Dissolving and solutions

When sugar is stirred into water it seems to vanish, but it is still there: it has dissolved. The sugar is the solute, the water is the solvent, and together they make a solution. Substances that dissolve are soluble; substances that don’t, like sand, are insoluble.

Key facts

  • Solute + solvent → solution. Sugar (the solute) dissolves in water (the solvent) to make sugar solution.
  • Soluble substances dissolve (salt, sugar, instant coffee). Insoluble substances don’t (sand, chalk, oil in water).
  • Mass is conserved: the mass of a solution = the mass of the solute + the mass of the solvent.
  • Stirring, heating and crushing the solute into smaller pieces all make it dissolve faster.
  • A saturated solution can’t dissolve any more solute at that temperature. Solubility is how many grams dissolve in 100 g of water; for most solids it rises with temperature.
  • Dissolving is reversible: evaporate the water and the solid is left behind.
  • Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).

Worked examples

Example 1

In salty water, which is the solute, which is the solvent and which is the solution?

  1. The salt dissolves: it is the solute.
  2. The water does the dissolving: it is the solvent.
  3. The mixture of the two is the solution: salty water.

Answer: salt: solute; water: solvent; salty water: solution

Example 2

25 g of sugar dissolves completely in 200 g of water. What is the mass of the solution?

  1. When sugar dissolves no mass is lost: the sugar particles are still there, spread between the water particles.
  2. Add the masses. 25 + 200 = 225 g

Answer: 225 g

Example 3

The solubility of potassium nitrate is 110 g per 100 g of water at 60°C and 32 g at 20°C. A saturated solution in 100 g of water is cooled from 60°C to 20°C. What mass of crystals forms?

  1. At 60°C, 110 g is dissolved in the 100 g of water.
  2. At 20°C only 32 g can stay dissolved.
  3. The rest comes out as crystals. 110 − 32 = 78 g

Answer: 78 g

Example 4

Give three ways to make a sugar cube dissolve faster in water.

  1. Stirring moves water past the sugar, bringing new water to it.
  2. Warmer water: the particles move faster, so the sugar breaks up and spreads out faster.
  3. Crushing it gives a bigger surface area for the water to work on.

Answer: stir it, use warmer water, and crush it into smaller pieces