Chemical Changes: Evidence & Conservation — Print · Grade 6 Science

Chapter: Matter, Mixtures & Water Properties

Chemical Changes: Evidence & Conservation

Learning goals

I can decide whether a change is chemical by comparing properties before and after.
I can explain why bubbles or a colour change are not proof on their own.
I can account for mass that appears to be gained or lost during a reaction.

Readings for this unit

The Clear Liquid and the Missing Salt

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

Burn 10.0 g of fine steel wool and weigh the ash. It comes out heavier, often around 13 g. Burn a 10.0 g candle and weigh what is left and it comes out lighter, often by almost the whole 10 g. The two results look like opposite laws of nature, and they are the same law seen from two sides.

In both cases the total mass of everything involved is unchanged. The steel wool combined with oxygen from the air, and oxygen has mass, so the solid that remains weighs more than the metal you started with. The candle also combined with oxygen, but its products, carbon dioxide and water vapour, floated away, so the balance lost track of them.

Antoine Lavoisier settled this in the 1770s by doing his reactions in sealed vessels. Nothing could enter or leave, and the mass before was always exactly the mass after. Mass is conserved; what varies is whether you remembered to weigh everything.

Build your understanding

Where this fits

The last lesson covered changes that leave a substance as it was. This one covers changes that produce something genuinely new, and the evidence that tells the two apart. Conservation of mass, introduced here, is the idea the whole of chemistry is built on and it comes back in chapter 7 when matter cycles through an ecosystem.

Use three flexible 40-45-minute sessions: the signs of chemical change and why they mislead; comparing properties before and after; conservation of mass in open and closed containers, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Signs, and why signs are not proof

The usual list of clues is worth knowing: a colour change, bubbles of gas, a temperature change you did not cause with a heater, light being given off, or a solid appearing when two clear liquids are mixed. Each of these does often accompany a chemical change. The trouble is that each also happens without one. Boiling water produces vigorous bubbles and is purely physical. Dissolving copper sulfate crystals turns clear water a deep blue and is purely physical, since evaporating the water returns the blue crystals unchanged. Ice melting absorbs a great deal of energy and cools its surroundings noticeably, and nothing new has been made at all.

So the clues are a reason to look more closely rather than a conclusion. The honest way to use them is as a prompt: something happened here that is worth investigating, and the investigation is a comparison of properties. A student who writes that a change was chemical because bubbles appeared has produced an observation and called it an inference, which is the error the first chapter of this course was about. The bubbles are real; what they mean still has to be established by other evidence. Some signs are more trustworthy than others, and it is worth ranking them. A solid appearing when two clear solutions are mixed is hard to explain physically, and so is a flame. A smell appearing where there was none before is also strong evidence. Colour change and bubbles sit at the weak end of the list, which is unfortunate because they are the two that students reach for first.

The balance tells the truth and the conclusion people draw from it does not. Nothing was destroyed — gas left the jar, which is why sealing it changes the reading and nothing else does.

Pause and explain

Why is the appearance of bubbles not enough to show that a chemical change has happened?

Check your reasoning after answering

Because bubbles also appear in purely physical changes. Water boiling produces vigorous bubbles of water vapour while remaining water throughout, and a dissolved gas can come out of solution when a liquid is warmed. Gas being released shows that something is happening, not that a new substance has been made, which has to be established by comparing properties.

Part 2 · The test that works

Compare what you have afterwards with what you had before, using the kind of properties from the last lesson. Iron is grey, strongly attracted to a magnet, hard, and has a density of 7.87 g/cm3. Rust is reddish-brown, crumbly, much less strongly magnetic and quite different in density. Those are not the same substance described in two moods; they are two different materials with different properties, so iron turning to rust is a chemical change. Compare that with iron being hammered into a thin sheet: still grey, still magnetic, still 7.87 g/cm3, so nothing new has been made.

The comparison works in every case you will meet. Sugar dissolved in water can be recovered unchanged by evaporating the water, so the sugar was always sugar. Sugar heated until it blackens cannot be recovered, and what is left is carbon, which does not taste sweet, does not dissolve and does not melt at the temperature sugar melts at, so a chemical change has occurred. Notice that this test asks about identity rather than about appearance or effort, which is why it gives the same answer no matter who performs it. One more pair worth having in mind, because it comes up constantly in cooking. Whisking egg white is physical: it traps air and changes the texture completely while the egg stays egg. Frying it is chemical: the clear liquid becomes an opaque white solid that will not return to liquid however it is cooled, because the proteins have been permanently rearranged into something new.

Pause and explain

How do the properties of iron and rust show that rusting is a chemical change?

Check your reasoning after answering

Because the two materials do not share properties. Iron is grey, hard, strongly magnetic and 7.87 g/cm3; rust is reddish-brown, crumbly, far less magnetic and of different density. A substance that has genuinely remained itself keeps its properties, as iron does when hammered flat, so a change in all of them means a different substance is now present.

Part 3 · Nothing is lost

Lavoisier's method was to do the reaction where nothing could escape. He sealed tin and air inside a glass vessel, weighed the whole thing, heated it until the tin turned to a grey powder, let it cool, and weighed it again: identical. Then he opened the vessel and heard air rush in, and the mass rose. The explanation is that the tin had combined with oxygen from the air inside, so the sealed total never changed, and opening the vessel let replacement air in which added mass. The experiment is clean because it removes the only way for the accounting to go wrong.

The steel wool case is the same reaction without the seal. Iron combines with oxygen from the surrounding air, and since the oxygen joins the solid rather than floating away, 10.0 g of steel wool becomes about 13 g of iron oxide. Students almost always predict a loss, because burning feels like destruction and ash looks like a remnant. Weighing it settles the matter, and it is one of the most useful surprises in the course: a belief that felt obvious is overturned in two minutes by a balance. The practical consequences are large. Rusting consumes an estimated several per cent of the world's annual steel production, and the whole business of painting, galvanising and oiling metal exists to keep oxygen and water away from iron. Knowing that rust is iron combined with oxygen rather than iron wearing away is what tells you that a barrier, rather than a repair, is the thing to reach for.

Pause and explain

Why does burning steel wool make it heavier when burning a candle appears to make it lighter?

Check your reasoning after answering

Because of where the products go. In both reactions the substance combines with oxygen, which has mass. The iron oxide stays on the balance, so 10.0 g of steel wool becomes about 13 g. The candle's products, carbon dioxide and water vapour, leave as gases, so the balance no longer holds them and records a loss that has not actually occurred.

Part 4 · Weighing everything

Conservation of mass is a statement about a closed system, which is the idea introduced at the end of chapter 1. If nothing enters or leaves the boundary you have drawn, the mass inside is constant through any chemical change, and this holds no matter how violent the reaction looks. The reason it is hard to believe at first is that almost every reaction you see in ordinary life is open to the air, so gases arrive or depart unweighed and the books appear not to balance.

You can demonstrate it directly. Put vinegar in a flask and a measured amount of baking soda in a small tube inside it, stopper the flask, and weigh the whole assembly, say 150.0 g. Tip the tube so the two mix; the mixture fizzes vigorously and carbon dioxide fills the flask. Weigh again with the stopper still in and the reading is still 150.0 g. Now take the stopper out, let the gas escape, and weigh a third time: the mass has fallen, perhaps to 148.5 g, and that 1.5 g is the carbon dioxide that walked out of your system. It is worth noticing how small that number is compared with the drama of the reaction. The flask fizzes energetically, foam rises, and the whole visible performance accounts for 1.5 g out of 150.0 g, about one per cent of the mass present. Reactions look far more destructive than they are, and the balance is a better guide to what happened than your eyes.

Pause and explain

A stoppered flask holds the same mass during a fizzing reaction but loses 1.5 g once opened. Explain both readings.

Check your reasoning after answering

While the stopper is in, the flask is a closed system: the carbon dioxide produced is still inside the boundary, so the total is unchanged even though a reaction has occurred. Removing the stopper lets that gas leave the system, and the 1.5 g drop measures its mass. No matter was destroyed at any point; it simply crossed a boundary.

Apply the whole unit · Balance the books

A student heats 24.0 g of magnesium ribbon in air. The white powder left behind has a mass of 40.0 g. The student concludes that the balance must be faulty because matter cannot be created.

Your task

Explain the result, state where the extra mass came from, and say what the student should have done.

What your response should show

Calculates the 16.0 g increase and identifies oxygen as its source
Explains that the rule covers all reactants, not only what sat on the balance
Rejects the faulty-balance conclusion using the chemistry rather than by assertion
Proposes a sealed system and says why that makes the conservation measurable

Response guidance — after your attempt

Calculate the increase in mass and name the substance that supplied it.
Explain why the result does not break conservation of mass.
Describe an experimental change that would make the conservation visible on the balance.

The mass rose by 40.0 minus 24.0, which is 16.0 g. That mass came from oxygen in the air, which combined with the magnesium to make magnesium oxide; the product contains every atom of the original magnesium plus the oxygen that joined it. Conservation of mass is not broken, because the rule applies to everything involved and the student weighed only what was sitting on the balance. The oxygen was drawn from the surrounding air and was never weighed before the reaction, so the accounting was incomplete rather than wrong. To make conservation visible, the student should carry out the reaction in a sealed container holding a fixed amount of air and weigh the whole container before and after: those two readings would match, and the apparent gain would disappear, because now the boundary of the system includes the oxygen that does the reacting.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

A chemical change makes a substance that was not there before, and the way to tell is to compare properties afterwards rather than to watch for bubbles or colour. However dramatic the change looks, the total mass of everything involved is exactly the same before and after.

Worked examples

Example 1 · A closed-system atom account

Explain new combinations and conservation, then distinguish this evidence from color alone.

Count A and B before and after.
Identify the changed particle combinations.
Use the stated closed boundary and mass record.

Answer: Two A atoms and two B atoms are represented on each side. Their combinations change from A₂ and B₂ to two AB particles. The closed-system mass remains 60 g. This model directly states a rearrangement; a color change alone would not supply the same evidence of substance identity.

Example 2 · Transfer to a different case

An exposed metal object gains mass during a stated reaction with oxygen. Does conservation fail?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. Oxygen from the surroundings enters the material. Account for both the metal and the added oxygen rather than treating the exposed object as a closed system.

Example 3 · Evaluate signs with an alternative explanation · a focused example

Do bubbles alone prove a new substance?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: No. A physical state change can also create bubbles; the gas identity and process need further evidence.

Avoid these mistakes

Misleading claim: Any bubbles or color change conclusively prove a chemical reaction.
Misleading claim: A mass change inside an open container means matter was destroyed.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

What does the particle model conserve?

Discuss the answer

The number of each kind of atom. New combinations form, but the represented atoms remain accounted for.

Do bubbles alone prove a new substance?

Discuss the answer

No. A physical state change can also create bubbles; the gas identity and process need further evidence.

Was 4 g of matter destroyed?

Discuss the answer

No. It crossed the chosen boundary as gas. The mass inside differs from the mass of the complete accounted system.

Why does a supplied record still need qualification?

Discuss the answer

The conclusion depends on the record’s stated method and boundaries; illustrative data are not proof that a real procedure was performed.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Chemical Changes: Evidence & Conservation · Model and explain

Assigned reading and preparation

Read The Clear Liquid and the Missing Salt and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Chemical Changes: Evidence & Conservation, study Parts 3–4 and attempt the worked case: Explain new combinations and conservation, then distinguish this evidence from color alone. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

Two A atoms and two B atoms are represented on each side. Their combinations change from A₂ and B₂ to two AB particles. The closed-system mass remains 60 g. This model directly states a rearrangement; a color change alone would not supply the same evidence of substance identity.

Activity 2 · Chemical Changes: Evidence & Conservation · Practice, revise and exit

Assigned reading and preparation

Read The Clear Liquid and the Missing Salt and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Chemical Changes: Evidence & Conservation, complete practice questions 1–6 and both choice checks, then answer independently: An exposed metal object gains mass during a stated reaction with oxygen. Does conservation fail? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. Oxygen from the surroundings enters the material. Account for both the metal and the added oxygen rather than treating the exposed object as a closed system.

Words to know

chemical change: A change that forms substances with different chemical identities.
conservation of mass: Matter remains accounted for when the complete relevant system and exchanges are included.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Every time you cook you are running chemical reactions: the browning of toast and the rising of bread come from new substances forming out of the reactants in the dough.

Exit check

An exposed metal object gains mass during a stated reaction with oxygen. Does conservation fail?

Check your explanation after answering

No. Oxygen from the surroundings enters the material. Account for both the metal and the added oxygen rather than treating the exposed object as a closed system.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

What does the particle model conserve?

Answer and explanation

The number of each kind of atom. New combinations form, but the represented atoms remain accounted for.