We will understand the state in which “no more can dissolve” and learn how to use the relationship between temperature and solubility to obtain substances.
Basic Knowledge
Solubility is expressed as the maximum mass of solute [g] that dissolves in 100g of water. An aqueous solution in which no more solute can dissolve is called a saturated aqueous solution.
Solubility changes with temperature. For many solids, solubility increases as temperature rises (for example, potassium nitrate dissolves about 32g at 20℃ and about 110g at 60℃). On the other hand, the solubility of table salt changes very little when the temperature rises (about 36g at 20℃ and about 37g at 60℃).
Recrystallization is a method of obtaining crystals by using the difference in solubility between temperatures. A large amount is dissolved at high temperature, and when the solution is cooled, the excess appears as crystals. Potassium nitrate is easy to recrystallize because its solubility changes greatly with temperature, while table salt is difficult to recrystallize because the change is small (table salt is obtained by evaporation).
Solubility（the maximum amount of solute [g] that dissolves in 100g of water）
Saturated aqueous solution（an aqueous solution in which no more solute can dissolve）
Solubility curve（a graph showing the relationship between solubility and temperature）
Recrystallization（a method of forming crystals using a temperature difference and increasing purity）
Crystal（a solid with a regular shape. Table salt forms cubes, and alum forms regular octahedra）
Precipitation（the phenomenon in which a dissolved substance emerges as crystals）
Further Exploration (Background and Significance)
Recrystallization is the purification method most relied on by chemists. Newly produced compounds often contain impurities, but by using differences in solubility between the desired substance and the impurities and obtaining the desired substance as crystals, the impurities can be removed.
The shape of crystals is determined for each substance: table salt forms cubes, alum forms regular octahedra, and copper sulfate forms blue orthorhombic crystals. This reflects the arrangement of atoms and ions (the ionic crystals studied in high-school chemistry). The solubility of gases behaves oppositely: it decreases as temperature rises (warming a carbonated drink causes it to lose its carbonation).
- Solubility = maximum amount that dissolves in 100g of water [g]
- Saturated aqueous solution = no more can dissolve
- Solubility of solids usually increases as temperature rises
- Table salt has a small temperature difference → difficult to recrystallize
- Potassium nitrate has a large temperature difference → easy to recrystallize
- Crystal shape is specific to each substance
- Gas solubility decreases as temperature rises
Points to Note (Easily Confused)
① Solubility is the amount relative to 100g of water, not the amount relative to 100g of solution. ② Use the evaporation method for table salt and the recrystallization method for potassium nitrate. ③ The solubility of gases is opposite to that of solids; it decreases as temperature rises. ④ Saturated means at the dissolving limit, while supersaturated means an unstable state exceeding the saturation limit (high-school content).
Practice
- State the definition of solubility.
- Why is potassium nitrate easy to recrystallize while table salt is difficult?
- Explain why a carbonated drink loses its carbonation when warmed.