A solution of salt water sits in the sun for 6 days. after 6 days, only the salt remains in the cup. Explain why this is an example of a mixture.

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t-nez | High School Teacher | (Level 3) Associate Educator

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Mixtures can be separated by physical means, while compounds can only change composition via a chemical reaction. Since the salt was separated by evaporation of the water without evidence of a chemical reaction we know that the salt water was a mixture. Evaporation, the process that removed the water from the salt, is a physical change.

Some other physical processes for separating mixtures are:

Distillation - separating liquids with different boiling points by boiling off each substance step-wise.

Magnetism - iron-containing particles can be separated out of a mixture using a magnet.

Screening - a screen can be used to separate substances that have particles of different sizes. 

An example of a compound (which is a pure substance) being separated by chemical means is the application of electric current to water to produce hydrogen and oxygen gases.