Pure Substance in Chemistry — Definition, Properties & Examples

Written by

Malcolm McKinsey

January 12, 2023

Fact-checked by

Paul Mazzola

Pure substance definition

Pure substances in chemistry are a classification of matter (substances) that are made up of all the same type of atom (elements), or all the same, molecules (covalent bonds), or all the same ionically bonded elements (compounds).

What is a pure substance?

In chemistry, pure substances are only two things: elements or compounds. Elements are substances made up of only one kind of atom. Currently, the Periodic Table of the Elements recognizes 118118 such pure substances. Compounds are molecules (covalently bonded) or units of ionic compounds.

Properties of pure substances

Whether element, molecule, or compound, pure substances share similar physical and chemical properties. These predictable properties make pure substances the foundation of chemistry because known interactions can be used to form new materials and new types of matter.

Physical properties

No matter where a given pure substance is found, it will have identical physical properties to every other pure substance of the same chemical makeup. Pure substances have a constant composition. Sodium chloride from the Dead Sea is physically the same as sodium chloride mined in Kansas.

This is a partial list of physical properties that can be observed, measured, and predicted for pure substances under normal conditions (room temperature and one earth atmosphere of pressure):

• State of matter

• Mass

• Compressibility

• Density

• Color

• Electrical and thermal conductivity

• Boiling point

• Melting point

• Heat capacity

• Vapor pressure (for liquids)

• Heat of vaporization (for liquids)

• Surface tension (for liquids)

Chemical properties

All pure substances will also show the same chemical properties regardless of where they are found. Chemical properties, meaning behavior of the pure substance during chemical changes, include:

• Flammability

• Toxicity

• Acidity or basicity

• Reactivity

• Solubility

• Electronegativity

• Ionization

• Coordination number (the number of atoms bonded to a given atom)

What is not a pure substance? {impure}

All matter is either a pure substance (atoms or compounds) or mixtures. Heterogeneous mixtures are not pure substances because they have variable compositions. Mixtures are formed by combining different substances.

Pure substance vs. mixture

There are two types of mixtures; homogeneous mixtures and heterogeneous mixtures.

Heterogeneous mixtures do not have the same characteristics as pure substances. For example, one sample of a heterogeneous mixture can have different physical and chemical properties than a second sample from the same mixture.

In addition, mixtures are made up of more than one type of atom, compound, or molecule, making them impure.

Examples of mixtures are a bowl of different fruits, a jar full of coins, and sand at the beach.

Pure substance examples

As said earlier, every one of the 118 elements on the Periodic Table is a pure substance, so you can start your search for examples with hydrogen, atomic number 1, and continue through to oganesson at 118.

Pure substances that form compounds and molecules include, well, everything around you that is not a mixture. For example, the molecule for sugar or sucrose, ${C}_{12}{H}_{22}{O}_{11}$ is a pure substance similar in sweetness to another pure substance, the molecule fructose, ${C}_{6}{H}_{12}{O}_{6}$. Millions of molecules, all pure substances, exist.

Baking soda (sodium bicarbonate, $NaHC{O}_{3}$) is a molecule, while lithium hydroxide ($LiOH$) is only a compound, not a molecule; hundreds of thousands of such compounds, all pure substances, exist.

Pure compounds

Atoms of different elements can bond together using ionic bonds, but these are not considered molecules. Instead, they are still pure substances and are often referred to just as units of the compound.

An example of a chemical compound that is a pure substance but not a molecule is sodium chloride, table salt, which is formed when a sodium atom gives up an electron to a chlorine atom, creating a positive sodium ion and a negative chlorine ion called chloride. This chemical reaction created the pure substance we know as table salt.

Here is the chemical formula for salt:

The pure substance formed, $NaCl$, is formed through electrostatic interactions, not the much stronger chemical bonds of covalently bonded atoms that form molecules.

Pure elements

Any substance composed entirely of one type of atom is considered an element and a pure substance. From the lightest element, hydrogen, to the heaviest element, oganesson.

The current iteration of the Periodic Table of the Elements, as approved by the International Union of Pure and Applied Chemistry (IUPAC), shows 118 elements, every one of them a pure substance.

Pure substance molecules

Covalent bonds (two or more atoms sharing electrons) create molecules. A molecule is an arrangement of two or more atoms forming the smallest identifiable unit of a pure substance. If the molecule is broken apart, it has neither the physical nor chemical properties of the pure substance.

Molecules exist in many degrees of complexity. The simple diatomic hydrogen is composed of just two hydrogen atoms forming ${H}_{2}$. Or ${O}_{2}$, which is the "dioxide" part of carbon dioxide. And the incredibly complex molecule known as PG5.

All compounds are pure substances. All molecules are types of compounds, but not all compounds are molecules.

Pure substances quiz

Don’t let chemistry confuse you! See if you can answer these questions, then check your work against our answers below.

1. What does pure substance mean in science?

2. What makes something a pure substance?

3. Please list at least two physical properties of pure substances.

4. Please list at least two chemical properties of pure substances.

5. Please list a pure substance that is an element, one that is a molecule, and a pure substance that is a compound but not considered a molecule.

If you struggled with these, refer back to the reading or do further research and questioning on your own.

1. In science, the term pure substance means matter that has a uniform composition.

2. Something is a pure substance if it has identical physical and chemical properties to all other examples of the same substance if it is made of only one type of atom (elements), one type of molecule (covalently bonded matter), or one type of compound (ionically bonded matter).

3. You may have mentioned any of these physical properties of pure substances: color, mass, weight, temperature, thermal and electrical conductivity, density, and state of matter.

4. You may have mentioned any of these chemical properties of pure substances: flammability, toxicity, acidity or basicity, reactivity, heat of combustion, or thermodynamic stability.

5. A pure substance that is an element is anything from the Periodic Table such as Einsteinium (99), one that is a molecule is sugar, ${C}_{12}{H}_{22}{O}_{11}$, and a pure substance that is a compound but not considered a molecule is sodium chloride, $NaCl$ (table salt).