Experiment Regarding Conservation of Mass

Early experiments related to mass helped lead to today’s understanding of conservation of mass.

Introduction

A chemical reaction occurs when two or more substances react and undergo chemical changes. During a chemical reaction, one or more new substances are formed. Some chemical reactions happen so slowly that an observer cannot see a change occurring. For example, the formation of rust from iron and oxygen is a gradual chemical change. However, others are more obvious. If you light a sparkler (see Figure 1), the magnesium in it combusts and the sparkler gives off heat and light. Other indicators of chemical reactions include bubbling, sound, production of an odor, or a change in color.

Figure 1

In all chemical reactions, matter and energy undergo changes. In the sparkler reaction, matter was converted into heat and light energy. A chemical reaction that produces heat is described as exothermic. Other types of reactions take up energy, and these are described as endothermic. Reactants undergoing endothermic reactions feel cool to the touch. Photosynthesis is a endothermic reaction in which the a plant takes in the sun’s energy to make glucose (Figure 2). Some of the most important work on changes of matter and energy was done by Antoine-Laurent Lavoisier (1743–94), a young Frenchman who conducted experiments in his home with the help of his wife, Marie Anne (1758–1836). In this experiment, you are going to learn more about Lavoisier and his research.

Figure 2

Time Required

30 minutes for Part A

30 minutes for Part B

Materials

• 1/2 teaspoon (tsp) baking soda
• 1/2 tsp citric acid
• 1 quart-size ZiplocTM freezer bag
• small beaker
• 40 milliliters (ml) water
• triple-beam balance or electronic scale
• access to the Internet
• science notebook

Wear gloves and goggles when working with chemicals.

Please review and follow the safety guidelines at the beginning of this volume.

Procedure, Part A

1. Access the Internet and carry out searches to learn more about Antoine-Laurent Lavoisier. Specifically, find answers the Analysis questions 1 through 8.

Procedure, Part B

1. Place 1/2 tsp baking soda and 1/2 tsp citric acid into the ZiplocTM bag.

2. Pour 40 ml water into the beaker.

3. Carefully set the beaker of water into the ZiplocTM bag. Do not spill the water.

4. Place the ZiplocTM bag and beaker of water on the triple-beam balance or electronic scale. Do not let the water spill.

5. In your science notebook, record the mass of the bag, dry chemicals, and beaker of water.

6. Remove the bag from the scale and seal it tightly. Tip the beaker of water so that it mixes with the dry chemicals. Continue to hold the bag in your hands and observe what happens. (While you are holding the bags, do not squeeze or knead it.)

7. When the chemical reaction stops, place the sealed bag on the scale. Record the mass of the bag and its contents in your science notebook.

8. Answer Analysis questions 9 through 15.

Analysis

1. When and where did Lavoisier live?

2. What was his job?

3. Explain the phlogiston theory.

4. What did Lavoisier think of the phlogiston theory?

5. What types of laboratory techniques did Lavoisier use?

6. What is the law of conservation of mass? How did Lavoisier’s work help establish it?

7. What was the role of Marie Anne Lavoisier in Antoine’s research?

8. What is a chemical reaction?

9. How do you know that a chemical reaction took place in Part B of the experiment?

10. Was the chemical reaction in the bag endothermic or exothermic?

11. Did the mass of the bag and its contents change between measurements?

12. According to the law of conservation of mass, the mass of materials in a closed system will remain constant, regardless of changes that take place within the system. Allowing for errors in massing that could have occurred during the experiment, did Part B demonstrate the law of conservation of matter? Explain your answer.

13. How does this experiment support Lavoisier’s work?

14. Lavoisier was very careful when he weighed chemicals. Why was it important for you to carefully weigh the bag before and after Part B of the experiment?

Chemistry as a science did not exist until the mid-1700s. Predecessors of chemists, the ancient alchemists, knew of only four elements: fire, water, earth, and air (see Figure 3). The earliest chemists focused their efforts on understanding burning, or combustion, which they considered to be the most important chemical reaction. These scientists understood that corrosion of metals and respiration in animals were also forms of combustion.

Figure 3

The four elements of alchemy: (a) earth, (b) air, (c) water, and (d) fire Phlogiston was believed to be the inflammable substance that every flammable material contained. When a flammable material burned, scientists thought that phlogiston was given off. For example, the combustion of wood was believed to produce two products, ash (which was called calx) and phlogiston. In a similar reaction, iron was thought to undergo a reaction to become rust (another form of calx) and phlogiston. Lavoisier found fault with the phlogiston theory. Through meticulous quantitative measurements, Lavoisier demonstrated that when a metal burned, its weight increased because air was absorbed. He also showed that combustion of calx (wood ash) with charcoal gave off air. Lavoisier’s work showed that phlogiston did not exist and that the total mass of matter does not change in a chemical reaction.

Connections

Lavoisier’s interest in science was not limited to demonstrating that matter is conserved in chemical reactions. He also developed a method of naming compounds that is still in use today. He dismissed the old four-element way of looking at things and defined an element as anything that cannot be broken down into simpler elements. Lavoisier also found that air is a mixture of several gases and that water is made of two components, oxygen and hydrogen.

Because Lavoisier did a lot of quantitative work, very carefully measuring reactants and products, he is recognized as a founder of stoichiometry, the field of science that quantifies the materials in a chemical reaction. Despite his intense interests in chemistry, Lavoisier never pursued chemistry as a career. He was formally trained in law and intensely interested in French politics. Most of his adult life, Lavoisier worked as a tax collector.

As a government employee, Lavoisier was very active is pushing for improvements that he felt were important. He supported the adoption of the metric system of uniform units of measurements. He also tried to introduce reforms to help the peasants whom he considered to be unfairly taxed. Some of Lavoisier’s opinions lead to false accusations that he was a traitor to the government and he was beheaded. Less than two years after his death, his named was cleared and apologies for his execution were extended to his wife.