If something is a physical property, it is possible to measure the property through observation that does not irreversibly changing the material that has the property. Chemical properties, on the other hand, are attributes of a substance that allow for a chemical change, often irreversible. They can't be observed without performing chemical experiments that result in chemically altering the material. When the experiment is complete, it is clear whether the material has the chemical property that the experiment was designed to detect. The more physical and chemical properties we define, the easier it is to identify the material in question accurately.

Generally, physical properties can be determined through more passive measurement (e.g. placing an item on a scale or noting qualitative features). Occasionally, physical properties will be measured with more active methods (e.g. measuring conductivity or resistance by applying a current of electrons).

Chemical reactions will nearly always require active application of some chemical compound, a different substance, or energy. Just as with physical reactions, there are some chemical properties, like oxidation (i.e. rust), which will display naturally through observation.

Throughout physical and chemical changes, we also want to define two terms: extensive properties and intensive properties. Extensive properties rely on the amount of matter present (e.g. mass or caloric energy), and intensive properties are only dependent on the identity of the substance.

We will enumerate some examples of chemical properties and other examples of physical properties to provide some context for studying new substances.

Density is a physical property. This is because it can be determined without performing chemical experiments. To find the density of a material, you have to know the volume and weight. The weight, in ounces or grams, can be found by weighing the material on a scale. The volume, in cubic inches or cubic centimeters, can be found by placing the material in a container filled with liquid and measuring the volume of the liquid that overflows. The resulting density is expressed in ounces per cubic inch or grams per cubic centimeter. For large materials, the corresponding density is expressed as pounds per cubic foot or kilograms per cubic meter. For liquids, density is described as pounds per gallon or kilograms per liter.

Solubility is a physical property. The reason is because it can be determined by simple observation and does not change the chemical composition of the material. For example, when salt dissolves in water, it is still salt. Whether a material is soluble in a solvent or not can be found by placing a sample of the material in the solvent, stirring and checking if it dissolves. If the material is soluble, the solubility is the maximum amount of material that dissolves in a solvent at a given temperature. The units of solubility are grams per 100 grams of solvent, grams per liter or moles per liter.

Color is a physical property. Why? Because determining the color of a material does not involve any chemical experiments or changes. Color is the result of some wavelengths of light being absorbed by the material and other wavelengths being reflected. For example, a material might absorb some green and blue light with the result that the material looks reddish. If it absorbs all shades equally, the material looks gray or black. If it reflects all light, it looks white. Color often becomes a more complicated property when color changes and energetic emission of color is involved.

Flammability is a chemical property. It involves chemical change. To determine whether a material is flammable, you test the material by applying heat. If it burns, the material undergoes a chemical reaction, demonstrating its flammability. Flammability testing is carried out on a small sample of the material, according to test protocols relevant to the type of flammability. For example, testing can be with an open flame applied beneath the sample, or the sample can be heated to see if it will burst into flame. Such tests can determine the combustion temperature, the heat of combustion, and the combustion byproducts as well as the flammability.

When we observe the reactivity of multiple substances, this can be indicative of a wide variety of interactions and properties. Common reactions typically alter the chemical bonds of reactants creating new chemical reaction products. This can also involve a change in states of matter. A common chemical reaction – that illuminates the composition and chemical identity of two substances – is the interaction between sodium bicarbonate and acetic acid (more commonly known as baking soda and vinegar). This reaction produces sodium acetate, water (in a liquid state), and carbon dioxide. Another common reaction involves the production of iron oxide (i.e. rust) from the interaction of iron and oxygen.

Some of these chemical reactions might also involve examples of physical changes (e.g. a change of state from liquid to gas).

Melting point is a physical property. Melting does not involve a chemical change. The melting point is the temperature at which a solid changes to a liquid. You can find it by heating a solid material and recording the temperature at which it melts. Typically, the temperature steadily rises until it reaches the melting point of the material. At this point, the temperature rises more slowly or even stops as the material absorbs the heat to produce the melting. When all the material has melted, the temperature continues to rise. In addition to the melting point, the heat of fusion for the material can be found if the heat added while the temperature remained steady is measured.

Boiling point is a physical property. Vaporization is a physical change of state that does not involve a chemical reaction. Heating a liquid until it vaporizes allows the determination of the boiling point of the material. When the liquid is heated steadily, the temperature of the liquid rises until it hits the boiling point. At the boiling point, the temperature stops increasing as the heat of vaporization is absorbed by the material and the liquid is changed to gas. If the gas is collected and condensed, it proves that the boiling point is, in fact, a physical property because the process can easily be reversed, and the original material can be recovered.