Coaching Centre
Solutions and solubility
Solute, solvent and solution, dissolving at the particle level, soluble and insoluble substances, water as the universal solvent, concentrated and dilute solutions, saturated solutions, solubility curves and how temperature affects solubility.
Stir a spoonful of sugar into a cup of tea and it vanishes. Keep adding spoonfuls and eventually it stops vanishing: a gritty layer builds up on the bottom. Now heat the tea and that layer disappears too. There's a limit to how much can dissolve, and the limit depends on temperature.
Solute, solvent, solution
- The solute is the substance that dissolves (the sugar).
- The solvent is the substance it dissolves in (the water in the tea).
- Together they make a solution.
A substance that dissolves in a solvent is soluble; one that doesn't is insoluble. Salt and sugar are soluble in water; sand and chalk are insoluble. Whether something dissolves depends on the solvent too: nail polish is insoluble in water but soluble in acetone (nail polish remover), and grease that water can't shift dissolves in eucalyptus oil.
Dissolving, particle by particle
When sugar dissolves, water particles bump into the sugar crystal and pull particles off its surface. The sugar particles spread out evenly between the water particles. They haven't disappeared or melted: they're just too small and too spread out to see. The mass is conserved, too. Dissolve 10 g of sugar in 100 g of water and you have exactly 110 g of solution.
You can make a solid dissolve faster by stirring, by heating the solvent, or by crushing the solid into smaller pieces so more of its surface touches the water.
Water dissolves more substances than any other common liquid, which is why it is called the universal solvent. That's useful (your blood carries dissolved nutrients and wastes; plants take in dissolved minerals) and a problem (pollutants dissolve in rivers just as easily).
Concentrated, dilute and saturated
The concentration of a solution is how much solute is dissolved in a certain amount of solvent, for example grams per 100 g of water. A concentrated solution has a lot of solute; a dilute solution has only a little. Adding more water dilutes a solution, which is what you do when you make up cordial.
A saturated solution holds the maximum amount of solute that can dissolve at that temperature. Add any more and it just sits on the bottom. The maximum mass that dissolves in 100 g of water is the substance's solubility.
- Keep adding salt at 20 °C until some stays on the bottom. Now heat the water to 100 °C. Does much more dissolve?
- Try potassium nitrate. Make a saturated solution at 80 °C, then cool it slowly. What happens, and why?
- Compare the solubility curves. Which substance's solubility changes most with temperature? Which barely changes?
Solubility curves
For most solids, solubility increases as temperature rises. A solubility curve shows how much dissolves in 100 g of water at each temperature. Any point on the curve is a saturated solution; below the curve is unsaturated.
Potassium nitrate's solubility is about 110 g per 100 g of water at 60 °C, and about 32 g at 20 °C.
A saturated solution in 100 g of water at 60 °C contains 110 g of dissolved potassium nitrate.
Cool it to 20 °C and the water can hold only 32 g. The extra 110 − 32 = 78 g comes out of solution as crystals.
Table salt is unusual: its solubility barely changes with temperature, from about 36 g at 20 °C to about 40 g at 100 °C. Gases are the opposite of most solids: they are less soluble in warm water. That's why warm soft drink goes flat faster, and why fish in a warm, still pond can run short of dissolved oxygen in summer.
Test yourself
In salt water, the salt is the
The solute is what dissolves.
A solution has some undissolved solid on the bottom that won't dissolve however long you stir. The solution is
It already holds as much solute as it can at that temperature.
Which would NOT make sugar dissolve faster?
Colder water slows dissolving.
20 g of salt is dissolved in 100 g of water. The mass of the solution is
Mass is conserved when something dissolves.
Potassium nitrate's solubility is 64 g at 40 °C and 32 g at 20 °C (per 100 g water). A saturated solution in 100 g of water is cooled from 40 °C to 20 °C. The mass of crystals formed is
64 − 32 = 32 g comes out of solution.
Label the solute and solvent particles.
Soluble or insoluble in water?
The liquid that does the dissolving is the . A solution with only a little solute is . The maximum mass of solute that dissolves in 100 g of water is its . For most solids this increases as rises. When a hot saturated solution cools, form.
Using the particle model, explain what happens when sugar dissolves in water, and why the mass of the solution equals the mass of sugar plus water.
Moving water particles bump into the surface of the sugar crystal and pull sugar particles away from it. The sugar particles spread out evenly in the spaces between the water particles, until they are too small and too spread out to see. No particles are created or destroyed when this happens, so the total mass of the solution equals the mass of the sugar plus the mass of the water.