The labels fell off
You have two identical jars in the kitchen. One holds table salt and the other baking soda, but both labels have fallen off. Both are white solids that look almost the same. How can you tell them apart?
Chemists rely on a substance’s properties性质. Every substance has its own set of identifying features, much like a person’s face, voice, and fingerprints.
Physical properties
A physical property物理性质 is one a substance shows without changing into a different substance. Common examples: color, state, odor, melting point, boiling point, hardness, density, solubility, electrical conductivity, thermal conductivity, and malleabilitymalleabilityThe ability to be hammered or rolled into thin sheets without breaking; being drawn into wire is called ductility. Most metals have both: copper is drawn into wire, and aluminum is rolled into foil. Lesson 1.4.3 returns to this..
Measuring a physical property doesn’t change the substance. Measure the density of an iron block, and it’s still iron.
Chemical properties
A chemical property化学性质 is one a substance shows only during a chemical change. Common examples: flammability, whether it supports burning, whether it breaks down when heated (stability), whether it reacts with oxygen or acids, and how corrosive it is.
The only way to find out whether something burns is to try lighting it, and afterward the part that burned has become different substances.
| Substance | Physical properties | Chemical properties |
|---|---|---|
| Oxygen | Colorless, odorless gas; not very soluble in water; slightly denser than air | Supports burning |
| Carbon dioxide | Colorless, odorless gas; dissolves in water; denser than air | Usually doesn’t burn or support burning; turns limewater cloudy |
| Iron | Silvery-gray metal; conducts electricity and heat; attracted to magnets | Rusts easily in damp air |
| Ethanol | Colorless liquid; distinctive smell; evaporates easily | Flammable |
Property or change? Read carefully
A property describes what a substance can do or tends to do. A change is something that is happening or has happened.
| Property | Change |
|---|---|
| Ethanol is flammable (chemical property) | The ethanol burned (chemical change) |
| Ethanol evaporates easily (physical property) | The ethanol evaporated (physical change) |
| Iron rusts in damp air (chemical property) | The iron skillet has rusted (chemical change) |
| Ice melts at 0 °C (physical property) | The ice melted into water (physical change) |
Sentences about properties often use words like “can,” “is able to,” “tends to,” “easily,” or describe what a substance generally does.
Properties determine uses
What a substance can be used for depends on its properties:
- Copper conducts electricity well, so it’s used for electrical wiring. (physical property)
- Nitrogen is very unreactive, so bags of chips are filled with it to keep the chips fresh. (chemical property)
- Dry ice absorbs a lot of heat as it sublimes, so it’s used to keep food frozen during shipping. (physical property)
- Hydrogen burns and releases a lot of heat, so it can be used as a fuel. (chemical property)
- Neon and other noble gases glow in different colors when electricity passes through them, so they’re used in neon signs. (physical property)
Try it: identify substances by their properties
Each set below has three unlabeled samples in random order. Click a test to run it, and the result is recorded in the table. Notice whether each test measures a physical or a chemical property. When you have enough evidence, choose a name for each sample:
Click a test above. The results will be recorded in this table.
A chemical property only shows up during a chemical change, so after a test like this, that portion of the sample has turned into something else.
Think about it: in the first set, which tests can’t tell the three samples apart at all? Which single test picks out the baking soda right away?
Example 1: Property or change?
Does each statement describe a physical property, a chemical property, a physical change, or a chemical change? (1) Ethanol evaporates easily. (2) The ethanol left in an open dish evaporated. (3) Ethanol is flammable. (4) Ethanol burns, forming carbon dioxide and water.
- (1) Ethanol evaporates easily. “Easily” signals a feature of ethanol itself. Evaporating forms no new substance, so this is a physical property.
- (2) The ethanol evaporated. This describes something that happened, and no new substance formed: physical change.
- (3) Ethanol is flammable. This is a feature of ethanol itself, and burning forms new substances, so it’s a chemical property.
- (4) Ethanol burns, forming carbon dioxide and water. This describes a process in which new substances form: chemical change.
Example 2: Oxygen or carbon dioxide?
You have two jars of colorless gas. One is oxygen and the other is carbon dioxide. How can you tell which is which?
- Try physical properties first. Both gases are colorless and odorless, so looking and smelling won’t help.
- Find a chemical property that differs. Oxygen supports burning; carbon dioxide doesn’t.
- Design the test. Lower a burning wooden splint into each jar. In oxygen it burns more brightly; in carbon dioxide it goes out.
- Another option. Pour a little limewater into each jar and swirl. The one that turns cloudy is carbon dioxide.
- Copper conducts electricity.
Conductivity shows up without any chemical change.
- Iron rusts in damp air.
This describes what iron tends to do, and rusting is a chemical change.
- Ice melts into water.
A process is happening, and no new substance forms.
- A candle is burning.
A process is happening, and it forms carbon dioxide and water.
- Hydrogen is flammable.
Flammability only shows up during burning, which is a chemical change.
- Carbon dioxide is denser than air.
Density is a physical property.
- The bread has gone moldy.
Something has happened, and new substances have formed.
- Gasoline evaporates easily.
This is a feature of gasoline itself, and evaporation is a physical change, so it's a physical property.
- An open bottle of concentrated hydrochloric acid loses mass over time.
Hydrogen chloride gas escapes from the solution into the air. No new substance forms.
Common mistakes
Mistake 1: Mixing up properties and changes. “Ethanol is flammable” is a chemical property; “the ethanol burned” is a chemical change. Watch for words like “can,” “tends to,” and “easily.”
Mistake 2: “You can see and smell evaporation, so it’s a chemical property.” Evaporating just turns a liquid into a gas without forming a new substance, so volatility is a physical property. The question is whether the property only shows up during a chemical change.
Mistake 3: “Testing a property never changes the substance.” Testing a physical property leaves the substance unchanged, but testing a chemical property turns the sample you tested into something else.
Quick check
Which of these is a chemical property?
Supporting burning only shows up during burning, a chemical change, so it's a chemical property. The other three are physical properties.
Which use depends mainly on a chemical property?
Nitrogen is unreactive and doesn't react with the food, which is a chemical property. A depends on conductivity, B on sublimation absorbing heat, and D on hardness, all physical properties.
Summary
- Physical properties show up without a chemical change: color, state, odor, melting point, boiling point, density, solubility, conductivity.
- Chemical properties only show up during a chemical change: flammability, supporting burning, stability.
- A property describes what a substance can do or tends to do; a change describes something happening or already done.
- Properties determine uses, and you can design tests based on different properties to identify substances.
On to AP (for later)
In Unit 3, AP explains differences in physical properties such as boiling point, melting point, and solubility using intermolecular forces and structure. Unit 4 studies which reactions a substance can undergo, its chemical properties. Identifying an unknown is also a classic AP lab task: choose tests that can tell the possibilities apart, then draw conclusions from the results. That’s AP Science Practice 2, Question and Method.