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Molecular Substance: Definition & Properties

Molecular Substance: Definition & Properties
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  • 0:00 Definition of…
  • 0:42 Three Types of…
  • 1:30 Understanding Covalent Bonds
  • 2:09 Molecular Substance Properties
  • 4:15 Lesson Summary
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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Hydrogen peroxide, water, and carbon dioxide all have something in common: They are examples of molecular substances. Discover what a molecular substance is, and learn about the different properties used to define these compounds.

Definition of Molecular Substances

Scientists estimate that 70% of our body's weight is made up of water. We need it for all parts of our body to function. Can you guess how we classify water, which plays such a large role in living organisms? It's a molecular substance, which is a substance with two or more atoms, the smallest units of matter, joined together by a covalent bond. A covalent bond is the link created through the sharing of electrons that holds these atoms together. We can group molecular substances into three broad categories based on the types of molecules that comprise them.

Three Types of Molecular Substance

Generally, small molecules don't have a lot of atoms in their structure. Molecular substances made of small molecules are typically room-temperature gasses and liquids. For instance, our pal water has small molecules. Also, the gasoline we put in our vehicles is made up of small molecules.

Unlike small molecules made of few atoms, large molecules are made of many atoms. The vitamins we take and even the hormones circulating in our bodies are great examples of large molecules.

A polymer is formed when smaller molecules join together to form one large molecule. Typically, polymers are relatively big in size, often much larger than the large molecules we discussed before. We can find them in items ranging from plastics to our human DNA.

Understanding Covalent Bonds

As we learned earlier, when you have a molecular substance, something has to hold each atom together. That something is a covalent bond. Covalent bonds are ideal in that they don't break easily and vary in length depending on the size of the atom. Let's take a better look at our pal water, as it's a great example of a small molecule that forms a covalent bond.

Covalent bonding between atoms in a molecule of water
water

A water molecule contains hydrogen (H) and oxygen (O) atoms. The little silver balls represent electrons between both atoms. With oxygen bearing two electrons and hydrogen having one, they will share their electrons to form a covalent bond.

Molecular Substance Properties

Let's explore some of the common physical properties, or properties that are measurable, applicable to all molecular substances.

The melting point is a temperature that indicates when a solid substance changes to a liquid. The boiling point is a temperature that indicates the point at which a liquid changes to a gas, or vapor. Molecular substances tend to have relatively low melting and boiling points, which explains why they are usually gasses and liquids.

As we've discussed, the covalent bonds within molecular substances are very strong. However, the physical properties of a molecular substance are not determined by these bonds. Rather, the physical properties depend on something called intermolecular force. What is intermolecular force? It's the force of attraction between atoms. When this force is weak, it contributes to the low melting and boiling points in molecular substances.

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