Physical & Chemical Properties and Changes
Learn to tell physical from chemical properties and changes using real evidence of new substances, and apply conservation of mass in open and closed systems.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Physical & Chemical Properties and Changes, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Ice melting and paper burning both look dramatic, but only one of them makes a new substance. That single question — is the chemical identity of the material still the same afterward? — is the dividing line between a physical change and a chemical change, and it is the idea this lesson is built around.
You will learn how to describe matter using physical properties (what you can observe without changing what the substance is) and chemical properties (how a substance behaves when it does change identity), how to read the evidence of a chemical reaction without being fooled by look-alike clues, and how conservation of mass lets you predict masses of reactants and products. These skills carry straight into balancing equations and stoichiometry later in the course.
You will learn how to describe matter using physical properties (what you can observe without changing what the substance is) and chemical properties (how a substance behaves when it does change identity), how to read the evidence of a chemical reaction without being fooled by look-alike clues, and how conservation of mass lets you predict masses of reactants and products. These skills carry straight into balancing equations and stoichiometry later in the course.
Physical Properties: Describing Matter Without Changing It
A physical property can be measured or observed while the substance keeps its chemical identity. Color, odor, melting point, boiling point, density, hardness, malleability, ductility, electrical conductivity, solubility, and physical state are all physical properties. Measuring the density of a copper wire leaves you holding copper wire.
Physical properties split into two useful groups. An intensive property does not depend on how much of the substance you have: density, melting point, and color are the same for a chip of gold and a gold bar. An extensive property does depend on amount: mass, volume, and length all change when you take a bigger sample. Only intensive properties are useful for identifying an unknown substance, which is exactly why a density measurement can tell you whether a metal sample is aluminum or zinc while its mass cannot.
Where students go wrong: they assume that because a property is measured with fancy equipment it must be chemical. The test is not the equipment. Ask instead, after the measurement, do I still have the same substance? If yes, the property is physical.
Physical properties split into two useful groups. An intensive property does not depend on how much of the substance you have: density, melting point, and color are the same for a chip of gold and a gold bar. An extensive property does depend on amount: mass, volume, and length all change when you take a bigger sample. Only intensive properties are useful for identifying an unknown substance, which is exactly why a density measurement can tell you whether a metal sample is aluminum or zinc while its mass cannot.
| Property | Type | Why |
|---|---|---|
| Density | Intensive, physical | Same value for any sample size; measuring it does not change the substance |
| Mass | Extensive, physical | Depends on how much you have |
| Boiling point | Intensive, physical | The liquid becomes vapor, but it is still the same substance |
| Flammability | Chemical | Can only be observed by burning the substance into new products |
Chemical Properties and the Evidence of a Chemical Change
A chemical property describes how a substance reacts to form different substances. Flammability, reactivity with acid, reactivity with water, toxicity, and resistance to corrosion are chemical properties. You cannot observe them without actually running the change — the only way to know magnesium is flammable is to burn some magnesium, which destroys the sample.
A chemical change (a chemical reaction) rearranges atoms into new combinations, so the products have different properties than the reactants. Common observable evidence includes formation of a gas (bubbling in a solution that is not boiling), formation of a precipitate (a solid appearing when two clear solutions mix), an unexpected and permanent color change, production or absorption of heat, emission of light, and a new odor. Chemical changes are also usually hard to reverse by simple physical means.
Every one of these clues has a look-alike that is purely physical, so evidence is suggestive, not proof. Bubbles rise out of boiling water, but that is water vapor, still . Bubbles rise out of an opened soda, but that is dissolved escaping — a physical process. Mixing blue and yellow paint produces green without any reaction. Water freezing releases heat. The reliable conclusion comes from asking whether the substances present at the end are chemically different from the ones you started with, and evidence like several clues appearing together, or an irreversible result, strengthens that case.
The most-missed example in class is dissolving. Salt dissolving in water is a physical change: the separates into ions that are still sodium and chloride, and evaporating the water gives the salt back. Compare that to zinc metal dissolving in hydrochloric acid, which produces hydrogen gas and zinc chloride — no amount of evaporating brings back zinc metal.
A chemical change (a chemical reaction) rearranges atoms into new combinations, so the products have different properties than the reactants. Common observable evidence includes formation of a gas (bubbling in a solution that is not boiling), formation of a precipitate (a solid appearing when two clear solutions mix), an unexpected and permanent color change, production or absorption of heat, emission of light, and a new odor. Chemical changes are also usually hard to reverse by simple physical means.
Every one of these clues has a look-alike that is purely physical, so evidence is suggestive, not proof. Bubbles rise out of boiling water, but that is water vapor, still . Bubbles rise out of an opened soda, but that is dissolved escaping — a physical process. Mixing blue and yellow paint produces green without any reaction. Water freezing releases heat. The reliable conclusion comes from asking whether the substances present at the end are chemically different from the ones you started with, and evidence like several clues appearing together, or an irreversible result, strengthens that case.
The most-missed example in class is dissolving. Salt dissolving in water is a physical change: the separates into ions that are still sodium and chloride, and evaporating the water gives the salt back. Compare that to zinc metal dissolving in hydrochloric acid, which produces hydrogen gas and zinc chloride — no amount of evaporating brings back zinc metal.
Conservation of Mass in Chemical and Physical Changes
In any chemical or physical change, atoms are neither created nor destroyed — they are only rearranged. The law of conservation of mass follows: the total mass of the reactants equals the total mass of the products.This makes chemical changes predictable arithmetic. If 10.0 g of calcium carbonate decomposes into 5.6 g of calcium oxide and carbon dioxide gas, the carbon dioxide must have a mass of 4.4 g, because 10.0 minus 5.6 equals 4.4.
The trouble comes with open systems, where gases can enter or leave the container. Burn a 5.0 g log and you might collect only 0.3 g of ash, and it looks like mass vanished. It did not — the carbon dioxide and water vapor escaped into the room. Weigh the log, the oxygen consumed, the ash, and every gas produced, and the totals match. Rusting shows the opposite illusion: an iron nail gains mass as it rusts because oxygen atoms from the air are incorporated into the iron oxide.
A good habit: before you claim mass changed, ask whether a gas could have entered or escaped. In a sealed container, the balance reading should not budge during a reaction — if it does, the seal leaked.
The trouble comes with open systems, where gases can enter or leave the container. Burn a 5.0 g log and you might collect only 0.3 g of ash, and it looks like mass vanished. It did not — the carbon dioxide and water vapor escaped into the room. Weigh the log, the oxygen consumed, the ash, and every gas produced, and the totals match. Rusting shows the opposite illusion: an iron nail gains mass as it rusts because oxygen atoms from the air are incorporated into the iron oxide.
| System | What happens to measured mass | Actual mass balance |
|---|---|---|
| Wood burning in open air | Appears to decrease | Gaseous products leave; total is conserved |
| Iron rusting in open air | Appears to increase | Oxygen from air joins the solid; total is conserved |
| Reaction in a sealed flask | Stays constant | Nothing enters or leaves |
A Decision Procedure You Can Reuse
When a problem describes a process, work through it in a fixed order rather than guessing from the vocabulary.
First, name the substance before and after. Melting butter: butter before, butter after — physical. Baking batter: flour, egg, and baking soda before; bread with carbon dioxide gas and new compounds after — chemical.
Second, check whether the process is a change of state or a change of form. Melting, freezing, boiling, condensing, subliming, crushing, grinding, cutting, bending, and dissolving are all physical. The particles move differently or are separated from each other, but the molecules themselves stay intact.
Third, look for evidence of new substances and check for a physical explanation of that same evidence. Gas produced from a solid dropped into acid is strong evidence of reaction; gas produced from a liquid on a hot plate is probably boiling.
Fourth, ask about reversibility by physical means. You can refreeze melted ice and re-condense steam. You cannot unburn toast.
Two more traps worth naming. Describing a property is not the same as describing a change: "gasoline is flammable" states a chemical property, while "the gasoline burned" describes a chemical change. And a physical property that changed is not automatically a chemical change — a copper wire hammered flat has a new shape, but it is still copper.
Finally, be careful with the word "new." A new color, new temperature, or new phase is not automatically a new substance. New substance means new chemical formula, new set of chemical properties. That is the standard every argument in this unit should meet.
First, name the substance before and after. Melting butter: butter before, butter after — physical. Baking batter: flour, egg, and baking soda before; bread with carbon dioxide gas and new compounds after — chemical.
Second, check whether the process is a change of state or a change of form. Melting, freezing, boiling, condensing, subliming, crushing, grinding, cutting, bending, and dissolving are all physical. The particles move differently or are separated from each other, but the molecules themselves stay intact.
Third, look for evidence of new substances and check for a physical explanation of that same evidence. Gas produced from a solid dropped into acid is strong evidence of reaction; gas produced from a liquid on a hot plate is probably boiling.
Fourth, ask about reversibility by physical means. You can refreeze melted ice and re-condense steam. You cannot unburn toast.
Two more traps worth naming. Describing a property is not the same as describing a change: "gasoline is flammable" states a chemical property, while "the gasoline burned" describes a chemical change. And a physical property that changed is not automatically a chemical change — a copper wire hammered flat has a new shape, but it is still copper.
Finally, be careful with the word "new." A new color, new temperature, or new phase is not automatically a new substance. New substance means new chemical formula, new set of chemical properties. That is the standard every argument in this unit should meet.
Key terms
- Physical property.
- A characteristic that can be observed or measured without changing the chemical identity of the substance, such as density, color, melting point, or malleability.
- Chemical property.
- A characteristic that describes how a substance reacts to form new substances, such as flammability or reactivity with acid; observing it requires changing the substance.
- Intensive property.
- A property whose value does not depend on the amount of substance present, such as density or boiling point; useful for identifying unknown substances.
- Extensive property.
- A property whose value depends on the amount of substance present, such as mass, volume, or length.
- Physical change.
- A change in form, state, or appearance in which the chemical identity of the substance is preserved, such as melting, boiling, grinding, or dissolving.
- Chemical change.
- A process in which atoms are rearranged so that one or more new substances with different properties are formed; also called a chemical reaction.
- Precipitate.
- An insoluble solid that forms when two solutions are mixed; its appearance is evidence that a chemical reaction has occurred.
- Law of conservation of mass.
- In any physical or chemical change, the total mass of the substances before the change equals the total mass after, because atoms are only rearranged.
Worked example
A student places a coiled strip of magnesium ribbon with a mass of 2.43 g into a crucible and heats it in air until it burns with a bright white light. The gray metal is gone, replaced by a white powder. After cooling, the white powder has a mass of 4.03 g. (a) Identify the type of change and cite two pieces of evidence. (b) Determine the mass of oxygen that combined with the magnesium. (c) The student says mass was created because the product weighs more than the metal. Correct this statement.
Part (a). Compare the substances before and after. Before: shiny gray magnesium metal. After: a crumbly white powder that will not conduct electricity or bend like a metal. Different properties mean a different substance, so this is a chemical change. Two pieces of evidence: emission of intense light and heat during the process, and a permanent change in appearance and properties from a malleable gray metal to a brittle white solid that cannot be turned back into magnesium by cooling or crushing.
Part (b). Apply conservation of mass. The reactants are magnesium plus oxygen from the air; the product is magnesium oxide.Substitute the known masses in grams: 2.43 plus the oxygen mass equals 4.03. Solving, the oxygen mass is 4.03 minus 2.43, which is 1.60 g of oxygen.
Part (c). No mass was created. The crucible is an open system, so oxygen molecules from the surrounding air were free to enter and react. Those 1.60 g of oxygen atoms were part of the total mass all along — they were just in the air and not sitting on the balance. If the reaction had been run inside a sealed container that already held the oxygen, the balance reading would not have changed at all. Atoms were rearranged, not created.
Part (b). Apply conservation of mass. The reactants are magnesium plus oxygen from the air; the product is magnesium oxide.Substitute the known masses in grams: 2.43 plus the oxygen mass equals 4.03. Solving, the oxygen mass is 4.03 minus 2.43, which is 1.60 g of oxygen.
Part (c). No mass was created. The crucible is an open system, so oxygen molecules from the surrounding air were free to enter and react. Those 1.60 g of oxygen atoms were part of the total mass all along — they were just in the air and not sitting on the balance. If the reaction had been run inside a sealed container that already held the oxygen, the balance reading would not have changed at all. Atoms were rearranged, not created.
Practice questions
Which of the following observations is the strongest evidence that a chemical change has occurred?
- Clear liquid water in an open dish slowly disappears over two days
- A shiny copper penny is hammered into a flat disc
- Two clear, colorless solutions are mixed and a yellow solid settles to the bottom
- Solid iodine crystals are heated and produce a purple vapor
Answer: Two clear, colorless solutions are mixed and a yellow solid settles to the bottom
A solid appearing when two clear solutions are combined is a precipitate — a substance that was not present before and has properties (insolubility, yellow color) different from either starting solution. That signals new substances formed. The other three are all physical changes: evaporation and sublimation of iodine are changes of state in which the molecules stay intact, and hammering copper changes only shape. Notice that the iodine choice includes a striking color change, which is a common reason students pick it, but the purple vapor is still iodine and it re-forms solid crystals on a cool surface.
A 15.0 g sample of a hydrated salt is heated in an open evaporating dish. Water vapor escapes and the remaining solid has a mass of 9.6 g. A student concludes that mass was destroyed. Explain what actually happened and calculate the mass of water driven off.
Answer: Mass was conserved; 5.4 g of water vapor escaped into the air. Total mass after the change is 9.6 g of solid plus 5.4 g of vapor, which equals the original 15.0 g.
The evaporating dish is an open system, so the water that left the salt is no longer sitting on the balance — but it still exists as vapor in the room. Conservation of mass says the total mass of products equals the mass of the reactant: 15.0 minus 9.6 gives 5.4 g of water. A complete answer names the system as open and identifies the escaping gas as the missing mass rather than saying the mass simply disappeared. If the same sample were heated in a sealed container, the balance reading would stay at 15.0 g.
Classify each of the following as a physical property, chemical property, physical change, or chemical change, and justify each answer in one sentence: (i) helium has a very low density; (ii) sodium metal reacts violently with water; (iii) a puddle freezes overnight; (iv) silver jewelry tarnishes to a black coating.
Answer: (i) physical property; (ii) chemical property; (iii) physical change; (iv) chemical change
Item (i) states a measurable characteristic of helium that can be found without turning helium into anything else, so it is a physical property, and an intensive one. Item (ii) describes how sodium behaves when it forms new substances, so it names a chemical property — note the wording states what sodium does react like, not that a specific reaction happened. Item (iii) is a change of state; the puddle is still water, so it is physical and reversible by warming. Item (iv) produces silver sulfide, a black compound with different properties from shiny silver metal, so a new substance formed and the change is chemical. The key distinction between the property items and the change items is whether the sentence describes a capability or reports an event that occurred.
FAQ
- Is dissolving sugar in water a physical or chemical change?
- Physical. The sugar molecules separate from one another and spread out among the water molecules, but each sugar molecule is unchanged. Evaporate the water and you recover the sugar. The general rule for dissolving is that if you can get the original substance back by a physical method like evaporation, the dissolving was physical. Dissolving zinc metal in acid is different — hydrogen gas bubbles off and you cannot recover zinc metal, so that one is chemical.
- Why does a rusted nail weigh more than a clean nail if mass is conserved?
- Because oxygen atoms from the air became part of the rust. Iron plus oxygen forms iron oxide, and the oxygen was never on the balance to begin with. The nail did not create matter; it absorbed matter from its surroundings. If you sealed the nail and a fixed amount of air in a closed flask and let it rust, the total mass of the flask and its contents would not change at all.
- Are bubbles always a sign of a chemical reaction?
- No, and this is one of the most common errors in this unit. Bubbles in boiling water are water vapor, still the same substance. Bubbles in an opened carbonated drink are dissolved carbon dioxide escaping, which is physical. Bubbles are strong evidence of a reaction when a gas appears that was not dissolved or being boiled off, such as when a solid tablet dropped into water fizzes at room temperature.
- How do I tell a chemical property from a chemical change on a quiz?
- Read the verb tense and the wording. A chemical property is a statement about what a substance can do or tends to do: gasoline is flammable, gold resists corrosion, potassium reacts with water. A chemical change reports something that actually happened: the gasoline burned, the potassium fizzed and produced hydrogen. Same chemistry, different kind of statement.
Learn this with a teacher, not a page
The Crimsora tutor teaches Physical & Chemical Properties and Changes live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.