Chemistry: Atoms First 2e · Essential Ideas
Physical and Chemical Properties
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In 30 seconds
How do you tell one substance from another? You look at its properties — the characteristics you can observe or measure. A Physical property Observable characteristic that does not change the substance's identity. Full entry → is observable without changing the substance's identity: color, odor, Density Mass per unit volume (m/V). Full entry →, melting point, hardness, conductivity. A Chemical property Characteristic shown only when a substance reacts to form new substances. Full entry → describes how a substance reacts to form new substances: flammability, reactivity with acid, corrosion, and toxicity show themselves only during a Chemical change Reaction producing one or more new substances. Full entry →.
The same pair of ideas applies to changes. A Physical change Change in form/appearance with no change in identity. Full entry → alters form or appearance but not identity — ice melting, sugar dissolving, bending a paper clip. A chemical change (chemical reaction) produces new substances with different properties — iron rusting, wood burning, food digesting.
Why this matters
- Identification. Physical properties like density and boiling point are fingerprints; density is especially useful because it is intensive — independent of sample size.
- Safety. Chemical properties tell you what a substance will do: is it flammable? corrosive? toxic? reactive with water? Knowing these properties before handling a chemical is the difference between a safe lab and an accident. This is why chemical labels carry hazard information.
- Materials selection. Surgical instruments must resist corrosion and sterilization; implants must be biocompatible; fire-retardant fabrics must be nonflammable.
- Exam value. "Physical or chemical change?" and "which is a chemical property?" are common early-course questions — easy points once the rule is clear.
The college version
Core Concepts
Physical properties
A physical property is measured or observed without changing what the substance is: color, odor, density, melting point, hardness, solubility, conductivity. Measuring them involves only physical changes — melt ice, dissolve salt, or bend copper and you still have the same substance. Physical properties split into extensive properties, which depend on the amount of matter (mass, volume), and intensive properties, which do not (density, temperature, melting point). Intensive properties are the ones useful for identification.
Chemical properties
A chemical property describes a substance's ability to react and form new substances: flammability (can it burn?), reactivity with acid (does it fizz?), corrosion (does it combine with oxygen and water?), reactivity with water, and toxicity. You cannot observe it by looking; you must trigger a reaction — gasoline's flammability appears only when it burns.
Physical vs. chemical changes
A physical change alters form or appearance without changing composition — melting, freezing, boiling, dissolving, crushing, cutting. Boiling water is still H₂O; crushed ice is still frozen H₂O.
A chemical change (chemical reaction) rearranges atoms into new substances with different properties. Signs include:
- a color change beyond simple mixing (rust turning orange)
- gas production (fizzing — vinegar + baking soda)
- a precipitate (a solid dropping out of solution)
- heat or light given off or absorbed (a burning match)
- an odor change (food spoiling)
None of these signs is absolute proof — boiling also bubbles — so the decisive test is: was a new substance formed?
Density: the intensive property that identifies
Density is mass per unit volume:
density = massvolume = mV
Common units are grams per milliliter (g/mL) or grams per cubic centimeter (g/cm3); since 1 mL = 1 cm³, the numbers match. Density is intensive: 1 mL or 1 L of water both give about 1.00 g/mL (at 4 °C). It explains why oil floats on water, why steel ships float (the hull traps air, lowering average density), and why ice floats (solid water is slightly less dense than liquid water — crucial for aquatic life in winter).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Physical property | Chemical property | Physical = observed without changing identity (melting point, density). Chemical = requires a reaction (flammability, reactivity with acid). |
| Physical change | Chemical change | Physical change keeps the same substance (ice → water). Chemical change makes new substances (wood → ash + gases). |
| Boiling (bubbles) | Chemical change (gas produced) | Boiling is a physical phase change of the liquid itself; fizzing from a reaction produces a new gas (CO₂ from vinegar + baking soda). |
| Melting | Dissolving | Melting: one substance changes phase (ice → water). Dissolving: a solute disperses in a solvent, forming a mixture. |
| Extensive property | Intensive property | Mass/volume depend on sample size (extensive); density/melting point do not (intensive). |
| Color change from mixing | Color change from reaction | Mixing blue and yellow paint is physical (no new substance); iron turning rust-orange is chemical. |
| Density | Weight | Density is mass per volume, independent of amount; weight is a force depending on gravity and amount. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Every material has a "profile card." Physical properties are things you can check without changing the stuff — color, whether it melts easily, how heavy it is for its size. Chemical properties are things the stuff does — whether it can burn, rust, or fizz with vinegar. Melting ice is a physical change because it is still water; burning wood is a chemical change because it turns into ash, smoke, and gases — brand-new stuff that was not there before.
Worked example
You are handed two unlabeled clear liquids and told one is pure water and the other is an aqueous salt solution. How do you tell them apart without drinking them?
Plan A — measure density. Weigh exactly 50.0 mL of each liquid. Pure water (at about 20 °C) has a density of 0.998 g/mL, so 50.0 mL should mass about 49.9 g. If one sample masses noticeably more — say 53.5 g — its density is higher:
density = mV = 53.5 g50.0 mL = 1.07 g/mL
The extra mass comes from dissolved salt, so that sample is the salt solution. Density works as an identifier because it is intensive: pure water's density is a fixed fingerprint (at a given temperature), while the solution's density varies with salt content.
Plan B — test with evaporation. Boil a few milliliters of each in separate dishes. Pure water evaporates completely, leaving nothing behind (a physical change). The salt solution leaves white salt crystals — the water evaporated, the salt did not.
Now classify everyday changes. Melting butter — physical: still butter, just liquid. Toasting bread — chemical: heat drives reactions making new brown, fragrant compounds. Dissolving sugar in tea — physical: the sugar is still sugar. A nail rusting — chemical: iron combines with oxygen and water to form rust, a new substance. Cutting paper — physical: same paper, smaller pieces.
Key takeaways
- Physical property: observable without changing identity (color, density, melting point, hardness, conductivity).
- Chemical property: observable only when the substance reacts (flammability, reactivity with acid, corrosion, toxicity).
- Physical change: identity unchanged — melting, freezing, boiling, dissolving, crushing, cutting.
- Chemical change: new substances form — rusting, burning, digesting, baking, fizzing with acid.
- Signs of chemical change: color change, gas, precipitate, heat/light, odor — but the decisive test is "is a new substance formed?"
- Extensive vs. intensive: extensive depends on amount (mass, volume); intensive does not (density, temperature, melting point). Intensive properties identify substances.
- Density = m/V; water ≈ 1.00 g/mL at 4 °C; oil floats because it is less dense; ice floats because solid water is less dense than liquid water.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Classify each as a physical or chemical property: (a) melting point, (b) flammability, (c) density, (d) reactivity with acid.
Show answer
(a) physical, (b) chemical, (c) physical, (d) chemical.
A substance is heated and melts, then the liquid is heated and ignites. Which step was a physical change and which a chemical change?
Show answer
Melting is a physical change (same substance, new phase). Igniting is a chemical change (combustion forms new substances — CO₂, H₂O, and other products).
What is the decisive test for deciding whether a change is chemical?
Show answer
Ask: has a new substance with different properties formed? If yes, it is a chemical change — regardless of what visible signs appeared.
A 25.0 mL sample of a liquid has a mass of 19.5 g. Calculate its density and identify whether it is likely water (about 1.00 g/mL), ethanol (about 0.789 g/mL), or mercury (13.6 g/mL).
Show answer
density = m/V = 19.5 g / 25.0 mL = 0.780 g/mL, which matches ethanol (about 0.789 g/mL) far better than water or mercury.
Give two signs that might indicate a chemical change, and explain why each sign alone is not proof.
Show answer
Gas bubbles or a color change can indicate a chemical change, but bubbles may be boiling (physical) and color may come from simple mixing (physical). Proof requires showing a new substance formed.
Why is density called an Intensive property Independent of the amount of matter (density, melting point). Full entry →, and why does that make it useful for identifying substances?
Show answer
Density does not depend on sample size: mass and volume both scale with amount, so their ratio stays fixed. That fixed "fingerprint" value lets you identify an unknown sample regardless of how much you have.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Physical property
- Observable characteristic that does not change the substance's identity.
- Chemical property
- Characteristic shown only when a substance reacts to form new substances.
- Physical change
- Change in form/appearance with no change in identity.
- Chemical change
- Reaction producing one or more new substances.
- Extensive property
- Depends on the amount of matter (mass, volume).
- Intensive property
- Independent of the amount of matter (density, melting point).
- Density
- Mass per unit volume (m/V).
Sources & references
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