Learning focus
Develop precise chemical explanations, connect observations to reactivity and structure, interpret industrial and environmental processes, and apply evidence to unfamiliar contexts.
Physical-constant test
Pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure. A pure substance shows sharp state-change temperatures.
Effect of impurities
Dissolved substances alter melting and boiling behaviour. Pressure also affects boiling point, so measurements must be compared under known conditions.
Distilled versus tap water
Distilled water is preferred in practical chemistry because it contains fewer dissolved ions and other chemical impurities. Tap water can introduce precipitates, extra ions or misleading test results.
Limits of the test
A measured boiling point close to 100 °C supports purity but does not detect every trace impurity. Thermometer calibration, pressure and heat loss affect the measurement.
High-value recall and connections
Essential recall: Why use distilled water in chemical tests? It contains fewer chemical impurities. What boiling point suggests pure water at standard pressure? 100 °C. Why might tap water interfere? Dissolved ions can react or form precipitates. Practical connection: Measure or interpret melting/boiling data for water samples. Identify controls and sources of uncertainty. Examination connection: State the pressure condition when using the 100 °C boiling point.
Practical or data skill
Measure or interpret melting/boiling data for water samples. Identify controls and sources of uncertainty.
Examination tip
State the pressure condition when using the 100 °C boiling point.
Review questions and suggested answers
Question 1
Why use distilled water in chemical tests?
Suggested answer
It contains fewer chemical impurities.
Question 2
What boiling point suggests pure water at standard pressure?
Suggested answer
100 °C.
Question 3
Why might tap water interfere?
Suggested answer
Dissolved ions can react or form precipitates.
