Separating Salts from Seawater
Contributors
Summary
The experiment "Separating Salts from Seawater" demonstrates the process of separating various salts from seawater. Students heat seawater to evaporate the water and precipitate solids. The procedure involves boiling seawater, allowing solids to settle, and using filtration and further evaporation to isolate different salts, primarily calcium carbonate, calcium sulfate, and sodium chloride. This experiment teaches concepts of mixtures, solubility, and chemical reactions, while emphasizing laboratory safety practices.
Keywords
Safety Precautions, Hazards, and Risk Assessment
See linked RSC resource
Teacher Recommendations or Piloting Data (if available)
Encourage the students to write down what they observe at each stage.
If no gradations are already present, teachers may wish to mark 250 cm3 beakers at the 60 cm3 level.
The artificial seawater contains calcium hydrogen carbonate owing to the reaction of limewater with excess carbon dioxide:
Ca(OH)2(aq) + 2CO2(g) → Ca(HCO3)2(aq)
When this solution is boiled, it soon precipitates calcium carbonate:
Ca(HCO3)2(aq) → CaCO3(s) + CO2(g) + H2O(l)
This is the identity of the predominant solid first left behind when the liquid is boiled. However, some calcium sulfate will also be present.
When hydrochloric acid is added to this solid, students should observe effervescence (fizzing), since the calcium carbonate is producing carbon dioxide gas:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The solid which continuously crystallizes out on further evaporation is sparingly soluble calcium sulfate, which is the predominant solid filtered off when 30 cm3 of seawater remains.
The more soluble sodium chloride precipitates out during the final stages of evaporation.
If no gradations are already present, teachers may wish to mark 250 cm3 beakers at the 60 cm3 level.
The artificial seawater contains calcium hydrogen carbonate owing to the reaction of limewater with excess carbon dioxide:
Ca(OH)2(aq) + 2CO2(g) → Ca(HCO3)2(aq)
When this solution is boiled, it soon precipitates calcium carbonate:
Ca(HCO3)2(aq) → CaCO3(s) + CO2(g) + H2O(l)
This is the identity of the predominant solid first left behind when the liquid is boiled. However, some calcium sulfate will also be present.
When hydrochloric acid is added to this solid, students should observe effervescence (fizzing), since the calcium carbonate is producing carbon dioxide gas:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The solid which continuously crystallizes out on further evaporation is sparingly soluble calcium sulfate, which is the predominant solid filtered off when 30 cm3 of seawater remains.
The more soluble sodium chloride precipitates out during the final stages of evaporation.
Link to external