What are Pure and Impure Substances?

A pure substance is a single chemical substance that is made up of only one type of element or compound. Pure substances can exist in different physical forms, such as solid, liquid, or gas, but their chemical properties remain the same.

The Two Types of Pure Substances

The two types of pure substances are elements and compounds.

Elements

Elements are substances that are made up of just one type of atom, and all elements are pure substances. In the image below, you can see an old copper teapot.

A vintage copper jug with a tapered, pointed lid. The jug has a textured surface and a curved handle, with visible signs of wear, giving it an aged appearance.

This is an example of a pure substance, made purely out of the element copper (Cu). If you zoom in on a pure substance, such as this copper teapot, you’ll see something similar to the diagram below.

A circular diagram labelled 'Element' with multiple evenly spaced red dots on a pale background.

The substance is made up of the same type of particles. In this case, it is the same type of atom.

Another example of a pure substance is the graphite found in pencil lead. When looking closely at the lead, you will see that it is made up of only carbon atoms. Graphite is a naturally occurring mineral form of carbon and is a pure substance.

Compounds

The other type of pure substance is compounds, which are made up of only one type of molecule. Compounds are formed when two or more elements chemically combine in a fixed ratio. If you zoom in on a compound, you will see something similar to the diagram below.

If we zoom in on a compound at a molecular level, we will see that atoms are bonded together in a specific arrangement. This arrangement of atoms can be represented by a chemical formula, such as CO2.

A circular arrangement of blue dots partially covering red circles, representing molecules of a compound. The word "Compound" is written below the arrangement.

  • This is different from mixtures, which can be separated by physical means

For example, when hydrogen and oxygen combine, they form water. If you zoom in on pure water, you’ll see water (H2O) molecules that make up the compound:

A molecular model of water with two red atoms connected to a central grey atom, representing the oxygen and hydrogen atoms respectively. The label "Water" is written below the model.

This is where the chemical formula (H2O) for water comes from. There is an oxygen atom in the middle and two hydrogen atoms attached to it.

  • In the image, the oxygen atoms are red and the hydrogen atoms are white

However, it is difficult to get substances that are completely pure, as they almost always contain trace amounts of other substances.

Impure Chemical Substance

As you can see below, if you drop salt into a beaker of pure water, the result looks like this:

A glass beaker filled with water and salt at the bottom. An enlarged, magnified section of the water and salt mixture shows individual molecules: green "Na+" (sodium) ions, pink "Cl-" (chloride) ions, and smaller "H2O" molecules interspersed. Labels identify "Water (H2O)" pointing to the water in the beaker and "Salt (NaCl)" pointing to the salt. The enlarged section also has labels for the individual ions and water molecules.

When you zoom in on the sodium chloride salt (NaCl) at the bottom of the beaker, you can see it hasn’t dissolved yet. The salt that hasn’t dissolved is a pure substance, consisting of just sodium chloride.

However, the water in the beaker is not pure because it has sodium and chlorine ions (from the salt) that have dissolved in the water. After mixing the beaker’s contents, no pure substances remain and the solution turns into salt water.

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