Elements, Compounds, and Mixtures

All matter can be sorted into a few basic categories, and telling them apart is a common test skill. The three you need are elements, compounds, and mixtures. They differ in how their atoms are combined, and once you see the pattern, sorting substances becomes quick.

Elements: One Kind of Atom

An element is a pure substance made of only one kind of atom. Oxygen, gold, hydrogen, and iron are all elements — each is built from a single type of atom and cannot be broken down into anything simpler by ordinary chemistry. The periodic table is simply a chart of all the known elements.

Compounds: Atoms Chemically Joined

A compound forms when two or more different elements are chemically bonded together in a fixed ratio. Water is a compound: two hydrogen atoms bonded to one oxygen atom. The important part is that a compound has new properties, different from the elements that make it. Hydrogen and oxygen are both gases, but combined as water they form a liquid. Because the atoms are chemically bonded, you cannot separate a compound just by physical means.

Mixtures: Just Combined, Not Bonded

A mixture is two or more substances physically combined but not chemically bonded. Because they are only mixed, each substance keeps its own properties, and you can usually separate them by physical methods like filtering, evaporating, or picking them apart. Salad, sand and water, and air are all mixtures. The key contrast: in a compound the atoms are bonded and hard to separate; in a mixture they are just mingled and easy to separate.

A quick test question strategy: if the substances are chemically joined in a fixed ratio with new properties, it is a compound. If they are simply combined and keep their own properties, it is a mixture.

Watch: A Short Video Lesson

FuseSchool walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for These Questions

1. Element = one kind of atom (cannot be broken down).
2. Compound = different elements chemically bonded, new properties, fixed ratio.
3. Mixture = substances combined but not bonded, keep their properties.
4. Compounds are hard to separate; mixtures separate by physical methods.

Practice

1. What is an element?
2. Is water an element, a compound, or a mixture?
3. What holds the atoms together in a compound?
4. Give one example of a mixture.
5. Why can you separate a mixture by physical methods?
6. How is a compound different from a mixture?

Answers

1. A pure substance made of only one kind of atom.
2. A compound.
3. Chemical bonds.
4. Any of: salad, sand and water, air, salt water.
5. The substances are not chemically bonded, so they keep their own properties.
6. A compound’s atoms are chemically bonded; a mixture’s are just combined.

Where This Fits in Your Science Prep

This builds on atoms and their parts and leads into states of matter and chemical reactions. See all topics on the Science Topics Hub.