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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!
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Sign up for freeWhich of the following are elements? Select all that apply and explain your reasoning.
If the molecular formula of a compound was C2H16O4, what is the correct empirical formula?
Which of the following are true of compounds?
True or False. Pure substances are made up of one type of atom or compound and have a fixed chemical composition with properties that do not change.
What are the two broad categories for the composition of matter?
True or False. All of the gold atoms found in a gold bar would be an example of a compound.
Element and Compounds are ____, while anything made up of two or more ______ are ______
Which of the following is not a mixture?
Which of the following are elements? Select all that apply and explain your reasoning.
If the molecular formula of a compound was C2H16O4, what is the correct empirical formula?
Which of the following are true of compounds?
True or False. Pure substances are made up of one type of atom or compound and have a fixed chemical composition with properties that do not change.
What are the two broad categories for the composition of matter?
True or False. All of the gold atoms found in a gold bar would be an example of a compound.
Element and Compounds are ____, while anything made up of two or more ______ are ______
Which of the following is not a mixture?
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Team Elemental Composition of Pure Substances Teachers
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!
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.
We are going to spend some time looking at elements and compounds separately and some common examples of each!
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:
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.
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:
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!
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.
Check out Law of Definite Proportions for a deep dive into this concept!
Percentage composition tells us by mass the percentage of an individual element present in the 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!
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!
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.
The main differences between compounds and mixtures are:
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!
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What are the 2 types of elemental pure substances?
The two types of pure substances are elements and compounds. Elements contain one type of atom while compounds are made up of two or more atoms of different elements. They both have a fixed chemical composition and unchanging properties.
How do you calculate the mass of a pure substance?
To calculate the mass of a pure substance, you can look up the atomic mass of the element(s) on the periodic table, multiply the mass by 1 g/mol, and then add them up if it's a compound.
What are 10 examples of pure substances?
Here are ten examples of pure substances. Five of the examples will be elements and five will be compounds. Gold, Iron, Tin, Hydrogen, and Carbon are all elements. Water, Alcohol, Ammonia, Baking Soda, and Carbon Dioxide are all compounds.
How many elements does a pure substance contain?
The amount of elements a pure substance contains depends on the type of pure substance. An element only contains one type of element. A compound can contain two or more elements.
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