Recall that the law of conservation of mass states that in a chemical reaction, mass cannot be created or destroyed. Since mass is a measure of the amount of matter, this law is also known as the law of conservation of matter.
Chemical equations show that mass (and therefore matter) is conserved in a chemical reaction. This means that if the total mass of the reactants in a chemical reaction is \(20\text{ g}\), then the total mass of the products will also be \(20\text{ g}\).
Examine the equation on each tab to see how the law of conservation of mass applies to both the types of atoms and number of atoms involved in a chemical reaction.
When a chemical reaction occurs and a new product is formed, it has not just appeared out of thin air. Instead, in a chemical reaction, the atoms in the reactants are rearranged to form the products. For example, when the carbonic acid in carbonated beverages is formed, the type of atoms needed to form this compound already existed in the beverage. Look again at the equation that represents this reaction.
\( \text{H}_{\text{2}}\text{O}_{(\text{l})} \)
\( + \)
\( \text{CO}_{2(\text{ g})} \)
\( \rightarrow \)
\( \text{H}_{\text{2}}\text{CO}_{\text{3}(\text{aq}\text{)}} \)
Water \((\text{H}_{2}\text{O})\) contains atoms of hydrogen \((\text{H})\) and oxygen \(\text{(O)}\). Carbon dioxide \((\text{CO}_{2})\) contains atoms of carbon \(\text{(C)}\) and oxygen \(\text{(O)}\). When carbon dioxide is dissolved in water, a solution of carbonic acid \((\text{H}_{2}\text{CO}_{3})\) is formed. Notice that the product, carbonic acid, is comprised of the same types of atoms—hydrogen, oxygen, and carbon. The same types of atoms are present before and after the chemical reaction. The only difference is that the atoms are rearranged to form the new substance.
Important Note
Type of atoms in the reactants = Type of atoms in the products
The same types of atoms are present after the reaction as before the reaction occurred. This is also true for the quantity of atoms. For example, when the carbonic acid in carbonated beverages is formed, the number of atoms needed to form this compound already existed in the beverage. Look again at the equation that represents this reaction.
\( \text{H}_{\text{2}}\text{O}_{(\text{l})} \)
\( + \)
\( \text{CO}_{2(\text{ g})} \)
\( \rightarrow \)
\( \text{H}_{\text{2}}\text{CO}_{\text{3}(\text{aq}\text{)}} \)
| Reactants | Product |
|---|---|
|
\(\text{2} \text{ H}\) \(\text{1 }\text{C}\) \(\text{3} \text{ O}\) |
\(\text{2} \text{ H}\) \(\text{1} \text{ C}\) \(\text{3} \text{ O}\) |
If you count the number of atoms on both sides of the equation, you will see that the quantity is the same, even though the chemical formula is not. Again, the only difference is that the atoms are rearranged to form the new substance.
Important Note
Number of atoms in the reactants = Number of atoms in the products
In our example reaction, the bonds between the atoms in the reactant are broken, the atoms are rearranged, and new bonds are formed to make products. The atoms remain the same in terms of identity and quantity--they just get rearranged in the reaction. If you count the number of each type of atom, as indicated by the subscripts, the quantity is the same on both the reactant and product sides of the chemical equation.
A chemical equation must show that matter is conserved in a chemical reaction. In other words, the chemical equation must show that the number of each type of atom on the reactant side is equal to the number of each type of atom on the product side.
Reminder
Number and type of atoms in the reactants = Number and type of atoms in the products
Let's Practice
How well can you interpret a chemical equation in terms of conservation of mass? Find out by completing this activity. Determine whether conservation of mass is obeyed in the example on each slide, then check your answer.
Ancient alchemists believed they could take ordinary metals, such as iron and lead, and expose them to high temperatures, changing them into gold.
\(\text{Fe}_{(\text{s})} + \text{O}_{2\ (\text{g})} \xrightarrow{\text{heat}} \text{Au}_{(\text{s})}\)
Is the law of conservation of mass obeyed in this example?
No. The law of conservation of mass is NOT obeyed in this example.
This reaction indicates that the reactants, iron \((\text{Fe})\) and oxygen \(\text{(O)}\), disappear in the presence of heat and a new element, gold \(\text{(Au)}\) appears. However, in a chemical reaction, only the atoms present in the reactants can end up in the products. No new atoms are created, and no atoms are destroyed.
\(\text{Au}_{\text{2}}\text{S}_{3(\text{s})} + \text{H}_{2(\text{g})} \rightarrow \text{Au}_{(\text{s})} + \text{H}_{2}\text{S}_{(\text{g})}\)
Is the law of conservation of mass obeyed in this example?
No. The law of conservation of mass is NOT obeyed in this example.
\(\text{Au}_{2}\text{S}_{3(\text{s})} + \text{H}_{2(\text{g})} \rightarrow \text{Au}_{(\text{s})} + \text{H}_{2}\text{S}_{(\text{g})}\)
| Reactants | Products |
|---|---|
|
\(2 \text{ Au}\) \(3 \text{ S}\) \(2 \text{ H}\) |
\(1 \text{ Au}\) \(1 \text{ S}\) \(2 \text{ H}\) |
If you count the number of atoms on both sides of the equation, you will see that the number of each type of atom on the reactant side is NOT equal to the number of each type of atom on the product side. Since the atoms are rearranged to form new substances in a chemical reaction, in order for the law of conservation of mass to be obeyed each type of atom in the reactants must be accounted for in the products.
\( \text{CH}_{\text{4}} \)
\( + \)
\( \text{2O}_{2} \)
\( \rightarrow \)
\( \text{CO}_{\text{2}} \)
\( + \)
\( \text{2H}_{\text{2}}\text{O} \)
Is the law of conservation of mass obeyed in this example?
Yes. The law of conservation of mass is obeyed in this example.
The reactants are comprised of carbon, hydrogen, and oxygen; the products are comprised of carbon, hydrogen, and oxygen. No types of atoms have appeared or disappeared in the products. There are also the same number of each element present in the products and the reactants.
