The physical properties of matter are any properties that can be perceived or observed without changing the chemical identity of the sample. In contrast, chemical properties are those that can only be observed and measured by performing a chemical reaction, thus changing the molecular structure of the sample.
Because physical properties include such a wide array of characteristics, they are further classified as either intensive or extensive and either isotropic or anisotropic.
Intensive and Extensive Physical Properties
Intensive physical properties do not depend on the sample's size or mass. Examples of intensive properties include boiling point, state of matter, and density. Extensive physical properties depend on the amount of matter in the sample. Examples of extensive properties include size, mass, and volume.
Isotropic and Anisotropic Physical Properties
Isotropic physical properties do not depend on the orientation of the specimen or direction from which it is observed. Anisotropic properties depend on the orientation. While any physical property could be assigned as isotropic or anisotropic, the terms are usually applied to help identify or distinguish materials based on their optical and mechanical properties.
For example, one crystal might be isotropic with respect to color and opacity, while another might appear a different color depending on the viewing axis. In a metal, grains might be distorted or elongated along one axis compared with another.
Examples of Physical Properties
Any property you can see, smell, touch, hear, or otherwise detect and measure without performing a chemical reaction is a physical property. Examples of physical properties include:
- Color
- Shape
- Volume
- Density
- Temperature
- Boiling point
- Viscosity
- Pressure
- Solubility
- Electric charge
Physical Properties of Ionic vs. Covalent Compounds
The nature of chemical bonds plays a role in some physical properties displayed by a material. The ions in ionic compounds are strongly attracted to other ions with opposite charge and repelled by like charges. Atoms in covalent molecules are stable and not strongly attracted or repelled by other parts of the material. As a consequence, ionic solids tend to have higher melting and boiling points compared with the low melting and boiling points of covalent solids.
Ionic compounds tend to be electrical conductors when they are melted or dissolved, while covalent compounds tend to be poor conductors in any form. Ionic compounds are usually crystalline solids, while covalent molecules exist as liquids, gases, or solids. Ionic compounds often dissolve in water and other polar solvents, while covalent compounds are more likely to dissolve in nonpolar solvents.
Chemical Properties
Chemical properties encompass characteristics of matter that can be observed only by changing the chemical identity of a sample—examining its behavior in a chemical reaction. Examples of chemical properties include flammability (observed from combustion), reactivity (measured by readiness to participate in a reaction), and toxicity (demonstrated by exposing an organism to a chemical).
Chemical and Physical Changes
Chemical and physical properties are related to chemical and physical changes. A physical change alters only the shape or appearance of a sample and not its chemical identity. A chemical change is a chemical reaction, which rearranges a sample on a molecular level.
Learning Objectives To separate physical from chemical properties. All matter has physical and chemical properties. Physical properties are characteristics that scientists can measure without changing the composition of the sample under study, such as mass, color, and
volume (the amount of space occupied by a sample). Chemical properties describe the characteristic ability of a substance to react to form new substances; they include its flammability and susceptibility to corrosion. All samples of a pure substance have the same chemical and physical properties. For example, pure copper is always a reddish-brown solid (a physical property) and always dissolves in dilute nitric acid to produce a blue solution and a brown gas (a chemical property). A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. Silver is a shiny metal that conducts electricity very well. It can be molded into thin sheets, a property called malleability. Salt is
dull and brittle and conducts electricity when it has been dissolved into water, which it does quite easily. Physical properties of matter include color, hardness, malleability, solubility, electrical conductivity, density, melting point, and boiling point.Physical Property
For the elements, color does not vary much from one element to the next. The vast majority of elements are colorless, silver, or gray. Some elements do have distinctive colors: sulfur and chlorine are yellow, copper is (of course) copper-colored, and elemental bromine is red. However, density can be a very useful parameter for identifying an element. Of the materials that exist as solids at room temperature, iodine has a very low density compared to zinc, chromium, and tin. Gold has a very high density, as does platinum. Pure water, for example, has a density of 0.998 g/cm3 at 25°C. The average densities of some common substances are in Table \(\PageIndex{1}\). Notice that corn oil has a lower mass to volume ratio than water. This means that when added to water, corn oil will “float.”
Table \(\PageIndex{1}\): Densities of Common Substances| blood | 1.035 |
| body fat | 0.918 |
| whole milk | 1.030 |
| corn oil | 0.922 |
| mayonnaise | 0.910 |
| honey | 1.420 |
Hardness helps determine how an element (especially a metal) might be used. Many elements are fairly soft (silver and gold, for example) while others (such as titanium, tungsten, and chromium) are much harder. Carbon is an interesting example of hardness. In graphite, (the "lead" found in pencils) the carbon is very soft, while the carbon in a diamond is roughly seven times as hard.
Melting and boiling points are somewhat unique identifiers, especially of compounds. In addition to giving some idea as to the identity of the compound, important information can be obtained about the purity of the material.
Chemical Properties
Chemical properties of matter describe its potential to undergo some chemical change or reaction by virtue of its composition. The elements, electrons, and bonds that are present give the matter potential for chemical change. It is quite difficult to define a chemical property without using the word "change". Eventually, after studying chemistry for some time, you should be able to look at the formula of a compound and state some chemical property. For example, hydrogen has the potential to ignite and explode given the right conditions—this is a chemical property. Metals in general have the chemical property of reacting with an acid. Zinc reacts with hydrochloric acid to produce hydrogen gas—this is a chemical property.
A chemical property of iron is its capability of combining with oxygen to form iron oxide, the chemical name of rust (Figure \(\PageIndex{2}\)). The more general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions of chemical changes are burn, rot, explode, decompose, and ferment. Chemical properties are very useful in identifying substances. However, unlike physical properties, chemical properties can only be observed as the substance is in the process of being changed into a different substance.
Table \(\PageIndex{2}\): Contrasting Physical and Chemical Properties| Gallium metal melts at 30 oC. | Iron metal rusts. |
| Mercury is a very dense liquid. | A green banana turns yellow when it ripens. |
| Gold is shiny. | A dry piece of paper burns. |
Example \(\PageIndex{1}\)
Which of the following is a chemical property of iron?
- Iron corrodes in moist air.
- Density = 7.874 g/cm3
- Iron is soft when pure.
- Iron melts at 1808 K.
Solution
"Iron corrodes in moist air" is the only chemical property of iron from the list.
Exercise \(\PageIndex{1A}\)
Which of the following is a physical property of matter?
- corrosiveness
- pH (acidity)
- density
- flammability
Exercise \(\PageIndex{1B}\)
Which of the following is a chemical property?
- flammability
- melting point
- boiling point
- density
Summary
A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. Physical properties include color, density, hardness, and melting and boiling points. A chemical property describes the ability of a substance to undergo a specific chemical change. To identify a chemical property, we look for a chemical change. A chemical change always produces one or more types of matter that differ from the matter present before the change. The formation of rust is a chemical change because rust is a different kind of matter than the iron, oxygen, and water present before the rust formed.