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!
We will go into more detail on all the components of matter, but for a more thorough review, check out Atomic Structure!
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.
Pure substances are matter that has a fixed chemical composition and properties that do not change.
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!
Elements are made up of only one type of atom and cannot be broken down into any other substance.
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
Some elements tend to exist in the form of diatomic molecules, which means two atoms of the same element are bonded together, such as hydrogen gas (H2), nitrogen gas (N2), oxygen gas (O2), fluorine gas (F2), chlorine gas (Cl2), bromine (Br2), and iodine (I2). These are still elements because the atoms are the same! Check out the Periodic table for more information on these molecules.
Compounds
The second class of pure substances are compounds!
Compounds are substances containing identical molecules that are made up of two or more atoms of different elements bonded together in a fixed structure. They can be chemically broken down into individual elements or simpler 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!
Compounds are always made up of molecules of two or more atoms bonded together. But, as we saw with diatomic molecules, not all molecules are compounds!
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
To check your understanding of the difference between elements and compounds, identify if the following are elements or compounds: NaF, Re, I2 NH3, Si
This should have been relatively easy! If not, review the above material and try again! The correct answers are Compound, Element, Element, Compound, Element!
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.
Percentage composition tells us by mass the percentage of an individual element present in the compound.
Fig.3-Conceptualization of percentage composition of an element in a compound.
The formula for percent composition is:
$$\%\,Composition=\frac{mass\,of\,element\,present\,in\,compound}{total\,molecular\,mass\,of\,compound}*100\%$$
Let’s play this out in a quick and simple example.
Find the percentage composition of sodium and chlorine in NaCl?
1) The first step is to determine the molecular mass of NaCl by using the periodic table to find the sum of Na and Cl atomic mass. Note, if there was more than one Na or Cl atom you would multiply the mass by that number.
The mass of Na = \(22.99\frac{g}{mol}\)
The mass of Cl = \(35.45\frac{g}{mol}\)
So the sum: $$22.99\frac{g}{mol}+ 35.45\frac{g}{mol}= 58.44 \frac{g}{mol}$$
2) The second step is to divide the individual atomic masses by the sum total molecular mass of the compound.
Na: \((\frac{22.99\frac{g}{mol}}{58.44\frac{g}{mol}})*100\% = 39.3\%\)
Cl: \((\frac{35.45\frac{g}{mol}}{58.44\frac{g}{mol}}) *100\%= 60.7\%\)
So, NaCl is made up of 39.3% Na and 60.7% Cl.
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
The simplest whole-number ratio of the elements in a compound is referred to as the 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.
To learn more about this relationship between empirical formula and percent composition check, Molecular and Empirical Formulas!
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.
Mixtures are comprised of two or more substances physically mixed together that don’t lose their individual chemical properties and can be separated back into individual substances through physical change.
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:
- In mixtures, pure substances are physically “mixed” together, whereas, in compounds, atomic elements are chemically bonded together.
- 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!
This is just a brief overview of the composition of mixtures. Check out the composition of mixtures to find out about the two types of mixtures and how to identify them!
Elemental Composition of Pure Substances - Key takeaways
- Matter can be classified according to its composition into two categories: pure substances and mixtures
- Pure substance refers to any matter with fixed chemical composition and its properties always remain constant.
- It can be divided into elements and compounds
- Elements are composed of one type of atom (think the periodic table of elements)
- Compounds are composed of two or more atoms of different elements bonded together (H2O, CO2)
- We can use percent composition and empirical formulas to figure out the ratio of elements in the compound
- Mixtures are made up of two or more pure substances physically mixed together.
References
- Brown, Theodore L, H E. LeMay, Bruce E. Bursten, Catherine J. Murphy, Patrick M. Woodward, and Matthew Stoltzfus. Chemistry: The Central Science. , 2015. Print.
- Fraser, Simon. Classifying Matter According to Its Composition, LibreTexts: Chemistry, 2020.
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