Distinguish one substance from a mixture and explain dissolving without disappearing.
Method and worked examples
A pure substance contains one chemical substance; a mixture contains multiple substances. A uniform appearance does not prove purity. A solution contains a solute dispersed through a solvent: in salt water, salt is the solute and water the solvent. Dissolved material remains present and contributes to the solution’s mass.
A suspension has dispersed particles that may settle or be trapped by a suitable filter. Dissolved salt generally passes through ordinary filter paper. At a specified temperature, a saturated solution contains the maximum solute that dissolves in the given solvent amount. Stirring can increase dissolving rate without increasing this maximum. Clear unknown liquids are not automatically pure or safe.
Example 1
Clear seawater looks uniform. Is it a pure substance?
No. It contains water and dissolved salts, so it is a mixture even though its components are not visible separately.
Example 2
A beaker contains 100g of water and dissolves 5g of salt with nothing lost. What is the solution mass?
The solution mass is 100+5=105g. Dissolving does not remove the salt’s mass.
Watch out:Its material remains dispersed in the water. In a closed process the solution mass includes both water and salt.
Pure substances, mixtures and solutions | Year 7 Science | Ascent Arcade