Why does mass decrease in a chemical reaction?

Perform two chemical reactions to see whether any mass changes occur in this microscale class practical

In this experiment, students measure the mass of various reactant solutions before and after reaction to see whether there has been any change in mass.

The practical should take approximately ten minutes.



  • Eye protection
  • Student worksheet
  • Part of a well-plate (see note 3 below)
  • Access to a balance that reads to 0.01 g


  • Sodium carbonate, 0.5 mol dm–3
  • Calcium nitrate, 0.5 mol dm–3
  • Hydrochloric acid, 1 mol dm–3
  • Magnesium ribbon
  • Marble chips (small)

Health, safety and technical notes

  1. Read our standard health and safety guidance.
  2. Wear eye protection throughout.
  3. Cut a three-well plate from the standard 24-well plate using a hacksaw. A class set can be cut from a single well-plate (see diagram below).
  4. Sodium carbonate, Na2CO3.10H2O, 0.5 mol dm–3 is of low hazard at this concentration. See CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB080.
  5. Calcium nitrate, Ca(NO3)2.4H2O, 0.5 mol dm–3 is of low hazard. See CLEAPSS Hazcard HC019B and CLEAPSS Recipe Book RB019.
  6. Hydrochloric acid, HCl(aq), 1 mol dm–3 is of low hazard. See CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.

Why does mass decrease in a chemical reaction?


Part 1: the reaction between sodium carbonate and calcium nitrate

  1. Put two plastic pipettes containing the solutions of sodium carbonate and calcium nitrate in the outer two wells of the mini well-plate.
  2. Place on a balance and record the mass.
  3. Put 20 drops of sodium carbonate solution into the middle well followed by 20 drops of calcium nitrate solution.
  4. Record any changes you see and write an equation for the reaction.
  5. Reweigh the complete apparatus and record the mass.

Is there a difference in the masses before and after the reaction? Explain your answer.

Part 2: the reaction between marble and hydrochloric acid

  1. Place one piece of marble chip and the pipette containing the hydrochloric acid in two of the wells in the mini well-plate.
  2. Add ten drops of hydrochloric acid to the well containing the marble chip.
  3. Record any changes you see and write an equation for the reaction.
  4. When the reaction has finished reweigh the complete apparatus and record the mass.

How do your answers compare with those in part 1? Explain your answers.

Teaching notes and expected observations

Students should find that there is a negligible difference in mass before and after mixing the sodium carbonate/calcium nitrate solution but there is some difference in mass in the magnesium or marble chip reaction with hydrochloric acid.

The success of this experiment depends on careful working by students and on the reliability of the balance and its proper use. Other combinations of substances could be examined and the experiment could be used as an investigation.

As an outcome of this experiment students should appreciate that matter is neither created nor destroyed in chemical reactions and that this is a very fundamental aspect of chemistry. It should also help them in balancing chemical equations!

Why does mass decrease in a chemical reaction?

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 does mass decrease in a chemical reaction?

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 does mass decrease in a chemical reaction?

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 does mass decrease in a chemical reaction?

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.