BananaLearnLearning LibraryGamesClassify melting an ice cube.
- The starting substance and liquid are both water.
- Cooling below its freezing point can restore solid ice.
- It is reversible under suitable cooling conditions, although the new ice may have a different shape.
State the change and the conditions
Melting ice can be reversed by cooling the liquid water until it freezes. Cutting a paper snowflake cannot be reversed simply by cooling or warming it. The question is whether the original material or useful starting form can be recovered under the stated conditions.
Reversible does not mean effortless or instantaneous. Water vapour can condense into liquid, but collecting all vapour from an open boiling pan is difficult. A classroom classification should name what is changed and what counts as recovering the starting state.
| Change | Possible recovery | What to notice |
|---|---|---|
| Ice melts | Freeze the water | Same substance changes state. |
| Salt dissolves in water | Evaporate water to recover salt | Recovering water too requires collection and condensation. |
| Paper is cut | Pieces cannot simply rejoin | No new substance is required for a practically irreversible change. |
| Egg is cooked | Ordinary cooling does not restore a raw egg | Cooking changes the material. |
Dissolving is different from disappearing
When salt dissolves, it is dispersed in the water and no longer visible as separate grains. It has not vanished. Evaporating the water can leave salt behind. Filtering a salt solution through ordinary filter paper does not remove the dissolved salt.
Sand mixed with water is different: the grains remain separate and can often be separated by settling or filtration. Choose the separation method from the material’s behaviour, not from the fact that both mixtures contain water.
Reversibility and chemical change are different questions
Many changes of state are reversible physical changes. Some physical changes, such as breaking a glass, are difficult to reverse in an ordinary classroom. Therefore “irreversible” is not a complete definition of a chemical change.
Evidence such as a new substance, gas production, lasting colour change or heat change can help investigate chemical change, but one sign alone may have another explanation. Bubbles in boiling water are water vapour; bubbles do not automatically prove a chemical reaction.
Record an observation before writing a conclusion
Use a simple record: starting material; action; observation; proposed way back; result. For an ice cube, write “solid water became liquid after warming”, rather than only “reversible”. Then explain that cooling can recover solid water.
Choose safe adult-led demonstrations. Do not taste unknown mixtures, burn materials or heat a closed container. An observation task with ice, a stretched elastic band within its limit, and folded paper can teach classification without flames.
Worked examples: follow each decision
Is tearing paper automatically a chemical change?
- Tearing changes size and shape; it does not by itself make a new substance.
- The original sheet cannot readily be recovered by simply reversing the action.
- This shows why “irreversible” does not automatically mean “chemical.”
A pupil says every irreversible change is chemical. Use a counterexample.
- Cutting paper changes its size and shape.
- The pieces do not simply rejoin, yet the material remains paper.
- Practical reversibility and formation of a new substance must be considered separately.
10 questions with explained answers
Attempt a short group of questions before opening the solutions. Use a drawing or written method where helpful. Explain the answer, then try a similar question with a detail changed.
Water freezes and later melts. Reversible?
Show answer and reasoning
Yes
Changing temperature can reverse the state change.
A nail rusts. Is cooling it enough to recover the original nail?
Show answer and reasoning
No
Rusting forms different substances.
Can melted ice be recovered as ice?
Show answer and reasoning
Yes, by freezing
The water changes state.
Will ordinary filtering remove dissolved salt?
Show answer and reasoning
No
Dissolved salt passes through ordinary filter paper with the water.
Does cutting paper necessarily create a new substance?
Show answer and reasoning
No
A difficult-to-reverse change can still be physical.
Do bubbles always prove a chemical reaction?
Show answer and reasoning
No
Boiling water produces vapour bubbles.
How could salt be recovered from salt water?
Show answer and reasoning
Evaporate the water
Collect and condense vapour as well if recovering water is required.
What is missing from “the change is reversible”?
Show answer and reasoning
The method and conditions for reversing it
Name how the starting material or form is recovered.
Compare sand in water with salt in water.
Show answer and reasoning
Sand remains as separate grains; salt dissolves.
Their different behaviour suggests different separation methods.
Why record observations before classifying?
Show answer and reasoning
To support the classification with evidence
A label alone does not show what happened.
Check what you can now explain
- Can you state a recovery method?
- Have you distinguished dissolving from vanishing?
- Does the evidence support your classification?
Keep these distinctions clear
- Do not define reversible as “easy to do”; it is about recovering the starting state/material.
- Avoid treating all physical changes as practically reversible.
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The examples teach the stated concept; check your school’s terminology and sequence when using the guide alongside a course.