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Reversible and irreversible changes - Science Year 5

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Introduction

Time-lapse animation of ice cubes gradually becoming liquid water in a glass. — Ice melting in a time-lapse sequence. The Commons record describes about 50 minutes of real change, compressed into a short repeating animation; its screen speed is not a real-time measurement. Moussa, public domain.

Ice can melt and its water can freeze again. Burned paper cannot be turned back into the original sheet simply by cooling. What is the useful difference, and how much can an observation really prove?

You will classify six changes with reasons, distinguish a physical change from a chemical change that produces new substances, and explain why bubbles or a colour change are clues rather than automatic proof. The examples include melting, freezing, dissolving, burning, rusting and vinegar reacting with bicarbonate of soda.

Recall: Dissolved salt can be recovered from its water solution by evaporation. The salt was not destroyed when it became invisible. Separating a mixture can recover components without making new substances.

Your final evidence is a reasoned classification table, a labelled model and an evaluation of an explicitly fictional observation record. Use the images, original animations and supplied examples; no pupil burning, hot cooking or chemical-mixing experiment is required.

Physical changes and recovery routes

Ask what changed and what can be recovered.

A physical change alters features such as state, shape or arrangement without changing the material into new chemical substances. In a reversible example, a suitable process can recover the original material. We do not mean that every detail of the object's previous shape must return automatically.

Given change	A possible recovery route	Reasoned classification
Ice becomes liquid water.	Cool the water sufficiently so that it freezes.	Reversible physical state change: the material remains water.
Liquid water freezes.	Warm the ice sufficiently so it melts.	Reversible physical state change: solid and liquid are forms of water.
A small amount of salt dissolves in water.	Evaporate water to retain salt; an open dish does not collect the water.	Physical mixing process: salt remains present and can be recovered in the stated example.

The processes need not happen at the same rate. Ice may melt over minutes in one setting and refreeze slowly in another. The introductory animation compresses a long observation into a few screen seconds and repeats it. Its return to the first frame is the animation looping, not evidence that the water spontaneously refroze in that glass.

Worked explanation: “The melted water has a different shape, so it must be a new substance.” Shape and state have changed, but the substance is still water. Freezing demonstrates a recovery route. A new shape is not enough to establish chemical change.

A model: Draw water particles in a solid arrangement and then in a liquid arrangement. Retain the same kind and number of symbols. The particles are not becoming a new type merely because they move differently. A drawing is a model of ideas too small to observe individually here, not a real photograph of particles.

Short film, then think critically: The original ABPI clip compares melting/freezing with examples that cannot simply be undone in an everyday setting. It includes cooking, mixing flour with milk and burning. Use its water example to explain recovery, then ask a more precise question: does difficulty separating a mixture by itself establish that new substances formed? No. “Hard to undo” and “chemical reaction” are different claims. The rest of this course tests the stronger claim with evidence.

You can complete the same check from the paragraphs and table without the film. None of its cooking or burning examples is a pupil practical instruction.

New substances: chemical changes and evidence

Chemical changes form new substances.

In a chemical reaction, starting substances change into different substances. Their atoms are rearranged; matter is not created from nothing or destroyed. At Year 5, focus on the difference between recovery of the same material and formation of new materials, rather than memorising complicated equations.

Given case	Relevant explanation	Why simple reversal is not the right model
Paper burns in air, producing ash and gases.	Burning forms new substances and transfers energy. Some material goes into the air.	Cooling the ash does not reconstruct the original sheet of paper.
Vinegar reacts with bicarbonate of soda.	The acid reacts with bicarbonate to form new substances, including carbon dioxide gas.	It is not merely bicarbonate dissolving unchanged; ordinary filtering or cooling does not restore the original reactants.
Iron rusts.	Iron reacts with oxygen in the presence of water, forming rust materials.	Drying the rusty object does not turn the rust back into the original iron.

These are chemical changes that are not simply reversed by the ordinary physical recovery methods in this lesson. “Irreversible” in this classroom context does not mean that no possible scientific or industrial process could ever transform any products again. It means the original materials are not recovered just by the simple reverse step, as with melting and freezing.

Bubbles: clue or proof? The known vinegar-and-bicarbonate reaction produces a new gas, carbon dioxide. But boiling water also bubbles, and air trapped in a material can escape into water. In those examples, bubbling alone does not establish a new substance. Use the known starting materials, the process and reliable supporting explanation.

The same caution applies to temperature and colour. A colour change can result from mixing coloured liquids. Melting and freezing involve energy transfer without creating a new substance. An observation can support an explanation without being a perfect one-step chemical test.

Watch an original scientific demonstration: The American Chemical Society's teacher overview compares how bicarbonate-containing powders react with vinegar. For our course, identify the observation of gas and connect it to the reliable explanation that a new substance forms. We are not repeating the provider's comparative powder experiment or assigning its measured results to our own class.

Learning boundary: Use the provided cases and a teacher-selected demonstration if offered. Do not burn materials, heat unknown substances, mix household cleaners or seal a gas-producing reaction in a container. A paper model can show the full system without performing a reaction. An image or film does not certify practical skills.

Evaluate clues, models and system boundaries

Separate observation, explanation and uncertainty.

Use three columns: What was observed? What explanation is supported? What is not established? For the ice animation, the visible observation is shrinking ice and increasing liquid. The supplied context identifies melting water. The compressed playback does not tell us an accurate classroom melting time or prove a freezing stage occurred.

A fictional mass record: A teacher's supplied record describes an open vessel containing a known vinegar-and-bicarbonate reaction. Vessel plus contents has mass 108.0 g before the reaction and 107.4 g after gas has escaped. The recorded difference is $108.0 - 107.4 = 0.6$ g. These numbers are invented for reasoning, not a practical report.

The gas left what was being weighed. The lower reading does not establish destruction of matter. A model of the full system would include material in the vessel and gas that moved into the surroundings. The mass difference alone also does not identify the gas: the carbon-dioxide identification comes from the supported explanation of the stated reaction. A real investigation would also check spills and measurement uncertainty. Do not seal a real reaction vessel to imitate a closed-system diagram.

Six-case classification challenge: Explain melting ice, freezing water, dissolving salt, burning paper, the vinegar/bicarbonate reaction and rusting iron. For each, identify a possible recovery route or explain the formation of new substances. The first three are physical examples with stated recovery routes; the latter three are chemical examples not simply reversed by our ordinary recovery methods.

A harder case: torn paper.

Tearing a sheet is hard to undo perfectly. That alone does not establish new chemical substances. You have changed its size and shape. “I cannot put it back neatly” is not the same evidence as “burning produced ash and gases”. Likewise, the difficulty of separating flour and milk does not by itself demonstrate a chemical reaction. Ask what the stronger claim would need before accepting it.

Worked correction: “There are bubbles, so all the water has turned into a new gas substance.” If the stated process is boiling pure water, the bubbles are water vapour; that is a physical state change. If the stated known process is vinegar reacting with bicarbonate, carbon dioxide is a new product. Similar-looking bubbles can arise through different processes.

Your final evidence card: Add the six-case classification table, a water-particle sketch and the checked fictional mass difference. Explain one misleading clue and one limitation. Have a peer or teacher review the reasoning if available, recording feedback only if it happened. A clear answer states the material, process, evidence and boundary; it does not rely on “looks different” alone.

Interactive Tasks

Quiz: Test Your Knowledge

Why is melting ice a reversible physical change in this lesson? (The material remains water and can freeze again) (!The water particles become a new chemical substance) (!Cooling the melt must always recreate the exact original cube shape) (!Any change that looks different must be chemical)

Which route can recover salt from the stated salt-and-water solution? (Evaporate water to retain the salt) (!Use an ordinary paper filter to trap all dissolved salt) (!Cool the solution and assume all salt has become pure water) (!Wait for salt particles to cease to exist)

What distinguishes a chemical change in this course? (Formation of new substances) (!Any change in an object's shape) (!Any process that is difficult to undo neatly) (!The presence of a liquid in the starting materials)

Why are bubbles alone not a complete chemical test? (Boiling water and escaping trapped air can also cause bubbles) (!All bubbles must contain carbon dioxide) (!Bubbling always proves that atoms were destroyed) (!A physical state change cannot produce a gas)

Which explanation fits the known vinegar-and-bicarbonate reaction? (New substances form, including carbon dioxide gas) (!Bicarbonate only changes into liquid bicarbonate by melting) (!Water is the only possible gas in every bubbling mixture) (!Ordinary paper filtration restores both starting reactants)

In the fictional open-vessel record, mass changes from 108.0 g to 107.4 g. What is the difference? (0.6 g) (!1.4 g) (!107.4 g) (!215.4 g)

What does the lower open-vessel reading establish about matter? (Material can leave the weighed system as gas without being destroyed) (!Exactly 0.6 g of atoms has ceased to exist) (!The gas identity is proven by the mass difference alone) (!Every chemical reaction must reduce the total mass of the full system)

What does the introductory animation's return to its first frame establish? (The animation loops rather than documenting a new freezing stage) (!The glass refreezes all water every few seconds in real time) (!Every ice cube melts in four seconds) (!The source contains a continuous real-time freezing experiment)

Why is torn paper a useful challenge case? (Difficulty restoring the sheet does not itself establish new substances) (!Tearing necessarily produces the same gases as burning) (!Every small paper piece is a new chemical substance) (!Any physical change must be easy to undo perfectly)

Which statement about rusting is accurate in the stated example? (New rust materials form and drying does not restore the original iron) (!Rust is merely liquid iron that will freeze back when cooled) (!A rough appearance identifies the exact cause of every patch in a photo) (!An ordinary paper filter turns rust back into iron)

Memory Game

Melting	Solid becomes liquid
Freezing	Liquid becomes solid
Physical change	Material changes state, shape or arrangement without forming new chemical substances
Chemical change	Starting substances become different substances
Observation	What is actually seen or measured
Inference	An explanation drawn from evidence

Drag and Drop

Ice becomes water	A physical state change with a freezing recovery route
Salt dissolves in water	A physical mixture with salt recoverable by evaporation
Paper burns	New ash and gases form
Vinegar reacts with bicarbonate	New products include carbon dioxide
Iron rusts	New rust materials form in the stated conditions

Crossword Puzzle

MELTING	State change from solid to liquid
FREEZING	State change from liquid to solid
PHYSICAL	Kind of change that does not form new chemical substances
CHEMICAL	Kind of change that forms new substances
BICARBONATE	The soda ingredient reacting with vinegar in the given example
EVIDENCE	Information used to support a conclusion

LearningApps

Optional extra practice. Choose an appropriate activity with your teacher. If the external collection is unavailable, use the native activities and independent tasks in this course.

Cloze Text

Open Tasks

Easy

Explain two water state changes: Explain melting and freezing with water examples and a reverse arrow. Check: the material remains water; the original cube's exact shape need not return.
Describe what the ice animation shows: Write an observation and distinguish the accelerated sequence from a real-time measurement. Check: shrinking ice and increasing liquid; repeating the file is not evidence that the glass refroze.
Recall a salt recovery route: Name solute and solvent in salt water and explain recovery by evaporation. Check: salt is solute, water solvent; an open dish retains salt but does not collect water as liquid.
Identify a claim that needs more evidence: Correct: This paper is hard to put back together, so tearing made a new chemical substance. Check: difficult reversal alone does not establish chemical change.

Standard

Classify six changes with reasons: Classify melting ice, freezing water, dissolving salt, burning paper, vinegar reacting with bicarbonate and rusting iron. Give a recovery route or new-substance explanation for each. Check: first three physical; latter three chemical in the stated examples.
Compare bubbling examples: Compare boiling water with the known vinegar-and-bicarbonate reaction. Explain why the gases have different origins and why the appearance of bubbles alone is insufficient. No practical reaction is required.
Calculate and explain an open-system difference: Use 108.0 g before and 107.4 g after gas escaped in the fictional record. Check: 0.6 g difference. Explain where matter can go and why the number alone does not identify the gas or prove destruction.
Draw a physical-change particle model: Use the same type and count of water symbols to represent solid and liquid arrangements. Add a key and one limitation. Check: arrangement changes; symbols do not become a different chemical substance.

Advanced

Review a short video's examples: Use the ABPI water and flour/milk examples to distinguish everyday difficulty of reversal from evidence of new substances. Acknowledge what the clip illustrates without treating a brief label as a complete chemical test.
Challenge a one-clue classification: A fictional note reports only bubbles, a colour change or cooling. Explain why each can have more than one cause and what contextual information would improve the conclusion. Do not invent a completed identification test.
Explain a full-system model: Create a paper diagram including the open reaction vessel and gas that moved into the surroundings. Explain why a lower vessel reading does not mean lower mass of all matter. Do not build or seal an actual reaction vessel.
Create a reasoned evidence card: Combine the six-case table, labelled model, checked fictional mass difference and a limitation. Identify a genuine revision after reviewing a misconception. Mark supplied observations and actual feedback honestly; no pupil burning or heating is required.

Learning Check

Reversible physical examples: Explain melting, freezing and salt dissolving, with appropriate recovery routes.
Chemical examples and evidence: Explain burning paper, rusting iron and the known vinegar-and-bicarbonate reaction.
Evaluate misleading clues: Why are bubbles and difficulty of reversal not complete chemical tests?
Interpret a fictional mass record: Calculate the difference between 108.0 g and 107.4 g and evaluate a destruction claim.
Animation and honest evidence: Explain one limitation of the introductory animation and one boundary on practical claims.

Worked checks and assessment guidance

Melting ice and freezing water change state without changing water into a new chemical substance; cooling or warming can reverse the state. Dissolved salt remains in a mixture and can be retained when water evaporates. These routes do not promise restoration of every original shape or collection of the water in an open dish.
Burning forms new substances including ash and gases. Iron reacts with oxygen in the presence of water to form rust materials. Vinegar's acid reacts with bicarbonate to form new substances including carbon dioxide. Ordinary cooling, drying or filtering does not restore these starting materials in the way that freezing reverses melting.
Boiling water and trapped air can give bubbles without a new substance forming. Tearing paper or difficulty separating a mixture does not by itself demonstrate new substances. Use the stated process and supported explanation, and distinguish an observation from an inference. A coloured photograph alone does not identify every material or its history.
The difference is 0.6 g. The stated open-system record says gas escaped beyond the vessel weighing boundary, so the lower reading does not establish destroyed matter. A full-system model includes the surroundings; the difference alone does not identify the gas or exclude spills and measurement uncertainty. No real sealed reaction is needed.
The source describes about 50 minutes compressed into a few screen seconds; playback speed and looping are not a real-time measurement or proof of spontaneous refreezing. A diagram, video and supplied fictional record show knowledge and reasoning, not a learner's performed practical or teacher-confirmed apparatus skill. Record actual feedback only if it happened.

Learning evidence

Keep the six-change classification with reasons, a labelled particle/system model and the checked fictional mass difference. Show accurate use of physical, chemical, reversible, observation and evidence. Explain at least one misleading clue and a justified limit. Mark supplied data as fictional and distinguish a generic diagram from an exact molecular model. A supported spoken explanation or labelled panels can show the same reasoning as extended writing. No learner burning, heating or mixing task is necessary, and an online result must not be recorded as an observed practical competence.

OER and Wikipedia

Department for Education: science programmes of study, Year 5 Properties and changes of materials. The six-case comparison, fictional mass record, critical questions and activities are original editorial teaching material.

American Chemical Society: Baking Soda vs Baking Powder supports the explanation of reaction products, including gas, and links the original demonstration below. The course uses the known reaction concept, not the provider's measured powder-comparison results. ACS: Conservation of Mass, teacher background supports the system-boundary explanation. ACS: Rust – the Crust that Makes Stuff Bust supports the role of iron, water and oxygen; its practical is not required here.

Melting animation: Melting icecubes, Moussa, public domain as recorded on Commons, used without modification. The source describes a roughly 50-minute time-lapse. The short file loops; no reverse freezing stage is inferred from that loop.

Rust photograph: Rust on iron, Laitr Keiows, CC BY-SA 3.0, used without modification. Its colour and texture do not establish the exact history of the object.

Short model video: Reversible and non reversible changes, Association of the British Pharmaceutical Industry, original 39-second clip. The publisher's page includes its transcript. The course explicitly distinguishes the clip's everyday difficulty-of-reversal examples from the stronger claim of new-substance formation.

Reaction video: Fifth Grade Lesson 3.1 Baking Soda vs Baking Powder, American Chemical Society, original teacher overview. US Grade 5 is the provider's classification, not a claimed equivalence to England Year 5. Both films are embedded through their original YouTube publishers without downloading or republishing; the text gives an independent study route. No pupil burning or reaction setup is needed.