Dissolving and solutions · Grade 4
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
25 g of sugar dissolves completely in 200 g of water. What is the mass of the solution?
Answer: 225 g
Example 2
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?
Answer: 78 g