Physical or chemical? The signs of change and the mass that never disappears
By the end of this lesson you will be able to
- Distinguish a physical change from a chemical change, and describe the five kinds of evidence that a chemical change has taken place.
- Identify the reactants and products in a reaction and write it as a word equation.
- State the law of conservation of mass and explain it in terms of atoms being rearranged, not created or destroyed.
- Explain why the mass of an open flask can appear to change during a reaction, and calculate a percentage change in mass.
- Define the terms precipitate and catalyst, and design an investigation that uses a balance to test the conservation of mass.
The puzzle
A candle burns for an hour and there is almost nothing left of it: it has, apparently, vanished. A pad of steel wool held in the same flame glows, crackles and ends up heavier than it started. Neither result makes sense if burning simply destroys things. For two thousand years the best minds in Europe believed that fire released a substance called phlogiston, and they could not explain the steel wool. Then a French tax collector bought the most accurate balance in the world.
Predict what a balance would show in each case: (1) an ice cube melting in a sealed bag; (2) a fizzy tablet dropped into water in an open beaker; (3) the same tablet dropped into water in a bottle with the lid screwed on; (4) a strip of magnesium burning in air, its ash collected. Say whether the reading goes up, down or stays the same, and why.
Compare your answer
(1) Stays the same: melting rearranges nothing, the same water molecules are just free to slide past one another. (2) Falls: the tablet makes carbon dioxide gas, which leaves the beaker and takes its mass with it. (3) Stays the same: the gas is still inside the bottle, pressing on the lid, and the balance weighs it. (4) Rises: the magnesium has combined with oxygen from the air, and the ash (magnesium oxide) contains atoms that were not on the pan before. In every case the mass of everything that took part is unchanged; the reading changes only when something invisible arrives or leaves. That is the whole lesson in four lines.
The science — structured notes
1.1 Two kinds of change
Matter changes in two fundamentally different ways. In a physical change the substance stays the same substance: ice melts to water, sugar dissolves in tea, a copper wire is bent, salt water is boiled dry. The particles move apart, mix or line up differently, but every molecule of water is still H₂O and every particle of sugar is still sugar. In a chemical change — a chemical reaction — the atoms are rearranged into new substances with different properties: iron rusts to a red flaky solid that is not iron; wood burns to gas and ash that could never be turned back into wood; milk sours; bread rises; a match head flares.
| Physical change | Chemical change | |
|---|---|---|
| New substance? | No — same particles, differently arranged | Yes — atoms rebonded into different molecules or lattices |
| Easily reversed? | Usually: freeze the water, evaporate the solvent, straighten the wire | Usually not: you cannot unburn toast or un-rust a nail by cooling it |
| Energy change | Small, and tied to state or mixing | Often large — heat, light, sound, electricity |
| Mass | Conserved | Conserved (but see 1.4 for the trap) |
| Examples | Melting, boiling, dissolving, crushing, magnetising, mixing sand and salt | Burning, rusting, cooking an egg, digestion, photosynthesis, fireworks, cement setting |
1.2 The five signs of a chemical change
You cannot see atoms rebond, so chemists use evidence. Five observations point to a reaction. None is proof on its own; two together usually are.
For each event, decide physical or chemical, and name the evidence that decides it.
(a) A silver spoon goes black in a drawer over months.
(b) Water vapour from a kettle condenses on a cold window.
(c) A drop of lemon juice on a pinch of bicarbonate of soda fizzes.
(d) Salt is stirred into a pot of boiling water and vanishes.
(e) A cold pack from a first-aid kit is squeezed and becomes icy.
Reveal answer
(a) Chemical: colour change to a new substance (silver sulfide) that polishing removes as a black powder — the silver has combined with sulfur compounds in the air.
(b) Physical: a change of state; the water on the glass is the same water that left the kettle.
(c) Chemical: a gas (carbon dioxide) is produced from a mixture that is not boiling; the bicarbonate is used up.
(d) Physical: dissolving; boil the water off and the salt is back, unchanged.
(e) Usually physical: most cold packs dissolve ammonium nitrate in water, which takes in energy without making a new substance. (Some packs use a genuine reaction — the question then hinges on whether a new substance forms, so a good answer says which.)
1.3 Reactants, products and the word equation
The substances you start with are the reactants; the new substances you finish with are the products. A word equation writes the change with an arrow that means “react to give”:
magnesium + oxygen → magnesium oxide
The arrow is not an equals sign and it is not reversible. The plus on the left means “reacts with”; the plus on the right means “and”. Every substance gets its proper name (magnesium oxide, not “the ash”; carbon dioxide, not “gas”). Lesson 6.2 turns these into symbol equations with formulas.
| Change you see | Word equation | Reactants | Products |
|---|---|---|---|
| A candle burns | wax + oxygen → carbon dioxide + water | wax, oxygen (from the air) | carbon dioxide, water (both gases at flame temperature — they leave) |
| Steel wool glows in a flame | iron + oxygen → iron oxide | iron, oxygen | iron oxide (a solid — it stays, so the mass rises) |
| Bicarbonate fizzes in acid | sodium hydrogencarbonate + hydrochloric acid → sodium chloride + water + carbon dioxide | sodium hydrogencarbonate, hydrochloric acid | sodium chloride, water, carbon dioxide |
| Limestone is roasted in a kiln | calcium carbonate → calcium oxide + carbon dioxide | calcium carbonate | calcium oxide (quicklime), carbon dioxide |
| A leaf in sunlight | carbon dioxide + water → glucose + oxygen | carbon dioxide, water | glucose, oxygen |
1.4 The law of conservation of mass
In 1789 Antoine Lavoisier stated the law that every chemist since has relied on: in a chemical reaction, the total mass of the products equals the total mass of the reactants. Mass is neither created nor destroyed. The reason is the atoms: a reaction breaks the bonds between atoms and makes new ones, but the atoms themselves are untouched. Count the atoms before and after and they match; each atom has a fixed mass; so the mass matches too.
The trap is the word total. A balance weighs only what is on its pan. If a product is a gas that floats away, or a reactant is a gas that arrives from the air, the reading changes while the law holds perfectly. Chemists describe the two situations as open and closed systems.
| System | What can cross the boundary | What the balance shows | Example |
|---|---|---|---|
| Closed | Energy only — no matter in or out | No change, ever | Tablet in a stoppered bottle; reaction in a flask with a balloon over the neck; a sealed glow-stick |
| Open, gas leaves | A gaseous product escapes | Reading falls by the mass of gas that left | Fizzing tablet in a beaker; candle burning; limestone in a kiln |
| Open, gas arrives | A gaseous reactant comes in from the air | Reading rises by the mass of gas that joined | Magnesium burning to magnesium oxide; iron rusting; steel wool in a flame |
A flask of dilute hydrochloric acid has a mass of 143.90 g. A student adds 8.40 g of sodium hydrogencarbonate and lets it fizz until the bubbling stops. The flask then reads 147.90 g. Find the mass lost, the percentage of the solid that left as gas, and show that the law of conservation of mass still holds.
Mass before the reaction: 143.90 + 8.40 = 152.30 g. Mass after: 147.90 g. Mass lost = 152.30 − 147.90 = 4.40 g.
Percentage of the solid that left: 4.408.40 × 100 = 52.4 %.
The 4.40 g is carbon dioxide gas that escaped through the open neck. Products = 147.90 g (still in the flask) + 4.40 g (in the room) = 152.30 g = reactants. Nothing was destroyed; the balance simply cannot weigh the air. Lesson 6.2 will show why the figure is 52.4 % and not some other number — it comes from the formula masses in the equation.
0.48 g of magnesium ribbon is burned in a crucible with a loose lid. The white ash has a mass of 0.80 g. Explain the gain and find the percentage increase.
Gain = 0.80 − 0.48 = 0.32 g. This is oxygen from the air that combined with the magnesium: magnesium + oxygen → magnesium oxide. The oxygen was not on the balance before; it is now, locked in the solid.
Percentage increase: 0.320.48 × 100 = 66.7 %. If the student had weighed the oxygen that came in, reactants (0.48 + 0.32) would equal products (0.80) exactly. The loose lid lets oxygen in while keeping the white smoke of magnesium oxide from escaping — a real gain slightly below 66.7 % usually means some ash was lost as smoke.
A student heats 10.0 g of powdered calcium carbonate strongly in an open crucible until no further change happens. The residue (calcium oxide) has a mass of 5.6 g.
(a) Write the word equation and state which product is missing from the crucible.
(b) Calculate the mass of gas released and the percentage of the original mass that it represents.
(c) Describe how the student could show, with the same reaction, that mass is conserved.
Reveal answer
(a) calcium carbonate → calcium oxide + carbon dioxide. The carbon dioxide is a gas and left the open crucible.
(b) Gas released = 10.0 − 5.6 = 4.4 g; percentage = 4.410.0 × 100 = 44 %. (The same 44 % appears for every carbonate that loses carbon dioxide from a formula mass of 100 — a number worth remembering.)
(c) Heat a sealed system — for example a small sample in a boiling tube fitted with a gas syringe or a deflated balloon, on the balance throughout — so the carbon dioxide is collected rather than lost. The reading will not change: 10.0 g before, 10.0 g after (5.6 g of solid + 4.4 g of gas in the balloon).
1.5 Two words you will need: precipitate and catalyst
A precipitate is an insoluble solid that forms when two solutions are mixed. Both solutions are clear; the moment they meet, the liquid turns cloudy or a coloured solid sinks. Silver nitrate solution added to tap water gives a white cloud of silver chloride — the standard test for chloride ions. The solid did not fall out of either solution; it was made when ions from one met ions from the other and formed a compound that water cannot hold.
A catalyst is a substance that speeds up a chemical reaction without being used up. Hydrogen peroxide solution decomposes to water and oxygen so slowly that a bottle lasts years; drop in a pinch of black manganese dioxide and it froths violently, and at the end you can filter the manganese dioxide out, dry it and weigh it — the same mass you put in. A catalyst is not a reactant (it is not in the equation, though it is often written over the arrow) and it is not a product. Living cells run on catalysts called enzymes; a car exhaust carries a catalytic converter of platinum; the fertiliser industry runs on iron (Lesson 6.5).
“A catalyst is used up slowly, which is why a catalytic converter eventually needs replacing.”
Reveal answer
False. By definition a catalyst is not used up: it takes part in the reaction and is regenerated, so it appears unchanged at the end. Converters fail because their platinum surface is poisoned — coated with lead, sulfur or soot so that the reacting gases can no longer reach it — or physically damaged by heat. The metal is still there; it has been blocked, not consumed. That is why scrap converters are valuable and the platinum is recovered.
1.6 Change and energy — the thread of this unit
Every change involves energy. A physical change needs only enough to loosen the forces between particles (melting, boiling, dissolving). A chemical change breaks bonds within particles and makes new ones, and because bonds store much more energy than the forces between particles, chemical changes move much more energy about: the flame of a candle, the cold of a chemical ice pack, the heat of setting cement. The statement of inquiry for this unit says it in one line: a change in matter is a consequence of energy differences between substances. Lesson 6.4 measures those differences; Lesson 6.5 shows what people build with them. First, in 6.2 and 6.3, you need to be able to write the change down.
…a student claims: “The mass of a match falls as it burns, so the law of conservation of mass is wrong.” Write the two sentences that answer this.
Reveal answer
The match head and wood react with oxygen from the air to form carbon dioxide and water vapour, which are gases and leave, so the balance loses the mass of those gases while the products still exist in the air. Weigh the match, the oxygen it uses and all the gases it makes inside a sealed container and the total is unchanged, because the atoms are only rearranged — the law describes the total mass of everything that takes part, not the reading for the solid you can see.
Try it — where does the fizz go?
A flask of excess hydrochloric acid stands on a balance. Add a weighed mass of sodium hydrogencarbonate, let the fizzing finish and read the mass lost. Run five masses of solid, three trials each, then seal the flask with a balloon and try again. Does the loss double when the mass doubles? Does the balloon change the loss to nothing? Does concentrated acid change the loss, or only the speed?
Suppose your open-flask results were: 2 g → 1.05 g lost, 4 g → 2.10, 6 g → 3.14, 8 g → 4.19, 10 g → 5.24 g; sealed, 6 g → 0.00 g; open with 4 mol/dm³ acid, 6 g → 3.13 g.
(a) Describe the relationship for the open flask, quoting values, and decide whether mass lost is proportional to mass of solid.
(b) Explain the sealed result and the concentrated-acid result using atoms.
(c) A student says the open flask “proves mass is not conserved”. Use the numbers to answer.
Reveal answer
(a) Mass lost increases with mass of solid, from 1.05 g at 2 g to 5.24 g at 10 g. Doubling the solid from 4 g to 8 g doubles the loss (2.10 to 4.19 g, within 0.01 g), and the line passes through the origin, so the relationship is directly proportional: the loss is 0.52 g for every gram of solid.
(b) Sealed: the carbon dioxide molecules are still inside the flask and balloon, so every atom that was on the pan is still on the pan — 0.00 g lost. Concentrated acid: the same number of solid particles react with the same result, so the same number of carbon dioxide molecules leave (3.13 versus 3.14 g, the same within the balance error); more acid particles per cm³ only means more collisions per second, so the loss happens faster, not more.
(c) The 3.14 g that “disappeared” is exactly the mass of carbon dioxide made from 6 g of solid, and the sealed run shows that when the gas is kept, nothing is lost. The open flask does not weigh the whole system; the closed one does, and it reads 0.00 g change. Mass is conserved — the balance was simply not weighing all the products.
Common mistakes to avoid
- “Bubbles mean a chemical change” — boiling bubbles are the same substance. Ask what gas it is and where it came from.
- “Dissolving is chemical because the solid disappears” — evaporate the water and it is back; no new substance.
- “Mass was lost in the reaction” — mass was lost from the flask; name the gas that carried it away.
- “The magnesium got heavier because of the heat” — heat has no mass; oxygen atoms from the air joined the solid.
- “The catalyst is used up” — it is recovered unchanged; it changes the rate, not the products.
- Percentage change with the wrong denominator — divide the change by the original mass of the substance that changed, not the mass of the flask.
How we found out — the balance that ended phlogiston
For most of the eighteenth century chemists explained burning with phlogiston, a substance supposedly released by anything that burned; ash was the wood with its phlogiston gone. The theory worked for candles and failed for metals, which gained mass when heated — so some argued that phlogiston had negative weight. In 1756 Mikhail Lomonosov in St Petersburg heated metals in sealed vessels and found no change in mass, but his result was published in Russian and ignored. In 1774 Joseph Priestley in England heated the red powder made from mercury and collected a gas in which a candle burned with a brilliant flame; he called it dephlogisticated air.
Antoine Lavoisier, a Paris tax official who spent his fortune on instruments, heard of the gas from Priestley over dinner and repeated the experiments with a balance accurate to a few milligrams. He sealed tin in a flask, weighed it, heated it until the metal was covered with white powder, and weighed the sealed flask again: no change. Then he opened it — air rushed in, and now the flask was heavier, by exactly the mass the tin had gained. The metal had taken something from the air, and it was the gas Priestley had found; Lavoisier named it oxygen. In 1789 his Traité élémentaire de chimie, illustrated by his wife and collaborator Marie-Anne Paulze Lavoisier, who translated the English papers and drew every piece of apparatus, stated that “nothing is created, nothing is destroyed” in a chemical change. The law was not a guess; it was what the balance said when the system was closed. Lavoisier was guillotined in 1794 during the Revolution. The balance outlived phlogiston, and chemistry became a science of weighing.
Explain why Lavoisier’s sealed-flask experiment could refute phlogiston when a hundred years of open-air burning had not, and what the rush of air into the opened flask showed.
Reveal answer
In the open air, mass could arrive from or leave to the atmosphere without being weighed, so any result could be explained by an invisible substance coming or going — phlogiston fitted whatever happened. Sealing the flask made the system closed: nothing could enter or leave, so if phlogiston left the tin the sealed flask should have lost mass. It did not, which showed that no substance had left. The rush of air on opening showed that the air inside the flask had been partly used up — its volume had fallen — and the mass gain on opening equalled the mass gained by the tin, so the tin had combined with a part of the air. A theory that predicted a loss met a measurement of zero, and the measurement won.
Where it is used now
- Soda ash at Sua Pan. Botswana Ash pumps brine from beneath the Makgadikgadi pans and turns it into sodium carbonate and salt. The plant is run on mass balances: every tonne of brine in is accounted for as product, waste or water out, because the law says it must be.
- Coal at Morupule. The power station burns well over a million tonnes of coal a year; conservation of mass says the carbon in it becomes about 3.7 tonnes of carbon dioxide per tonne of carbon, and that number is how the country’s emissions are calculated for its climate reports.
- Cement. A cement kiln roasts limestone, losing 44 % of its mass as carbon dioxide (the reaction in the Apply check above), which is why cement making is one of the largest industrial sources of the gas worldwide.
- Baking and brewing. Bread rises because yeast or baking powder makes carbon dioxide; a loaf weighs less than its dough. A bottle of ginger beer sealed too early can burst — a closed system with a gas product.
- Forensics and food safety. A silver nitrate precipitate test for chloride, a colour change on a test strip, a mass gain on a rusting sample: the five signs of chemical change are the working tools of every testing laboratory in Gaborone.
Science now — and the scientist you need to be
What is happening in 2026
Lavoisier’s law is now enforced by computers. Chemical plants run continuous mass balances — sensors on every pipe, reconciled every second, with an alarm if the atoms in do not match the atoms out, because a mismatch means a leak. Climate accounting works the same way at the scale of a planet: the carbon in every tonne of fuel sold must reappear as carbon dioxide somewhere, and satellites now measure it over cities and power stations to check the paper accounts. Machine-learning models trained on millions of published reactions predict what products will form and in what mass before anyone opens a bottle, and the first automated laboratories run those predictions overnight, weighing everything — but every one of those predictions is constrained by the rule that atoms are conserved, because a model that predicted mass appearing from nowhere would be discarded on day one. The oldest law in chemistry is the first line of the newest code.
The mindshift
What you need to be able to do
Three examinable skills from this lesson: classify a change and name the evidence (Criterion A, always with the new substance named); calculate a percentage change in mass with the right denominator and a unit-free answer; and plan a conservation-of-mass investigation for Criterion B in which the balance is the instrument, the system is stated as open or closed, and every control variable carries a value. Examiners report every session that “same equipment” is not a control variable and that a hypothesis without a “because” scores one mark at most.
See an example
Candle: 14.2 g before, 12.9 g after — 1.3 g of wax became carbon dioxide and water vapour (wax + oxygen → carbon dioxide + water), and the mass gained from the oxygen also left in the gases. Bottle: 523.6 g sealed, 519.8 g flat — 3.8 g of dissolved carbon dioxide escaped (a physical change: the gas was dissolved, not made). The AI’s first answer said the candle’s mass “was converted to heat”; when challenged it corrected itself and gave the equation with the gases named — the correction is the answer that earns the marks.
Key terms
| Physical change | A change in which no new substance forms: state, shape, dissolving, mixing. Particles rearrange but keep their identity. |
| Chemical change (reaction) | A change in which atoms are rebonded into one or more new substances with different properties. |
| Reactant | A substance that is used up in a reaction; written on the left of the arrow. |
| Product | A new substance formed by a reaction; written on the right of the arrow. |
| Word equation | reactants → products, using full chemical names, with the arrow meaning “react to give”. |
| Law of conservation of mass | The total mass of the products of a chemical reaction equals the total mass of the reactants, because atoms are neither created nor destroyed (Lavoisier, 1789). |
| Closed system | A system that no matter can enter or leave; its mass cannot change. |
| Open system | A system that matter can enter or leave, e.g. a flask open to the air; the balance reading changes if a gas leaves or arrives. |
| Combustion | Burning: a reaction with oxygen that releases energy as heat and light. |
| Precipitate | An insoluble solid formed when two solutions are mixed. |
| Catalyst | A substance that increases the rate of a reaction without being used up; recovered chemically unchanged at the end. |
Think like a scientist
Criterion B A class is given: sodium hydrogencarbonate (bicarbonate of soda), 2 mol/dm³ hydrochloric acid, conical flasks, balloons, measuring cylinders, a balance reading to 0.01 g, a spatula, weighing boats and a stopwatch. Design an investigation into how the mass of sodium hydrogencarbonate added to excess acid affects the mass lost from an open flask, and use it to test the law of conservation of mass.
(a) Write a research question. [2]
(b) Write a hypothesis with scientific reasoning and a testable prediction. [3]
(c) Identify the independent and dependent variables, each with its unit and the instrument used to measure it, and explain why “how much the mass changes” is not an acceptable statement of the dependent variable. [4]
(d) Identify three control variables. For each, state its value and how it is kept the same. [6]
(e) Write a numbered method another student could follow, including the values of the independent variable, repeats, what is calculated, and one safety point specific to this experiment. [8]
(f) Explain how you would extend the investigation with the balloons to show that mass is conserved. [2]
Model answer
(a) How does the mass of sodium hydrogencarbonate (2.00 to 10.00 g) added to 100 cm³ of excess 2 mol/dm³ hydrochloric acid affect the mass lost from an open conical flask (g) when the reaction is complete?
(b) If the mass of sodium hydrogencarbonate is increased, then the mass lost from the open flask will increase in direct proportion, because the reaction sodium hydrogencarbonate + hydrochloric acid → sodium chloride + water + carbon dioxide produces one carbon dioxide molecule for every unit of solid that reacts, and the carbon dioxide is a gas that escapes through the open neck; twice the solid gives twice the gas. Prediction: 8.00 g of solid will lose about twice the mass that 4.00 g loses, and a graph of mass lost against mass of solid will be a straight line through the origin.
(c) Independent variable: the mass of sodium hydrogencarbonate, in g, weighed in a weighing boat on the balance to 0.01 g. Dependent variable: the mass of the flask and contents, in g, read from the balance before the solid is added and again when the fizzing has stopped; the mass lost is calculated as the difference. “How much the mass changes” names no quantity, unit or instrument, and it is a calculated result, not the reading; the dependent variable must be the balance reading from which the change is worked out.
(d) Volume and concentration of acid: 100 cm³ of 2 mol/dm³ hydrochloric acid measured with a measuring cylinder for every run, so that the acid is always in excess and every gram of solid reacts. Temperature of the acid: room temperature, about 22 °C, checked with a thermometer, because warm acid holds less dissolved carbon dioxide and could change the loss slightly. Time allowed: the flask is read only when no bubbles have appeared for 30 s, timed with the stopwatch, so that every run is complete. (Also acceptable: the same flask, so that the neck size and the mass of gas trapped above the liquid are the same, and the flask is not swirled during the reaction.)
(e) 1. Measure 100 cm³ of 2 mol/dm³ hydrochloric acid with a measuring cylinder into a 250 cm³ conical flask and place it on the balance. 2. Weigh 2.00 g of sodium hydrogencarbonate in a weighing boat on the balance. 3. Record the mass of the flask plus acid plus boat of solid together (mass before). 4. Tip the solid into the flask, return the empty boat to the pan, and start the stopwatch. 5. When no bubbles have appeared for 30 s, record the balance reading (mass after). 6. Calculate mass lost = mass before − mass after. 7. Repeat steps 1–6 with 4.00, 6.00, 8.00 and 10.00 g of solid — five values of the independent variable. 8. Do three trials at each mass with fresh acid and calculate the mean mass lost, to 0.01 g, excluding any anomalous result. 9. Plot mean mass lost (y) against mass of solid (x) with a line of best fit. Safety: 2 mol/dm³ hydrochloric acid is an irritant and the fizzing can spray droplets, so wear eye protection, add the solid slowly, and keep the flask on the bench, not held in the hand.
(f) Repeat one mass (for example 6.00 g) with the solid placed inside a balloon that is then stretched over the neck of the flask; weigh the whole assembly, tip the solid in by lifting the balloon, and weigh again when the fizzing stops. The balloon inflates but the reading should be unchanged (0.00 g lost), showing that the mass “lost” in the open runs is the carbon dioxide that escaped, and that when the system is closed the mass of products equals the mass of reactants.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
A student says that melting candle wax and burning candle wax are both chemical changes because in each case the wax disappears. Explain why she is right about one and wrong about the other.
Reveal answer
Melting is physical: the wax becomes liquid wax, the same substance, and solidifies again on cooling [1]. Burning is chemical: the wax reacts with oxygen to form new substances, carbon dioxide and water, which escape as gases [1]. Disappearing is not the test — the test is whether a new substance has formed, and in burning it has [1].
State the law of conservation of mass and explain it in terms of atoms.
Reveal answer
The total mass of the products of a chemical reaction equals the total mass of the reactants [1]; a reaction only rearranges atoms into new combinations — no atoms are created or destroyed, and each atom keeps its mass, so the total is unchanged [1].
Copper carbonate is a green powder. When 12.4 g is heated in an open test tube it turns black and the residue has a mass of 8.0 g. Calculate the mass of gas released and the percentage mass loss, and name the gas.
Reveal answer
Gas released = 12.4 − 8.0 = 4.4 g [1]; percentage loss = 4.4 ÷ 12.4 × 100 = 35.5 % [1]; the gas is carbon dioxide (copper carbonate → copper oxide + carbon dioxide) [1].
A student writes: “When iron rusts it gains mass because the rust is heavier than iron. This breaks the law of conservation of mass but only slightly.” Identify three errors.
Reveal answer
The mass gain comes from oxygen (and water) from the air combining with the iron, not from rust being “heavier” as a material [1]; the law is not broken at all — iron + oxygen gained = rust formed exactly [1]; a law cannot be broken “slightly”: the extra mass was simply not on the balance before because it was in the air [1].
True or false, with a reason: “A precipitate is a solid that was dissolved in one of the solutions and settles out when they are mixed.”
Reveal answer
False [1]: a precipitate is a new insoluble compound made when ions from one solution combine with ions from the other; neither solution contained it before they were mixed [1].
0.50 g of manganese dioxide is added to hydrogen peroxide solution; oxygen is released rapidly. Describe how you would show that the manganese dioxide is a catalyst and not a reactant, and state two results you would expect.
Reveal answer
When the fizzing stops, filter the mixture, wash and dry the black solid and weigh it [1]; expected result 1: its mass is still 0.50 g — it was not used up [1]; expected result 2: adding it to fresh hydrogen peroxide makes the reaction fast again, and hydrogen peroxide without it still releases oxygen, just very slowly — so it changes the rate only, not the products [1].
Summary
- A physical change keeps the same substance (state, shape, dissolving); a chemical change rebonds atoms into new substances with new properties.
- Five signs of a chemical change: colour change, gas given off, precipitate, temperature change, light or flame — each a clue, none alone a proof; name the new substance.
- Reactants → products; write word equations with full chemical names.
- Mass is conserved because atoms are rearranged, not created or destroyed. A closed system shows no change; an open flask loses mass when a gas leaves and gains it when a gas arrives — percentage change = change ÷ original × 100.
- A precipitate is a new insoluble solid from two solutions; a catalyst speeds a reaction and is recovered unchanged.
Next time → Writing the change down: formulas, symbol equations, why 2H₂O is not H₂O₂, and balancing by counting atoms.