Solutions and Dissolving

Some materials spread right through a liquid until you can no longer see them. We say they dissolve, and the clear mixture that results is a solution.

• A material that dissolves in water is soluble — sugar, salt, instant coffee, Oros powder.
• A material that does not dissolve is insoluble — sand, chalk dust, cooking oil, iron filings.

In a solution, the material that dissolves is the solute and the liquid it dissolves into is the solvent. Water dissolves so many things that it is called the universal solvent — which is also why river water is so easily polluted.

Three things speed dissolving up: stirring, warming the liquid, and crushing the solid into smaller pieces. All three do the same job — they get more of the solid touching more of the water.

Worked example. Two learners each drop one sugar cube into $200$ mL of water.

• Learner A uses cold water and does not stir.
• Learner B crushes the cube, uses warm water and stirs.

B's sugar dissolves far faster. Crushing gave the water more sugar surface to work on, warmth made the particles move about more quickly, and stirring kept fresh water sweeping past the sugar instead of letting sweet water sit around it.

But notice: both cups end up equally sweet. Stirring and heating change how fast the sugar dissolves, not how much dissolves in the end.