Physical vs. Chemical Properties · Grade 8 Science

Physical vs. Chemical Properties

Learning goals

• I can distinguish a physical property from a chemical property.
• I can classify a change as physical or chemical based on evidence, including calculating density.
• I can plan a mixture separation, evaluate recovery evidence and account for incomplete collection.

Readings for this unit

• A container that works once

Every substance has a set of physical properties — traits you can observe or measure without turning the substance into anything new. Color, hardness, texture, melting point, boiling point, and density (mass ÷ volume) are all physical properties. You can slice a loaf of bread, bend a copper wire, melt an ice cube, or dissolve sugar in tea, and in every one of those cases the material is still fundamentally the same stuff, just in a different form. A change like that — where no new substance appears — is a physical change, and it's often reversible.

Density is worth a closer look, because it's a physical property you'll calculate often. It measures how much mass is packed into a given amount of space (density = mass ÷ volume), and it explains everyday puzzles like why a massive steel ship floats while a small pebble sinks — what matters isn't total weight but how tightly the mass is packed compared with water. A key point: density belongs to the material itself, so a tiny gold ring and a giant gold bar have exactly the same density.

A chemical property, by contrast, describes how a substance behaves when it reacts and turns into something different. Flammability (will it burn?), reactivity with acid (will it fizz or dissolve?), and the tendency to rust are all chemical properties. Here's the catch: you can only truly observe a chemical property by letting the substance change — you can't see that wood is flammable until some of it actually burns.

When a substance genuinely reacts and becomes one or more new substances with new properties, that's a chemical change. Burning wood, rusting iron, and digesting your lunch are all chemical changes: the original material is gone, its atoms rearranged into different substances entirely. And unlike most physical changes, chemical changes are usually very hard to reverse — there's no simple way to un-burn a log or un-cook an egg.

So how do you tell the two apart? Ask a single question: did a genuinely new substance form? Melting butter is physical — it's still butter, just liquid, and it hardens right back if it cools. Burning that same butter is chemical — it becomes soot, smoke, and gases that will never turn back into butter. Watch out for one common trap, though: a dramatic-looking change of state, like water boiling into steam, is still only a physical change, because that steam is nothing but water in gas form.

A physical property can be observed without changing a substance's chemical identity. Measure melting point, density or electrical conductivity under stated conditions. Color alone rarely identifies a substance because different substances can have similar colors.

Chemical properties describe possible reactions, such as burning in oxygen or reacting with an acid. The property exists before the test, but observing it normally involves a chemical change. A chemical reaction rearranges atoms into new substances.

Separate the evidence from the conclusion. Cutting metal changes its size; rusting makes new substances. Dissolving sugar in water usually keeps the sugar molecules intact. Ask whether composition changed, rather than whether the change looks dramatic or is easy to reverse.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can the properties of a material help us explain it and choose a useful application? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Separate a property from a change

A property describes how a substance behaves under stated conditions. A physical property can be measured without changing chemical identity, such as density or electrical conductivity. A change is something that happens to a sample. Cutting a copper wire changes its length, while conductivity remains a property of the copper.

Read the visual model

Is flammability a change or a property?

Visual reasoning — after your attempt

Flammability is a chemical property describing a tendency to burn. Actual burning is a chemical change producing new substances.

Make two notebook columns: what can be measured, and what happened. Melting point belongs in the first column; a sample melting belongs in the second. Give the temperature or other test conditions when they matter.

Pause and explain

Is flammability a change or a property?

Check your reasoning after answering

Flammability is a chemical property describing a tendency to burn. Actual burning is a chemical change producing new substances.

Part 2 · Ask whether chemical identity changed

A physical change can alter size, shape, arrangement, or state while retaining the substance. A chemical change produces substances with different chemical identities by rearranging atoms. The decision concerns composition, not whether the event looks dramatic or whether it is easy to reverse.

Read the visual model

Why is reversibility alone an unreliable rule?

Visual reasoning — after your attempt

Some physical changes are hard to undo, such as breaking glass, and some chemical reactions can be reversed under appropriate conditions. Chemical identity is the deciding idea.

Boiling water makes water vapor, so it is a physical change. Rusting iron produces iron-containing compounds with oxygen, so it is chemical. Breaking a glass is difficult to undo but does not, by itself, create a new substance.

Pause and explain

Why is reversibility alone an unreliable rule?

Check your reasoning after answering

Some physical changes are hard to undo, such as breaking glass, and some chemical reactions can be reversed under appropriate conditions. Chemical identity is the deciding idea.

Part 3 · Build a comparison from several measurements

One property rarely establishes identity. Different substances may share a color or approximate density. Collect a pattern of evidence using comparable conditions: appearance, density, melting behavior, or a suitable approved test. Treat a proposed identity as a claim that should agree with all relevant evidence.

Read the visual model

The slope of this mass-versus-volume graph is density, not speed. Both axes name measured properties. Use several properties, with conditions stated, before deciding whether a substance’s identity has changed.

For a 12 cm³ sample following this pattern, predict mass. Can that result alone identify the substance?

Visual reasoning — after your attempt

Predicted mass is 12×3 = 36 g. Density alone is insufficient because different substances can share similar values; compare additional characteristic properties.

Two white powders may look identical but behave differently in water. Describe the observations without tasting or handling unknown chemicals. A label, a known safe reference sample, and a controlled test are stronger evidence than appearance alone.

Pause and explain

Do two equal densities prove two samples are the same substance?

Check your reasoning after answering

No. Different substances can have similar densities, and measurements have uncertainty. Additional independent properties and known comparison samples are needed.

Part 4 · Describe dissolving at the particle level

When sugar dissolves, sugar molecules disperse among water molecules. The mixture looks uniform at the everyday scale, but the sugar has not simply vanished. For this familiar example, dissolving keeps the sugar molecules intact. Some substances can also react when mixed with water, so dissolving should not be treated as one universal chemical rule.

Read the visual model

How can a clear liquid contain more than one substance?

Visual reasoning — after your attempt

Particles can be mixed too finely to distinguish by eye. A clear sugar solution contains both water molecules and dispersed sugar molecules.

A particle drawing of sugar water should show both labeled water and sugar particles. Evaporation can leave sugar behind, provided heating does not cause decomposition. Explain the chosen example rather than claiming every clear solution contains only water.

Pause and explain

How can a clear liquid contain more than one substance?

Check your reasoning after answering

Particles can be mixed too finely to distinguish by eye. A clear sugar solution contains both water molecules and dispersed sugar molecules.

Mixtures and physical separation

Sand and salt can be combined without automatically forming a new compound. A physical combination needs investigation, not a reaction claim based only on mixing.

A mixture contains substances physically combined in variable proportions. A solution is a mixture that is uniform at the visible scale because one substance is distributed among particles of another. A solute is the dissolved substance and a solvent is the dissolving medium. Salt dissolved in water remains matter even when it is not visible. A compound has a chemical composition; changing the mixture proportions is different from changing a compound’s formula.

Separation can use differences in physical properties. A filter traps sufficiently large suspended particles but ordinarily does not remove dissolved salt. Decanting means carefully pouring off a liquid while leaving settled material. Evaporation can leave dissolved salt behind while water enters the surroundings; it does not destroy water or create new salt atoms. Separation behavior, recovered characteristic properties and mass accounting supply evidence for a physical combination.

Plan before observing: specify known substances, their amounts, the property used for separation, the recovery method and how records will be kept. Test the components and their combination with matched water amounts and stirring/settling time. A sample’s clear appearance does not establish purity or drinking safety. Incomplete recovery may reflect transfer loss or dissolved material left behind, rather than a chemical reaction. Report all recovered and unrecovered matter within the chosen boundary.

Evidence to interpret

Original components	Recovered sand (g)	Recovered salt (g)
5.0 g sand + 3.0 g salt	4.8	2.9

Use this method safely

Adult support for material selection and spills. Use only clean known nonhazardous samples, avoid dust inhalation and keep small items from younger children. Do not taste, drink, heat or mix unknown chemicals. Use provided records when balance, filters or safe samples are unavailable.

Investigate or evaluate the supplied records

1. Use clean approved sand and table salt, or the supplied records. Record the known components and masses.
2. Compare each component and the mixture in equal room-temperature water volumes; record dissolving/settling observations.
3. Filter through a suitable paper filter into a clean collection vessel and record which component remains.
4. Leave a small labeled salt-solution sample in a safe tray to evaporate at room temperature over several days; no stove or boiling is needed.
5. Compare recovered solids with original component records and account for incomplete recovery.
6. Record actual measurements separately from supplied values and propose a repeated improved method.

Explain and apply

Plan and, if safe materials are available, conduct the sand/salt comparison. Use the supplied recovery record to argue whether it is consistent with a physical mixture, and explain why a mass shortfall is not automatically a reaction.

• Specify components and a property-based method.
• Use recovered identity and mass evidence.
• Explain a limitation or controlled improvement.

Reasoning and marking guidance — after your attempt

The separate-component tests and recovery of sand and salt are consistent with a physical combination. The illustrative recovered masses are 4.8+2.9=7.7 g versus 8.0 g originally added, leaving 0.3 g unaccounted for in that collection. Transfer residues, remaining dissolved salt or reading uncertainty can explain a shortfall. Better rinsing, complete collection and repeated matched tests can improve the account; the data do not establish a new compound.

Apply the whole unit · Classify with evidence

Illustrative observations: ice melts into liquid water; a glass breaks into pieces; iron develops a new reddish coating after exposure to moist air; sugar dissolves and is recovered after gentle evaporation.

Your task

Classify each event and explain the chemical-identity evidence. Identify a property used in an observation and explain why a dramatic appearance or difficult reversal is not enough to classify a change.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

Melting, breaking, and the stated sugar dissolution are physical changes; rusting is chemical because new iron compounds form. State identity evidence rather than relying on size or reversibility. Color is an observed physical property, but color change alone would not establish a chemical reaction without more evidence.

Scientific references and adapted teaching resources

• OpenSciEd 7.1 Chemical Reactions & Matter — public unit overview
• OpenSciEd 6.2 Thermal Energy — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Use familiar pictured events and ask for because-statements. Keep properties, observations, and conclusions in separate columns. Do not ask learners to taste samples or test unknown household chemicals. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation. The illustrated study “Mixtures and physical separation” is required curriculum content. Teach it before the associated application or distribute it over the assigned unit sessions; it is not an optional enrichment. Its practice/review/quiz response provides evidence of understanding. Model/data work must not be recorded as an actual physical experiment.

Density

density = mass ÷ volume

mass in grams (g); volume in cm³; density in g/cm³.

Worked examples

Avoid these mistakes

• An irreversible-looking change is not automatically a chemical reaction.

Words to know

physical property

A characteristic you can observe or measure without changing what the substance is, like color, density, or melting point.

chemical property

A characteristic that describes how a substance reacts or changes into a different substance, like flammability or reactivity with acid.

physical change

A change in form or appearance where no new substance is created.

chemical change

A change where one or more new substances with new properties are formed.

density

A physical property found by dividing an object's mass by its volume (density = mass ÷ volume).

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

The one-question test for a chemical change

Students mix these up constantly, so here's the key question: Is a new substance with new properties formed? If yes, it's a chemical change; if no, it's physical. Crushing chalk into powder is physical — it's still chalk. But chalk fizzing in vinegar makes a new gas (carbon dioxide) — that's chemical. Tearing paper is physical; burning it into ash and smoke is chemical.

The clues that a chemical reaction happened

You can't always see bonds breaking, but chemical changes leave fingerprints: a color change (a nail rusting), a gas (bubbles that aren't from boiling), a precipitate (a solid forming when two liquids mix), light or heat released or absorbed, and — the big one — it's hard to reverse. Any one clue is a hint; several together are strong evidence.

Why 'hard to reverse' is the giveaway

Physical changes are usually easy to undo: melt ice, then refreeze it; dissolve salt, then evaporate the water to get it back. Chemical changes are stubborn because new bonds have formed. You can't un-bake a cake or un-burn a log by cooling them, because the atoms have been rearranged into entirely new molecules. That difficulty of reversal is often the clearest sign a chemical change occurred.

The sneaky in-between: dissolving

Is dissolving sugar in water physical or chemical? It's a classic trick question — the answer is physical. The sugar molecules are still sugar; they've just spread out among the water molecules, and you can get the sugar back by evaporating the water. No new substance formed. This trips people up because it looks dramatic, but 'looks dramatic' isn't the test — 'new substance' is.

In the real world

Firefighters need to know which materials are flammable (a chemical property) so they can fight fires safely and predict how they'll spread.

Exit check

Which case changes substance identity? Explain the relevant mechanism and one model limitation.