Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level 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.
Questions students ask about Distilled Water
Why does distilled water boiling at exactly 100°C not prove it is pure water?
A sharp boiling point shows a sample is pure, but it does not identify which pure substance it is. Many pure substances have their own sharp, fixed boiling point at standard atmospheric pressure. The correct conclusion is "the data are consistent with pure water", not "this proves it is water".
Why is distilled water used in the laboratory instead of tap water?
Tap water has been treated to make it safe to drink, but it still contains dissolved chemical impurities that survive treatment, such as chloride. Those impurities could react with reagents and give misleading results. Distilled water has fewer dissolved chemical impurities, so it is the water used in practical chemistry.
What are the three stages of treating a domestic water supply, and what does each one remove?
Sedimentation and filtration remove solids; carbon filtration removes tastes and odours; chlorination kills harmful microbes. Each stage removes only what it targets, so treated tap water is safe to drink but is not chemically pure — dissolved substances remain, which is why laboratory work still uses distilled water.