Lesson 1.2 · Unit 1 · HS-PS1-2
Physical and chemical change, and why the usual signs can mislead
The distinction sounds easy: a physical change alters the form, a chemical change alters the substance. In practice students are handed a list of five signs and told to spot them, and every one of those five signs has a physical impostor. The reliable question is not what you saw but whether a new substance is present at the end.
- A physical change alters form without producing a new substance. Melting, dissolving, grinding and boiling are physical: the particles are the same particles, differently arranged or separated.
- A chemical change produces at least one new substance, with different properties, because bonds between atoms were broken and new ones formed.
- The five usual signs are evidence, not proof: gas evolved, a precipitate formed, color changed, temperature changed, or light emitted. Each has a physical process that mimics it.
- The decisive test is whether the original substance can be recovered by physical means. Dissolved salt evaporates back out unchanged; burned wood does not reassemble.
- An extensive property depends on how much you have (mass, volume, total energy). An intensive property does not (density, melting point, color, conductivity). Only intensive properties identify a substance, which is why density is useful and mass is not.
- Physical and chemical properties are described differently. A physical property can be observed without changing the substance's identity; a chemical property can only be observed by changing it, which is why "flammable" is a chemical property.
Where students lose marks: treating bubbling as automatic proof of a chemical change. Boiling water bubbles vigorously and produces nothing new. Ask what the gas is, not whether there is one.
The problem. A burning candle is sometimes given as an example of a physical change and sometimes of a chemical one. Settle it.
Step one: notice that more than one thing is happening. The question is badly posed as stated, because a burning candle involves several distinct processes at once. Separating them is the whole answer.
Step two: the wax near the flame melts. Solid wax becomes liquid wax. The substance is unchanged: cool it and it solidifies into wax again, with the same melting point. Physical.
Step three: the liquid wax is drawn up the wick. This is capillary action moving a liquid. No new substance. Physical.
Step four: the wax at the top of the wick vaporizes. Liquid to gas. Still wax, now as a vapor. Physical.
Step five: the wax vapor burns. Here the hydrocarbon molecules react with oxygen and become carbon dioxide and water vapor. These have entirely different properties from wax and cannot be turned back into wax by cooling. Chemical.
Step six: apply the recovery test to confirm. Hold a cold dry surface in the flame and a film of liquid condenses on it, which is water, not wax. A limewater test on the gases above the flame turns the limewater cloudy, indicating carbon dioxide. Neither product is the starting material, and no amount of cooling returns them to wax.
Step seven: state the answer. The candle shows both. Three physical changes supply fuel to one chemical change, and the physical steps exist because the wax has to be a vapor before it can burn at a useful rate. Saying only "chemical" misses why a candle needs a wick at all.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- State the defining difference between a physical and a chemical change.
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A chemical change produces at least one new substance; a physical change does not
- List the five common signs of a chemical change.
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Gas evolved, precipitate formed, color change, temperature change, light emitted
- Classify density as intensive or extensive, and say why it matters.
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Intensive: it does not depend on how much you have, so it can be used to identify a substance
- Classify these as physical or chemical changes: melting ice, souring milk,
shredding paper, digesting food.
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Physical, chemical, physical, chemical
- Is flammability a physical or a chemical property? Explain in one sentence.
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Chemical, because you can only observe it by changing the substance into something else
- Bubbles form when hydrochloric acid is poured on limestone, and also when water is
heated to boiling. Explain how you would tell the two situations apart
experimentally.
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Identify the gas and check whether the starting material survives. Collect the gas from each: the gas from the limestone turns limewater cloudy and does not condense on cooling, so it is carbon dioxide, a substance that was not present before. The gas from the boiling water condenses back to liquid water on a cold surface, so it is the same substance in a different state. Then check the residue: the limestone has partly disappeared and a different solid remains, while cooling the steam returns all the water. One produced something new and the other did not. Identify the gas and test whether the original substance can be recovered by cooling
- Dissolving sodium chloride in water absorbs a little heat, and dissolving it
changes a clear liquid into a clear solution. Two of the five signs are arguably
present. Explain why dissolving is nevertheless a physical change.
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Because the salt is still there and can be recovered unchanged. Evaporate the water and sodium chloride crystals come back with the same melting point, density and crystal form as the ones that went in, which would be impossible if they had been converted into something else. The temperature change comes from the energy needed to separate ions from the lattice against the energy released when water molecules surround them, and neither step forms new bonds between different elements. The signs are evidence that something happened, not evidence of what happened. The salt is recoverable unchanged by evaporation, so no new substance was formed
- A student says mass is a good way to identify an unknown liquid. Explain the error
and give a better property.
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Mass is extensive, so it depends entirely on how much of the liquid happens to be in the container and tells you nothing about what the liquid is. Any liquid can have a mass of 50 g. An identifying property must be intensive, so that the value is the same for a drop and for a barrel. Density works, because it is mass per unit volume and is fixed for a pure substance at a given temperature, as do boiling point and refractive index. In practice you would measure two or three intensive properties, since a single value can be shared by more than one substance. Mass is extensive; use an intensive property such as density or boiling point
- A firework emits light and heat, and a glow stick emits light but gets slightly
cooler. Are both chemical changes? Explain.
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Yes, both are. The direction of the energy flow does not decide the question; the production of new substances does. In the firework an exothermic reaction releases energy as heat and light, and the products are oxides quite different from the reactants. In the glow stick two substances are mixed and react, and the energy released by bond rearrangement leaves as light rather than heat, so the surroundings do not warm. Both produce new substances that cannot be separated back into the originals by physical means, which is the test. Both are chemical; energy direction does not decide it, new substances do
- Explain why the candle in the worked example needs a wick, in terms of physical and
chemical changes.
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Solid wax does not burn at a useful rate, because combustion happens where fuel molecules and oxygen molecules can mix freely, which is in the gas phase. A block of wax presents only its surface, so the reaction is slow and self-extinguishing. The wick performs the physical steps that solve this: heat from the flame melts wax at the base, capillary action carries liquid wax upward, and the heat at the tip vaporizes it, delivering a steady supply of wax vapor to the flame. The chemical change then has fuel in the phase it requires. The physical processes exist to feed the chemical one. The wick melts, transports and vaporizes the wax so the chemical change has gaseous fuel