Observable Physical Properties of Matter
Observable physical properties of matter are characteristics that can be seen, felt, measured or otherwise detected without changing what the substance is. Color, shape, texture, odor, melting point and density are common examples. In brief: you can describe a piece of copper by its reddish-orange color, its shine, its ability to bend and its melting point of 1,085 °C, and none of these tests turns the copper into a different substance.
Scientists use physical properties to describe, compare and identify materials. This article explains the difference between qualitative and quantitative properties, lists the most useful physical properties, shows how to identify an unknown material, and corrects some typical misunderstandings.
What is a physical property?
A physical property can be observed or measured without altering the chemical identity of the substance. When you measure the length of an iron nail or its mass, the nail is still iron afterwards. When ice melts, it becomes liquid water, which is a different state but the same substance.
This contrasts with a chemical property, such as the ability of iron to rust, which can only be observed when the substance reacts and forms something new. Chemical properties are covered in later articles.
Qualitative and quantitative properties
Properties can be described in words or with numbers.
- Qualitative properties are described without numbers, such as color, odor, taste (never taste lab chemicals), texture, shine and clarity.
- Quantitative properties are measured and given as numbers with units, such as mass, volume, density, melting point, boiling point and length.
Qualitative descriptions are quick but depend on the observer. Quantitative measurements are more reliable, since different people measuring the same sample should obtain similar numbers.
Which physical properties are used most often?
Properties you can observe directly
Color, luster (shiny or dull), shape, texture, state at room temperature (solid, liquid or gas), odor and transparency. A fresh piece of sodium looks silvery and soft, whereas sulfur is a brittle yellow solid.
Properties that need measurement
Mass, volume, density, melting point, boiling point, solubility in water and electrical conductivity. These are especially useful because each pure substance has characteristic values.
Properties of how a material behaves
Hardness, brittleness, ductility (can be drawn into wires), malleability (can be hammered into sheets), elasticity and viscosity (resistance of a liquid to flow). Gold is both ductile and malleable, while glass is hard but brittle.
Physical properties of common substances
| Substance | State at 20 °C | Melting point (°C) | Boiling point (°C) | Density (g/cm3) |
|---|---|---|---|---|
| Water | Liquid | 0 | 100 | 1.00 (at 4 °C) |
| Ethanol | Liquid | −114 | 78 | 0.79 |
| Aluminum | Solid | 660 | 2,470 | 2.70 |
| Iron | Solid | 1,538 | 2,862 | 7.87 |
| Copper | Solid | 1,085 | 2,562 | 8.96 |
| Lead | Solid | 327 | 1,749 | 11.34 |
Values are rounded and are given for pure substances. Impurities change melting and boiling points, which is one way to tell that a sample is not pure.
How can properties identify an unknown material?
A single property is rarely enough because different substances can share a color or a similar density. A reliable identification combines several properties.
Worked example: identifying a metal cube
A shiny gray metal cube has sides of 2.0 cm and a mass of 63.0 g. It attracts a magnet only weakly or not at all.
- Calculate the volume: 2.0 × 2.0 × 2.0 = 8.0 cm3.
- Calculate the density: 63.0 g ÷ 8.0 cm3 = 7.9 g/cm3.
- Compare with the table: 7.9 g/cm3 matches iron at 7.87 g/cm3.
- Check another property: iron is strongly magnetic, but the cube is not, so the match is doubtful.
The density is close to that of iron, but the lack of magnetism suggests a different material, for example a non-magnetic steel alloy. The example shows why several properties should be combined before drawing a conclusion.
Intensive and extensive properties
Some properties depend on the amount of sample, and others do not.
- Extensive properties depend on how much matter is present: mass, volume and length. Doubling the sample doubles the value.
- Intensive properties do not depend on the amount: density, color, melting point and boiling point. A drop of water and a bucket of water both boil at 100 °C at standard pressure.
Intensive properties are the most useful for identifying substances for exactly this reason.
Why did early chemists record properties?
Before atoms and molecules were understood, the only way to tell materials apart was by their appearance and behavior. Craftspeople and early chemists tested metals by their color, hardness and how they responded to heat. According to a well-known story, Archimedes used the idea of density to test whether a crown was pure gold, although the details of the story are uncertain. Later, the systematic measurement of properties such as density and boiling point became the basis of modern chemical analysis.
Which mistakes are common?
Mistake 1: Treating color as proof of identity
Many different substances are white powders. Color is useful, but it must be combined with other tests.
Mistake 2: Thinking a physical change always keeps the same appearance
Melting or dissolving changes appearance dramatically, yet the substance is still the same. Appearance and identity are different things.
Mistake 3: Confusing mass with density
A large block of aluminum can have more mass than a small block of lead, but lead still has the higher density.
Mistake 4: Smelling or tasting unknown chemicals
Observation in the laboratory must be safe. Never taste a sample, and only waft vapors gently toward your nose when your teacher instructs you to do so.