a separating funnel is used: the bottom tap is opened to let the bottom liquid flow out first, then closed just before the top liquid arrives.

  Example: separating water and oil from a water + oil mixture.

Towards purification

These techniques make it possible to separate the components of a mixture in order to obtain purer substances.

Pure substance

A pure substance contains only one type of molecule. 

In a solid, these identical molecules are often arranged in an orderly manner (crystals).

Modèle Solide Pur

In a liquid, they move freely but all remain identical.

Modèle Liquide Pur

The mixture

A mixture contains several types of molecules. 

In a solid mixture, particles of different natures coexist.

Modèle Solide Mélange

In a liquid mixture, different molecules move together.

Modèle Liquide Mélange

   Example: 
- A bag filled only with red marbles = pure substance. 
- A bag filled with red, green and yellow marbles = mixture.