Cambridge IGCSE Chemistry · Syllabus 0620 · Chemistry of the Environment
Distilled Water
What is Distilled Water?
Water that has been purified by distillation, so that it contains fewer dissolved chemical impurities than tap water. It is used in practical chemistry because dissolved substances in tap water could react with the reagents being used and give misleading results. Distillation removes dissolved solids; it is not the same process as the treatment of a domestic water supply.
This definition is part of the Chemistry of the Environment chapter in Cambridge IGCSE Chemistry.
Common mistakes with Distilled Water
- “Distilled water is used in the laboratory because tap water is dirty.” Defect Wrong reason; “dirty” is not a chemical statement. Chemistry Tap water has been treated and is clear. The problem is the dissolved chemical impurities that survive treatment. Repair “Distilled water contains fewer chemical impurities, which could otherwise react with the reagents.” Transfer Why does silver nitrate give a faint precipitate with tap water? Dissolved chloride is present.
- “Distilled water is sterile.” Defect Confuses a separation with a disinfection. Chemistry Distillation separates by boiling point. Killing microbes is what chlorination does, in a different process for a different purpose. Repair Claim only “fewer chemical impurities”. Transfer Which stage of domestic treatment deals with microbes? Chlorination.