Why is it important to understand the law of conservation of mass?

The Law of Conservation of Mass states that mass cannot be created or destroyed. The mass of reactants and products of any chemical reaction must stay the same.

The law of conservation of mass states that in a chemical reaction mass is neither created nor destroyed. For example, the carbon atom in coal becomes carbon dioxide when it is burned. The carbon atom changes from a solid structure to a gas but its mass does not change. Similarly, the law of conservation of energy states that the amount of energy is neither created nor destroyed. For example, when you roll a toy car down a ramp and it hits a wall, the energy is transferred from kinetic energy to potential energy.

Teach about the conservation of energy and mass with these classroom resources.

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Chemistry is a physical science that studies matter, energy and how they interact. When studying these interactions, it's important to understand the law of conservation of mass.

  • Simply stated, the law of conservation of mass means matter cannot be created or destroyed, but it can change forms.
  • In chemistry, the law is used to balance chemical equations. The number and type of atoms must be the same for both reactants and products.
  • Credit for discovering the law may be given to either Mikhail Lomonosov or Antoine Lavoisier.

The law of conservation of mass is that, in a closed or isolated system, matter cannot be created or destroyed. It can change forms but is conserved.

In the context of the study of chemistry, the law of conservation of mass says that in a chemical reaction, the mass of the products equals the mass of the reactants.

To clarify: An isolated system is one that does not interact with its surroundings. Therefore, the mass contained in that isolated system will remain constant, regardless of any transformations or chemical reactions that occur—while the result may be different than what you had in the beginning, there can't be any more or less mass than what you had prior to the transformation or reaction.

The law of conservation of mass was crucial to the progression of chemistry, as it helped scientists understand that substances did not disappear as result of a reaction (as they may appear to do); rather, they transform into another substance of equal mass.

History credits multiple scientists with discovering the law of conservation of mass. Russian scientist Mikhail Lomonosov noted it in his diary as a result of an experiment in 1756. In 1774, French chemist Antoine Lavoisier meticulously documented experiments that proved the law. The law of conservation of mass is known by some as Lavoisier's Law.

In defining the law, Lavoisier stated, "Atoms of an object cannot be created or destroyed, but can be moved around and be changed into different particles."

  • Okuň, Lev Borisovič (2009). Energy and Mass in Relativity Theory. World Scientific. ISBN 978-981-281-412-8.
  • Whitaker, Robert D. (1975). "An historical note on the conservation of mass." Journal of Chemical Education. 52 (10): 658. doi:10.1021/ed052p658

Why is it important to understand the law of conservation of mass?

Figure 3: A forest system

Because of conservation of mass, if inputs exceed outputs, the biomass of a compartment increases (such as in an early successional forest). Where inputs and outputs are equal, biomass maintains a steady level (as in a mature forest). When outputs exceed inputs, the biomass of a compartment decreases (e.g., a forest being harvested).

The availability of individual elements can vary a great deal between nonliving and living matter (Figure 5). Life on Earth depends on the recycling of essential chemical elements. While an organism is alive, its chemical makeup is replaced continuously as needed elements are incorporated and waste products are released. When an organism dies, the atoms that were bound in biomolecules return to simpler molecules in the atmosphere, water and soil through the action of decomposers.

Each organism has a unique, relatively fixed, elemental formula, or composition determined by its form and function. For instance, large size or defensive structures create particular elemental demands. Other biological factors such as rapid growth can also influence elemental composition. Ribonucleic acid (RNA) is the biomolecular template used in protein synthesis. RNA has a high phosphorus content (~9% by mass), and in microbes and invertebrates RNA accounts for a large fraction of an organism's total phosphorus content. As a result, fast-growing organisms such as bacteria (which can double more than 6 times per day) have especially high phosphorus content and therefore demands. By contrast, among vertebrates structural materials such as bones (made of calcium phosphate) account for the majority of an organism's phosphorus content. Among mammals, black-tailed deer (Odocoileus columbianus; Figure 6) have a relatively high phosphorus demand due to their annual investment in calcium- and phosphorus-rich antlers. Failure to meet elemental demands can lead to poor health, limited reproduction, and even extinction. The extinction of the majestic Irish Elk (Megaloceros giganteus) is thought to have been caused by the shortened growing season that occurred during the last ice age, which reduced the availability of the calcium and phosphorus these animals needed to grow their enormous antlers.

Why is it important to understand the law of conservation of mass?

Figure 4: All types of natural and even human-designed systems can be evaluated as ecosystems based on conservation of mass.

Individual organisms, watersheds, and cities receive materials (inputs), transform them, and export them (outputs) sometimes in the form of waste.

Obtaining the resources required for metabolism, growth, and reproduction is one of the central challenges of life. Animals, particularly those that feed on plants (herbivores) or detritus (detritivores), often consume diets that do not include enough of the nutrients they need. The struggle to obtain nutrients from poor quality diets influences feeding behavior and digestive physiology and has led to epic migrations and seemingly bizarre behavior such as geophagy (feeding on materials such as clay and chalk). For example, the seasonal mass migration of Mormon crickets (Anabrus simplex) across western North America in search of two nutrients: protein and salt. Researchers have shown that the crickets stop walking once their demand for protein is met (Figure 7).

Why is it important to understand the law of conservation of mass?

Figure 5: Comparison between elemental composition of the Earth's crust and the human body

The flip side of the struggle to obtain scarce resources is the need to get rid of excess substances. Herbivores often consume a diet rich in carbon — think potato chips, few nutrients but lots of energy. Some of this material can be stored internally, but this is a limited option and excess carbon storage can be harmful, just as obesity is harmful to humans. Thus, animals have several mechanisms for getting rid of excess elements. Excess nutrients are released in feces or urine or sometimes it is respired (i.e., released as carbon dioxide). This release of excess nutrients can influence both food webs and nutrient cycles.

Why is it important to understand the law of conservation of mass?

Figure 6: Components of an animal's mass balance

This black-tailed deer consumes plant material rich in carbon but poor in other necessary nutrients, such as nitrogen (N). The deer requires more N than is found in its food and must cope the surplus a surplus of carbon. As a result, it must act to retain N while releasing excess carbon to maintain mass balance. Carbon and N mass balances suggest that deer waste should be carbon rich and low in N. Boxes show the abundance of N (green boxes) relative to carbon (gray boxes) in the diet, deer, and deer waste products.