Elemental Composition of Pure Substances

Imagine you were trying to describe types of doughnuts to your friend. You could choose to describe the doughnuts based on their physical state: donut holes, glazed donuts, and cream-filled donuts. You could also describe them according to what they are made of yeast donuts, cake donuts, or sourdough donuts. This is a loose, fun example of how we can classify matter! We can classify matter according to the physical state (solid, liquid, gas), or we can classify it according to its composition as a pure substance or mixture!

• We are going to do a quick overview of the classification of matter according to the composition of pure substances and mixtures
• Then, we will zoom in on the definition of pure substances
• After, we will look at some pure substance examples
• We will also explore the percentage composition and empirical formulas of pure substances
• Finally, we will briefly investigate the composition of mixtures

Classification of Matter According To Composition

When you think about classifying matter, you likely think of the three states of matter: solid, liquid, and gas. But, thinking back to the donut metaphor, we can also classify matter according to what it is made of (aka its chemical composition)! So, what makes up matter? Well, on a molecular level, all matter is made up of atoms that can form elements, compounds, and molecules! Even with all these different components, matter can be split into two broad categories based on its composition: pure substances and mixtures. We will spend most of our time on pure substances, but briefly cover mixtures! So, let’s dive into pure substances!

Definition of Pure Substances

A simple way to think about pure substances is as something that is made up of only one type of matter that always has the same properties, such as melting point, boiling point, density, etc.

Can you guess what are pure substances in your day-to-day life? I bet you may have salt (NaCl) and tin cans (Sn) in your pantry! These are perfect examples of pure substances because salt is made up of purely NaCl molecules, and tin is made up of only tin atoms.

But wait, you may have noticed a difference between salt and tin and asked how exactly salt is only made up of one type of matter when it’s two different atoms bonded together?

Pure substances can be further divided into two classes: elements and compounds.

Pure Substance Examples

We are going to spend some time looking at elements and compounds separately and some common examples of each!

Elements

If you think elements sound familiar, you are correctly thinking of the 118 organized in the periodic table. Tin is an example of an element!

Some common examples of elements are:

• Hydrogen (H), Oxygen (O), and Nitrogen (N) - usually occur as diatomic molecules
• Gold (Ag), Iron (Fe), and Copper (Cu) - are metals that usually exist by themselves
• Sodium (Na), Potassium (K), and Lithium (Li) - naturally occur only in compounds

Fig.1-Elements contain one type of atom

Compounds

The second class of pure substances are compounds!

Thinking back to our salt and tin example, table salt is a compound. Every sample of table salt will always be composed of two bonded atoms (sodium and chlorine), but through a chemical change, it can be broken down into sodium and chlorine atoms!

Can you think of any common examples of compounds?

Here are some:

• Water -H₂O
• Carbon dioxide -CO₂
• Baking soda -NaHCO₃
• Alcohol - C₂H₆O

Fig.2-H₂O contains two different atoms from two different elements

Before we define the other category of matter known as mixtures, it is essential to understand how to find out the exact percentage of elements that make up a compound!

Percentage Composition and Empirical Formula

Often, if we analyze a compound, we will want to determine the ratio of all the different elements present in the compound relative to the compounds as a whole. You may think, doesn't the ratio change depending on how much of the compound we have? Well, because of a concept known as the law of definite proportions, compounds will always be composed of a fixed ratio of elements by mass regardless of the size of the sample! We can determine this mass ratio by calculating the percentage composition of the compound.

Fig.3-Conceptualization of percentage composition of an element in a compound.

The formula for percent composition is:

$$\%\,Composition=\frac{mass\,of\,element\,present\,in\,compound}{total\,molecular\,mass\,of\,compound}*100\%$$

This is a very simple demonstration of how to find percent composition. Please check out Molecular and Empirical Formulas, where we will cover more advanced examples!

Empirical Formula

So, let’s say a compound had a molecular formula of C₆H₁₀O₄. The empirical formula would be C₃H₅O₂. It’s similar to reducing fractions down to their simplest form!

We can often use percentage compositions to determine the empirical formula of a compound.

Composition of Mixtures

As we come to an end, we are going to look at the second type of substance: mixtures!

To understand mixtures, let’s say we took NaCl and dissolved it into water. What does the combination of these two pure substances create? The mixture of saltwater.

Fig.4-Gold and Iron are elements that when mixed together become a mixture and yet maintain their individual chemical properties.

The main differences between compounds and mixtures are:

1. In mixtures, pure substances are physically “mixed” together, whereas, in compounds, atomic elements are chemically bonded together.
2. This means that salt and water have the same chemical properties before and after creating saltwater. However, when the compound NaCl, is separated into its atomic components, pure sodium becomes a shiny metallic solid, and chlorine is a greenish gas that, when bonded together, forms a new compound of white crystals of salt with new properties.

Some examples of mixtures include things like soil, tea/juices, gasoline, and gold mixed with iron!