Water purity and why laboratories use distilled water
Compare melting and boiling under controlled pressure, then explain how fewer impurities improve a chemical test.
Section C10.1 · Water
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish pure substances from solutions and read a thermometer scale.
- Explain boiling and melting as physical changes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are the school pure-water reference temperatures at standard atmospheric pressure?
What you will learn
- Describe school-level purity testing through water's melting and boiling points at stated pressure.
- Explain why distilled water is preferable to tap water in practical chemistry.
A reference value needs reference conditions
Pure water melts at about 0°C and boils at about 100°C at standard atmospheric pressure in the school reference. A sharp melting/boiling point consistent with that reference supports purity; impurities can change the temperatures or broaden melting behaviour. A positive chemical water test alone does not give this information.
Pressure changes the boiling point, so a temperature measured at a different pressure cannot be compared uncritically with 100°C. Thermometer calibration, heating method and measurement resolution also affect observations. A melting/boiling comparison provides school-level evidence, rather than proving absence of every trace contaminant or microbe. State the conditions and avoid treating one approximate reading as a complete analytical certificate.
Distillation reduces interfering dissolved substances
Distilled water contains fewer chemical impurities than ordinary tap water in the required comparison. Tap water may contain dissolved mineral compounds or treatment substances that can react with a reagent, contribute ions or change a measurement. Distilled water reduces those unwanted contributions when preparing solutions, rinsing suitable apparatus or performing tests.
The required reason is fewer chemical impurities, not that tap water contains no water or must always be unsuitable for drinking. In an experiment intended to test a sample's chloride ions, adding water that itself contains chloride can contaminate the result. Use the appropriate laboratory water for the method, and distinguish sample composition from substances accidentally introduced by preparation.
Sources for this lesson
Pause and explain
A sample boils below 100°C at a lower pressure. What should be checked first?
Worked example
A boiling sample reads 100°C at standard atmospheric pressure. What purity conclusion is reasonable, and why might distilled water be used to prepare a chemical test?
Show the worked solution
- The reading agrees with the school pure-water boiling reference under the stated pressure.
- It supports a comparison but does not alone exclude every trace impurity or microbial risk.
- Distilled water has fewer interfering chemical impurities than tap water in practical chemistry.
Answer The boiling reading supports the reference comparison; distilled water reduces chemical interference, without either observation certifying drinking safety.
Try a topic test
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how melting and boiling behaviour can provide school-level evidence about water purity.
Give me a hint
Include the reference values and pressure rather than one isolated number.
Compare my reasoning
- Use about 0°C for pure-water melting and 100°C for boiling at standard atmospheric pressure.
- Compare the sample's measured behaviour using appropriate pressure and apparatus conditions.
- A changed value or broadened behaviour can suggest impurities, but the comparison is not a complete trace-contaminant or microbial analysis.
Compare melting near 0°C and boiling near 100°C at standard atmospheric pressure, retaining measurement limits.
Look for these in your work
- Both state-change comparisons are stated.
- Reference conditions are explicit.
A chloride-ion experiment uses tap water known in the supplied scenario to contain chloride. Explain the unwanted contribution and why distilled water is preferred.
Give me a hint
Separate the ions introduced during preparation from ions in the intended sample.
Compare my reasoning
- The preparation water can itself contribute chloride ions.
- A resulting observation may partly reflect that added impurity rather than only the sample.
- Distilled water's fewer chemical impurities reduce that interference.
Tap water adds possible interfering chloride; distilled water reduces unwanted chemical contributions.
Look for these in your work
- The contamination source is identified.
- The required distilled-water reason is clear.
A sample is described as pure because it gave a water colour test and boiled at 98°C without a recorded pressure. Evaluate what the evidence establishes and what is missing.
Give me a hint
Detection, pressure and a purity comparison answer different questions.
Compare my reasoning
- The colour test supports water present.
- The boiling temperature needs pressure and suitable measurement information before comparison with a reference.
- Neither fact alone proves full purity; use controlled property comparisons and an appropriate assessment for any further claim.
Water presence is supported; missing pressure and measurement conditions prevent the proposed purity conclusion.
Look for these in your work
- The positive test is not discarded.
- The unsupported purity claim is narrowed with a specific missing condition.
Common mistakes
- Ignoring pressure when comparing a boiling point with 100°C.
- Calling a positive water colour test a purity measurement.
- Explaining distilled water only as being warmer or more drinkable.
- Confusing dissolved impurities in preparation water with the intended sample.
What can you explain now?
Why is distilled water used rather than tap water in the stated practical-chemistry comparison?
Compare with the explanation
It contains fewer chemical impurities that could interfere with the experiment.
Unwanted ions or substances can affect reactions and observations.
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Make it stick
State the pure-water reference temperatures with pressure, then explain distilled water by the effect of unwanted chemical impurities on an experiment.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.