When is mass conserved

We say that mass is always conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning. This is because no atoms are created or destroyed during chemical reactions. The principle of conservation of mass allows you to work out the mass of one substance in a reaction if the masses of the other substances are known.

For example:. It eventually dissolves in water and is taken up by an algal cell, where it is then consumed by a copepod. Labels also indicate the length of time that the atom spends in each compartment. Figure 2: Ecosystems are represented as a network of various biotic and abiotic compartments, connected through the exchange of materials and energy. Every compartment has inputs and outputs. Life involves obtaining, utilizing, and disposing of elements. The biomolecules that are the building blocks of life proteins, lipids, carbohydrates, and nucleic acids are composed of a relatively small subset of the hundred or so naturally occurring elements.

Living organisms are primarily made of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. And each of these important elements cycle through the Earth system. Ecosystems can be thought of as a battleground for these elements, in which species that are more efficient competitors can often exclude inferior competitors.

Though most ecosystems contain so many individual reactions, it would be impossible to identify them all, each of these reactions must obey the Law of Conservation of Mass — the entire ecosystem must also follow this same constraint. Though no real ecosystem is a truly closed system, we use the same conservation law by accounting for all inputs and all outputs.

Scientists conceptualize ecosystems as a set of compartments Figure 2 that are connected by flows of material and energy. Any compartment could represent a biotic or abiotic component: a fish, a school of fish, a forest, or a pool of carbon. Mass balance ensures that the carbon formerly locked up in biomass must go somewhere; it must reenter some other compartment of some ecosystem. Mass balance properties can be applied over many scales of organization, including the individual organism, the watershed, or even a whole city Figure 4.

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. 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. 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.

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. 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. This release of excess nutrients can influence both food webs and nutrient cycles. 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. Ecologists have often used naturally delineated ecosystems, such as lakes or watersheds, for applying mass balances. A forested watershed receives inputs of carbon through photosynthesis, inputs of nitrogen from nitrogen-fixing bacteria, as well as through the deposition of atmospheric nitrogen, inputs of phosphorus from the slow weathering of bedrock, and inputs of water from precipitation.

Outputs include gaseous pathways e. Outputs also include material transport across ecosystem boundaries, such as the movement of migratory animals or harvesting trees in a forest. This landscape has similar-sized, discreet watersheds drained by streams and underlain by impermeable bedrock. By installing V-notch weirs, investigators could precisely and continuously measure stream discharge. By measuring the concentration of nutrients and ions in stream water, they could quantify the losses of these materials from the ecosystem.

After calculating inputs to the ecosystem by sampling precipitation, dry deposition, and nitrogen fixation , they could also construct mass balances. Additionally, researchers could experimentally manipulate these watersheds to measure the effects of disturbance on nutrient retention.

In , an entire experimental watershed was whole-tree harvested, resulting in large increases in nitrate and calcium losses relative to an uncut reference watershed Figure 8. For many students the idea that matter is conserved is not a natural one. They observe that sugar disappears when mixed with water, a large log burns away to a small amount of ash, cars rust and big holes appear, water boils away, frost and condensation appear from apparently nowhere and trees grow apparently from nothing but the soil.

It may seem to students that matter disappears or appears during processes such as dissolving, burning, evaporation, boiling, rotting, respiration, rusting, condensation and growth of plants. Invisible gases are involved in many of these processes leading to many of these alternative student conceptions.

Students also often believe that matter is exchanged or converted into energy for example, they believe that wood turns into heat during combustion and food turns into energy when we metabolise it or they confuse the energy of food listed on packets as kilojoules with the weights of the listed ingredients. If a liquid evaporates inside a sealed container then they believe that the combined weight of the container and the liquid will be reduced by the weight of the liquid because it has apparently disappeared.

Although at this level the majority of students will have an understanding of the particle nature of atoms, for many the numbers of atoms are not conserved during chemical reactions. For example, the number of atoms appears to grow in the bark of trees, and their numbers drop during processes such as combustion or decay and increase during photosynthesis.

The idea of indivisible atoms helps to explain the conservation of matter. If the number of atoms stays the same no matter how they are rearranged, then their total weight stays the same.

In all physical and chemical changes, the total number of atoms remains the same, hence when substances interact with one another, combine or break apart, the total weight of the system remains the same.

Growing plants obtain their new carbon the great majority of their dried weight from carbon dioxide i. When we lose weight by dieting or exercise, most of the loss is from breathing out the carbon atoms metabolised from our fat as carbon dioxide. When a liquid evaporates in a sealed container, the weight stays the same; the gas particles are affected by gravity in the same way as tennis balls and they consequently hit the bottom surface of the container with more force than they hit the top.

In your teaching of conservation of matter and hence weight, students need to be encouraged to change their views from those based on their everyday experiences to more scientific views such as the idea that there are only a fixed number of particles in the world and these building blocks are continually being rearranged into new things. This is a difficult, abstract idea and we can use analogies to assist students with understanding. One of the difficulties is that closed systems involving changes such as combustion and respiration are almost impossible to set up and weigh in a classroom.

You can disprove by experiment only a few of the common alternative conceptions in this area. It is important to discuss a variety of change situations that appear to involve non-conservation of matter and to revisit this issue in other topics such as ecosystems, food and diet, and sources of energy. Students should be encouraged to make predictions about these processes in a supportive classroom environment where they can be assisted to develop new theories, critically analyse their understandings and those of others, and compare these with scientific views presented by the teacher.