State water's melting and boiling reference temperatures
Use 0°C and 100°C with their pure-water and standard-atmospheric-pressure conditions.
Core and Extended; Physics 0625, 2026–2028, section 2.2.3. Explain energy input during melting/boiling, water's standard-pressure temperatures, condensation/solidification, surface evaporation and liquid cooling. Detailed boiling/evaporation comparisons, rate factors and cooling a contacted object belong to the labelled Supplement lessons. Written practice is self-checked
Before you begin
- Distinguish melting from boiling and evaporation.
- Read Celsius temperatures and a heating curve.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are the school reference values for pure water at standard atmospheric pressure?
What you will learn
- Know water's melting and boiling temperatures at standard atmospheric pressure.
- Recognise pressure, composition and process conditions before interpreting a reading.
Remember values with conditions
For pure water at standard atmospheric pressure, the school reference melting temperature is 0°C and the boiling temperature is 100°C. The melting value refers to the solid/liquid transition, while the boiling value refers to liquid/gas boiling. Say the pressure condition with the values; a different pressure can change the boiling temperature.
The 0°C value is not absolute zero. Liquid water can exist between the reference transitions, and it can evaporate below 100°C. Knowing that a sample is at 20°C does not exclude surface evaporation. A temperature label alone also does not show that all the sample has finished a transition or is a single phase.
Observations need more than one number
A mixture or dissolved substances can change transition behaviour, so the pure-water values are not a universal rule for every liquid labelled water-containing. For a supplied pure-water standard-pressure model, apply the listed values; for an unspecified situation, avoid certifying an exact transition from one reading without composition and pressure information.
Vapour is gaseous water and is not itself a visible white cloud. Visible mist consists of small liquid droplets formed when vapour condenses. Bubbles observed before sustained boiling can also include dissolved gas, so 'any bubble proves boiling at exactly 100°C' is too strong. The question must provide enough process and pressure evidence.
Pause and explain
Does 100°C have to be the boiling temperature for water at every pressure?
Worked example
State the school reference melting and boiling temperatures of pure water at standard atmospheric pressure.
Show the worked solution
- Identify the solid-to-liquid reference.
- Identify the liquid-to-gas boiling reference.
- Retain the specified purity and pressure conditions.
Answer Melting: 0°C; boiling: 100°C, at standard atmospheric pressure.
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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.
Give water's reference melting and boiling temperatures and the required pressure condition.
Give me a hint
Attach the pressure condition to both named transitions.
Compare my reasoning
- For the school pure-water reference, melting is at 0°C.
- Boiling is at 100°C.
- These values use standard atmospheric pressure.
0°C melting and 100°C boiling at standard atmospheric pressure.
Look for these in your work
- Name both processes.
- Retain the conditions.
A bowl of pure water is at 20°C and standard atmospheric pressure. Must surface evaporation be absent because it is below 100°C?
Give me a hint
Distinguish evaporation from reaching the boiling point.
Compare my reasoning
- 20°C is below the boiling reference.
- Surface particles can still escape from the liquid.
- The reading does not rule out evaporation.
No; evaporation can occur at 20°C.
Look for these in your work
- Use the correct process.
- Avoid treating boiling temperature as an evaporation threshold.
A sample contains dissolved material and is at an unspecified pressure. Can its exact boiling temperature be certified as 100°C from the label 'water' alone?
Give me a hint
The reference requires composition and pressure conditions.
Compare my reasoning
- 100°C is the pure-water standard-pressure reference.
- Composition and pressure in this case do not match confirmed conditions.
- An exact boiling temperature needs further evidence.
No; the necessary reference conditions are not established.
Look for these in your work
- State the missing conditions.
- Avoid an unsupported exact value.
Common mistakes
- Retain standard atmospheric pressure.
- 0°C is not absolute zero.
- Evaporation can occur below the boiling point.
What can you explain now?
Can water evaporate at 20°C?
Compare with the explanation
Yes.
Surface evaporation does not require the liquid to reach its boiling temperature.
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Make it stick
Recall the values with conditions and explain why surface evaporation and visible mist are different evidence.
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.