Trace a sequence of physical state changes
Keep substance identity and a closed-system mass account through several state changes.
Section P2.1.1 · States of matter
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
- Name melting, freezing, vaporisation and condensation.
- Distinguish an entire closed system from one component inside it.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In a closed system, which mass includes both liquid water and water vapour?
What you will learn
- Name consecutive state changes without reversing their direction.
- Separate a physical change of state from loss of material or a chemical reaction.
Follow the actual sequence
A sequence solid → liquid → gas → liquid contains melting, vaporisation and condensation. The gas in this water example is still water, not a conversion into air. A change of state changes the physical form; it does not by itself change water into a different chemical substance. The same material can return to a previous state when the appropriate conditions are changed.
Do not skip the stated intermediate states. If a described sample first freezes and then melts, the changes are liquid to solid followed by solid to liquid. A change of temperature within one state is not an additional state-change arrow. The diagram is a qualitative route, not a temperature scale or an energy budget.
Track the system boundary
In a closed system with no material entering or leaving, a physical change of state does not change total mass. If a closed container contains 25 g of water before some evaporates, the total water mass remains 25 g across liquid and vapour. The liquid portion's mass can decrease while the vapour portion's mass increases. Constant total mass does not imply constant liquid mass.
An open puddle can lose water mass because vapour moves into the surroundings. That is material leaving the chosen puddle, not destruction of matter during evaporation. These are written model accounts, not instructions to heat a sealed vessel. Actual heating and observation require suitable apparatus and teacher supervision; a sealed heating experiment is not proposed here.
Sources for this lesson
Pause and explain
In a closed model, 30 g of water contains 8 g vapour. What is the liquid mass?
Worked example
A closed model starts with 25 g of liquid water. Later 5 g is vapour. Find the liquid mass, then name the change if all vapour becomes liquid again.
Show the worked solution
- The closed model retains 25 g total water.
- Liquid mass is 25 − 5 = 20 g.
- Vapour returning to liquid undergoes condensation.
Answer 20 g liquid; condensation; 25 g total remains.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Name solid water → liquid water → water vapour → liquid water, and state whether state change alone makes a new chemical substance.
Give me a hint
Follow every directed step and retain the water identity.
Compare my reasoning
- Solid to liquid is melting.
- Liquid to gas is vaporisation; gas to liquid is condensation.
- These state changes alone do not make a different chemical substance.
Melting, vaporisation, condensation; it remains water.
Look for these in your work
- Name each direction.
- Preserve substance identity.
A closed written model has 40 g total water, including 12 g vapour. Find liquid mass and total mass after all vapour condenses.
Give me a hint
Keep a component account within the conserved total.
Compare my reasoning
- Initial liquid mass is 40 − 12 = 28 g.
- Condensation transfers the 12 g into the liquid component.
- The resulting liquid and total water masses are both 40 g.
Initially 28 g liquid; after condensation 40 g liquid and 40 g total.
Look for these in your work
- Avoid double-counting vapour.
- Use the closed-system condition.
A puddle's liquid mass decreases during drying. Another student says this contradicts the closed-system mass account. Resolve the apparent contradiction.
Give me a hint
Identify whether the chosen puddle allows matter to leave.
Compare my reasoning
- A puddle is an open subsystem.
- Water vapour can leave it and enter the surroundings.
- Mass conservation applies to an account that includes the transferred water, so the observations do not contradict it.
The puddle loses material to its surroundings; matter is not destroyed.
Look for these in your work
- State the boundary.
- Distinguish transfer from destruction.
Common mistakes
- Water vapour is water, not air.
- Total mass and liquid mass differ.
- A written closed model is not an instruction to heat a sealed vessel.
What can you explain now?
Does an open puddle losing water prove that matter is destroyed?
Compare with the explanation
No.
Water can leave the puddle as vapour and enter the surroundings.
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Make it stick
Trace several state changes, preserve material identity and explain the difference between a component and a closed-system total.
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.