Solute, solvent and solution
A solution is a homogeneous mixture: the same all the way through. It has two parts:
- Solute: the substance that dissolves (usually the smaller amount), e.g. sugar.
- Solvent: the substance it dissolves in (usually the larger amount), e.g. water.
When water is the solvent, it is an aqueous solution. Water dissolves so many things that it is called the universal solvent. Other solvents: alcohol, acetone (nail-polish remover).
Solutes and solvents can be solid, liquid or gas: salt in water (solid in liquid), oxygen in water (gas in liquid), alloys like brass (solid in solid), air (gas in gas).
What happens when something dissolves
Water particles pull on the solute particles and pull them apart. The solute spreads out as single molecules or ions, far too small to see.
Properties of a true solution
- Clear and see-through (it can be coloured, like copper sulfate).
- Particles do not settle on standing.
- It passes through filter paper unchanged.
- Particle size below about 1 nm.
A suspension (sand in water) has big particles that settle. A colloid (milk) has middle-size particles that do not settle but scatter light.
Dissolving is not the same as melting: sugar dissolves in water at room temperature, it does not melt.
Concentration
Concentration = amount of solute in a given amount of solution. A dilute solution has little solute; a concentrated one has a lot.
- Mass percentage = mass of solute ÷ mass of solution × 100
- Mass of solution = mass of solute + mass of solvent
- Mass per volume = mass of solute (g) ÷ volume of solution (L), in g/L
Adding more solvent (dilution) lowers the concentration, but the amount of solute stays the same.
Solubility, saturation and temperature
Solubility is the largest mass of solute that dissolves in 100 g of solvent at a given temperature.
- Unsaturated: more solute can still dissolve.
- Saturated: no more dissolves at that temperature; extra solid stays at the bottom.
- Supersaturated: holds more than normal after careful cooling; a tiny crystal makes the extra come out.
A solubility curve plots solubility against temperature. For most solids it rises with temperature. For gases it falls: warm fizzy drinks go flat faster, and warm water holds less oxygen for fish.
Stirring and smaller grains make a solid dissolve faster, but do not change how much can dissolve. Energy changes on dissolving can make the solution warm up or cool down.
'Like dissolves like': water dissolves salts and sugar; oil does not mix with water but dissolves in petrol.
Separating a solution
Filtering does not work on a solution, because the particles are too small.
- Evaporation: heat until the solvent leaves; the solid remains.
- Crystallisation: heat to make a saturated solution, then cool slowly; pure crystals form (used for copper sulfate, sugar).
- Distillation: boil, then cool the vapour to collect the pure solvent (used for drinking water from sea water).
- Chromatography: separates dissolved coloured substances, like inks.
Key formulas and definitions
- Solution = solute + solvent
- Mass % = (mass of solute ÷ mass of solution) × 100
- Mass of solution = mass of solute + mass of solvent
- Concentration (g/L) = mass of solute ÷ volume of solution in L
- Solubility = max mass of solute per 100 g solvent at a given temperature
Worked examples
1. 15 g of sugar is dissolved in 135 g of water. Find the mass percentage.
Mass of solution = 15 + 135 = 150 g. Mass % = 15 ÷ 150 × 100 = 10 %.
2. How much salt is in 250 g of a 4 % salt solution?
Salt = 4 % of 250 = 0.04 × 250 = 10 g. Water = 240 g.
3. 12 g of sodium hydroxide is dissolved to make 500 mL of solution. Find the concentration in g/L.
500 mL = 0.5 L. Concentration = 12 ÷ 0.5 = 24 g/L.
4. Solubility of a salt at 20 °C is 36 g per 100 g water. How much can dissolve in 50 g of water?
Half the water, so half the solute: 18 g.
5. A saturated solution at 60 °C has 110 g solid in 100 g water. It is cooled to 20 °C where solubility is 32 g. How much solid crystallises?
110 − 32 = 78 g comes out as crystals.
6. Why can't you get salt back from salty water by filtering?
Dissolved salt is in the form of tiny ions that pass through the filter paper holes with the water. You must evaporate the water.
Common mistakes
- Dividing by the mass of solvent instead of the mass of solution in mass %.
- Saying sugar 'melts' in tea; it dissolves.
- Thinking stirring increases solubility; it only speeds up dissolving.
- Assuming gases dissolve more in hot water; they dissolve less.