Physical vs Chemical Properties of Matter
A physical property is a characteristic of a substance that can be observed or measured without changing its identity, while a chemical property describes how a substance can change into a different substance in a chemical reaction. In brief: the melting point of iron (1,538 °C) is a physical property, and the ability of iron to rust in moist air is a chemical property.
Properties are the clues chemists use to identify substances and predict their behavior. This article separates properties into physical and chemical ones, explains why a chemical property can only be seen during a reaction, compares examples in tables, works through a classification exercise and explains the link between properties and uses.
What are physical properties?
Physical properties can be observed or measured while the substance stays the same. Color, density, hardness, melting point, boiling point, electrical conductivity, solubility and state at room temperature are physical properties. When you measure the boiling point of ethanol, you heat it, but it is still ethanol afterwards, as the vapor condenses back into the same liquid.
Physical properties are either intensive, such as density and melting point, which do not depend on the amount, or extensive, such as mass and volume, which do.
What are chemical properties?
A chemical property describes the ability of a substance to take part in a chemical reaction and become a new substance. Flammability, reactivity with acids, reactivity with oxygen, tendency to rust or tarnish and the ability to decompose when heated are chemical properties.
To observe a chemical property you have to attempt the reaction, and the original substance is used up in the process. You cannot find out whether paper is flammable by merely looking at it. You find out by trying to burn it, and then the paper is gone.
Physical and chemical properties compared
| Feature | Physical property | Chemical property |
|---|---|---|
| Identity of substance during observation | Unchanged | Changes into new substances |
| How it is found | Observation or measurement | Reaction with another substance or conditions |
| Sample after the test | Can often be reused | Used up or altered |
| Examples | Density, color, melting point | Flammability, reactivity with water |
Examples for common substances
| Substance | Physical properties | Chemical properties |
|---|---|---|
| Sodium | Soft, silvery, melts at 98 °C | Reacts violently with water; burns in air |
| Gold | Yellow, malleable, melts at 1,064 °C | Very unreactive; does not rust |
| Hydrogen | Colorless gas, very low density | Burns in air to form water |
| Marble (calcium carbonate) | White, hard solid | Fizzes with acid; decomposes when strongly heated |
| Wood | Brown, fibrous, less dense than water | Flammable; rots |
| Helium | Colorless gas, lower density than air | Does not burn or react |
Worked example: classifying properties
Decide whether each statement is a physical or chemical property, and say why.
- “Copper conducts electricity.” Conduction can be tested without changing the copper. This is physical.
- “Silver tarnishes in air containing sulfur compounds.” A new substance, silver sulfide, forms. This is chemical.
- “Ethanol boils at 78 °C.” Boiling is a change of state, and the ethanol remains ethanol. This is physical.
- “Magnesium burns with a bright white light.” It forms magnesium oxide. This is chemical.
- “A block has a mass of 54 g and a volume of 20 cm3.” The density is 54 ÷ 20 = 2.7 g/cm3, a physical property that matches aluminum.
A helpful test is the question: “Do I need to change the substance into something else to observe this property?” If the answer is yes, it is a chemical property.
How do properties decide the use of a material?
Engineers select materials by combining physical and chemical properties. Aluminum is used for aircraft because it has low density (physical) and forms a protective oxide layer that resists corrosion (chemical). Gold is used for jewelry and electrical contacts because it is malleable and does not tarnish. Helium fills balloons because it is less dense than air and is chemically inert, and hydrogen, although lighter, is flammable and is therefore rarely used for this purpose.
How did chemists begin to describe properties?
Ancient metalworkers recognized the properties of metals by working them, and alchemists described substances by color, smell and behavior in fire. In the 18th century, chemists such as Antoine Lavoisier introduced quantitative measurements, including mass and density, to describe substances. Later, the systematic collection of properties led to the periodic table by Dmitri Mendeleev in 1869, which grouped elements with similar physical and chemical properties.
Which mistakes are common?
Mistake 1: Treating a change of state as chemical
Melting, boiling and freezing are physical changes, and the related properties are physical.
Mistake 2: Saying that a chemical property is the result of the reaction
A chemical property is the ability to react, such as flammability. The new substances are products of a chemical change.
Mistake 3: Mixing up color changes
The color of a substance is physical. A color change during a reaction, such as a copper roof turning green, is evidence of a chemical change.
Mistake 4: Thinking that chemical properties can be measured without a reaction
Chemical properties can only be seen when the substance takes part in a reaction.