It may seem as though burning destroys matter, but the same amount, or mass, of matter still exists after a campfire as before. Look at Figure \(\PageIndex{1}\) below. It shows that when wood burns, it combines with oxygen and changes not only to ashes, but also to carbon dioxide and water vapor. The gases float off into the air, leaving behind just the ashes. Suppose you had measured the mass of the wood before it burned and the mass of the ashes after it burned. Also suppose you had been able to measure the oxygen used by the fire and the gases produced by the fire. What would you find? The total mass of matter after the fire would be the same as the total mass of matter before the fire. The law of conservation of mass was created in 1789 by a French chemist, Antoine Lavoisier. The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. For example, when wood burns, the mass of the soot, ashes, and gases equals the original mass of the charcoal and the oxygen when it first reacted. So the mass of the product equals the mass of the reactant. A reactant is the chemical reaction of two or more elements to make a new substance, and a product is the substance that is formed as the result of a chemical reaction (Video \(\PageIndex{1}\)). Matter and its corresponding mass may not be able to be created or destroyed, but can change forms to other substances like liquids, gases, and solids. If you witness a 300 kg tree burn to the ground, there are only ashes left after the burn, and all of them together weigh 10 kg. It may make you wonder where the other 290 kg went. The missing 290 kg was released into the atmosphere as smoke, so the only thing left that you can see is the 10 kg of ash. If you know the law of conservation of mass, then you know that the other 290 kg has to go somewhere, because it has to equal the mass of the tree before it burnt down. If heating 10.0 grams of calcium carbonate (CaCO3) produces 4.4 g of carbon dioxide (CO2) and 5.6 g of calcium oxide (CaO), show that these observations are in agreement with the law of conservation of mass.
\[\begin{align*} \text{Mass of the reactants} &= \text{Mass of the products} \\[4pt] 10.0\, \text{g of } \ce{CaCO3} &= 4.4 \,\text{g of }\ce{CO2} + 5.6\, \text{g of } \ce{ CaO} \\[4pt] 10.0\,\text{g of reactant} &= 10.0\, \text{g of products} \end{align*} \nonumber \]
Because the mass of the reactant is equal to the mass of the products, the observations are in agreement with the law of conservation of mass.
Potassium hydroxide (\(\ce{KOH}\)) readily reacts with carbon dioxide (\(\ce{CO2}\)) to produce potassium carbonate (\(\ce{K2CO3}\)) and water (\(\ce{H2O}\)). How many grams of potassium carbonate are produced if 224.4 g of \(\ce{KOH}\) reacts with 88.0 g of \(\ce{CO2}\)? The reaction also produces 36.0 g of water.
Answer 276.4 g of potassium carbonateThe Law is also applicable to both chemical and physical changes. For example, if you have an ice cube that melts into a liquid and you heat that liquid up, it becomes a gas. It will appear to have disappeared, but is still there.
Summary
- Burning and other changes in matter do not destroy matter.
- The mass of matter is always the same before and after the changes occur.
- The law of conservation of mass states that matter cannot be created or destroyed.
Contributions & Attributions
Binod Shrestha (University of Lorraine)
LICENSED UNDER
In reactions under normal laboratory conditions, matter is neither created nor destroyed, and elements are not transformed into other elements. Therefore, equations depicting reactions must be balanced; that is, the same number of atoms of each kind must appear on opposite sides of the equation. The balanced equation for the iron-sulfur reaction shows that one iron atom can react with one sulfur atom to give one formula unit of iron sulfide. Chemists ordinarily work with weighable quantities of elements and compounds. For example, in the iron-sulfur equation the symbol Fe represents 55.845 grams of iron, S represents 32.066 grams of sulfur, and FeS represents 87.911 grams of iron sulfide. Because matter is not created or destroyed in a chemical reaction, the total mass of reactants is the same as the total mass of products. If some other amount of iron is used, say, one-tenth as much (5.585 grams), only one-tenth as much sulfur can be consumed (3.207 grams), and only one-tenth as much iron sulfide is produced (8.791 grams). If 32.066 grams of sulfur were initially present with 5.585 grams of iron, then 28.859 grams of sulfur would be left over when the reaction was complete. The reaction of methane (CH4, a major component of natural gas) with molecular oxygen (O2) to produce carbon dioxide (CO2) and water can be depicted by the chemical equation CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Here another feature of chemical equations appears. The number 2 preceding O2 and H2O is a stoichiometric factor. (The number 1 preceding CH4 and CO2 is implied.) This indicates that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. The equation is balanced because the same number of atoms of each element appears on both sides of the equation (here one carbon, four hydrogen, and four oxygen atoms). Analogously with the iron-sulfur example, we can say that 16 grams of methane and 64 grams of oxygen will produce 44 grams of carbon dioxide and 36 grams of water. That is, 80 grams of reactants will lead to 80 grams of products. The ratio of reactants and products in a chemical reaction is called chemical stoichiometry. Stoichiometry depends on the fact that matter is conserved in chemical processes, and calculations giving mass relationships are based on the concept of the mole. One mole of any element or compound contains the same number of atoms or molecules, respectively, as one mole of any other element or compound. By international agreement, one mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams (this is called the molar mass) and represents 6.022140857 × 1023 atoms (Avogadro’s number). One mole of iron contains 55.847 grams; one mole of methane contains 16.043 grams; one mole of molecular oxygen is equivalent to 31.999 grams; and one mole of water is 18.015 grams. Each of these masses represents 6.022140857 × 1023 molecules.
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