Lab: Conservation of Matter

Objective

Mass is a measure of the amount of matter in an object. Indirectly, mass tells you something about how many atoms are in a particular object. Mass is conserved in chemical reactions and physical changes. In chemical reactions atoms and molecules are rearranged to make different materials but no matter is created or destroyed. That is why scientists say that mass is conserved. In physical changes there is no molecular rearrangement: the atoms and molecules just change phases (from a solid to a liquid or from a liquid to a gas or the reverse of one of these).

Scientists have a (very strongly supported) hypothesis that mass is neither created from nothing nor can it be completely destroyed in chemical or physical changes. The origin of this idea can be traced back to Antoine Lavoisier and his wife Marie Anne who performed the first experiments which proved the principle in the 18^th century.

In this lab it is your job to do experiments to either prove or disprove the hypothesis that mass is conserved in chemical and physical changes.

Overview

In this brief lab you will perform several simple experiments. In each experiment you will find the mass of the material(s) before a physical or chemical change and after the change. One experiment will be performed by your instructor as a demonstration.

Materials

Gather materials as you need them. You will be creating some aspects of your experiments yourself and this list is meant to suggest items that may be useful.

Safety

• Wear goggles or risk sitting out the lab
• When your teacher burns the Mg do not look directly at it; it is very bright and could damage your eyes
• Do not attempt to operate a Bunsen burner without instruction
• Tie back long hair and take off dangling jewelry while working with matches and flames
• Keep all paper away from flames; keep a neat work station and be proactive about preventing accidental fires
• Do not touch the chemicals: wash well with water immediately if you touch anything accidentally
• When mixing the chemicals make sure the bag is sealed and pointed away from fellow students
• Feeling the bag while the reaction proceeds is OK; but do not squeeze the bag!

Procedure

Your task today is to test this hypothesis: Mass is neither created nor destroyed in chemical or physical changes.

Remember to record your observations in your lab notebook or on a piece of paper in your binder before you leave class. When making observations be sure to use all of your senses except taste. Never taste anything in the chemistry lab.

Write down what you do in a few words in your notebook. You will use the following experiments to test the hypothesis:

1. Determine whether mass is gained, lost, or stays the same when your teacher burns magnesium.
2. Determine whether mass is gained, lost, or stays the same when an ice cube melts.
3. Determine whether mass is gained, lost, or stays the same when water is boiled for five minutes.
4. Determine whether mass is gained, lost, or stays the same when you burn a candle for five minutes.
5. Determine whether mass is gained, lost, or stays the same when citric acid reacts with baking soda in a closed plastic bag. (See the detailed procedure on the next page).

Generalized Procedure for Testing the
Hypothesis of the Conservation of Matter

This procedure is written so that you can apply it to any of the objectives for the lab. Further specific instructions are provided for the final objective which involves the chemical reaction of citric acid with baking soda.

1. Measure the mass to the nearest 0.01 g of any container you are using for the materials involved in your experiment.
2. Measure the mass to the nearest 0.01 g of the materials you have before you carry out the experiment. Be sure to subtract the mass of any container. This is your initial mass (symbolized by m_i).
3. Carry out the experiment. Some suggestions:
   • Your teacher will carry out the procedure for burning magnesium as a demonstration. At the same time, the procedure will serve as a model for your own experiments.
   • Ice cubes melt faster when heated but be aware of the possibility of evaporation of the liquid water.
   • Water boils quickly over a Bunsen burner flame. Use all due caution in operating the burner and ask your teacher for help setting up an appropriate way to heat the water using a burner. You may also use an electric hot plate, which is safer and easier to use.
   • Be aware that any wax that drips from the candle must be included in the mass you measure after your experiment is complete. You may use a piece of aluminum foil to catch drips but be sure to measure its mass so you can subtract it later.
4. When the experiment is complete measure the mass of the materials to the nearest 0.01 g. Subtract the mass of any containers. This is your final mass (symbolized by m_f).

Citric Acid Reaction Procedure

1. Take a moment to describe on paper the two chemicals you will be reacting.
2. Place the ½ tsp baking soda and ½ tsp citric acid into the zip-top bag. Shake the bag to mix them together. Does anything happen? Make some observations at this point.
3. Put about 20 mL of water from the tap into the 50-mL beaker.
4. Add a few drops of the indicator solution provided by your teacher. Describe the color of the solution in your notes.
5. Put the bag with the chemicals and the beaker with water onto the lab balance and record the mass to the nearest 0.01 g.
6. Carefully put the beaker full of water inside the bag without spilling any water on the dry chemicals. Seal the zip-top bag while pushing as much air out of it as possible. Once you are sure it is sealed, mix the water with the dry chemicals.
7. Hold the bag so that your hand is near the reaction but do not squeeze the bag. Also, do not open the bag. What do you observe? Record what your senses tell you, writing down as many observations as you can. Allow others in your group to feel it as well.
8. Did the bag inflate when you did the reaction? Think about what may have caused this and write your observations in your lab notebook. Estimate the volume of the bag in liters and write this down.
9. Place the closed bag back on the balance and find its mass again when it looks like the reaction has finished. Record the mass to the nearest 0.01 g.

Data Table

Use the data table below to collect your measurements and calculations. Make sure to also write down observations from your work in the lab. A keen eye and a sharp focus may notice things which will help to explain the results you find.

In order to quantify any changes you find you will calculate the Change in Mass: Δm = m_f - m_i. This means that change in mass (Δm) equals the the final mass (m_f, the mass after the change) minus the initial mass (m_i, before the change). If the answer is a positive number then the experiment shows a gain of mass. If the answer is a negative number then the experiment shows a loss of mass.

It is possible to quantify any difference as a quantity called a Percent Difference using the formula below. This is the percent change in mass.

   Δm
 —————— × 100%
   mi

1. The mass gained/lost is significant and you have evidence that the Law of Conservation of Matter is false.
2. There is a significant change in mass due to the circumstances of the change. Matter may become impossible to measure on a balance, perhaps by becoming a gas. This would mean that the experiment is poorly designed to test the Law of Conservation of Matter.

Post-lab Questions

Create a Google Doc to be turned in using the Google Classroom page. Include:

1. Your neatly typed data table.
2. The answers to the following questions.

1. Which of the experiments was a chemical reaction? Describe each one in terms of what each reactant and what each product of the reaction was. Justify your choices based on whether or not new substances were formed. (If new substances came into being while others were used up then you observed a chemical reaction).
2. Which of these experiments was a physical change? Justify your choices based on the idea of phase changes (reversible changes of a substance between solid, liquid and gas forms).
3. Did burning magnesium show a change in mass? Give the numerical result from your lab data, showing initial mass, final mass, and the change in mass. Explain the change in mass based on your own observations. Also, describe what happened to the atoms of magnesium involved so that you can explain a change in mass.
4. Did melting ice show a change in mass? Give the numerical result from your lab data, showing initial mass, final mass, and the change in mass. Explain the change in mass based on your own observations. Also, describe what happened to the molecules of water involved so that you can explain a change in mass.
   If there was no change in mass then explain why not.
5. Did boiling water show a change in mass? Give the numerical result from your lab data, showing initial mass, final mass, and the change in mass. Explain the change in mass based on your own observations. Also, describe what happened to the molecules of water involved so that you can explain a change in mass.
   If there was no change in mass then explain why not.
6. Did burning candle show a change in mass? Give the numerical result from your lab data, showing initial mass, final mass, and the change in mass. Explain the change in mass based on your own observations. Also, describe what happened to the molecules of wax involved so that you can explain a change in mass.
   If there was no change in mass then explain why not.
7. Did the reaction between citric acid and baking soda show a change in mass? Give the numerical result from your lab data, showing initial mass, final mass, and the change in mass. Explain the change in mass based on your own observations. Also, describe what happened to the molecules involved so that you can explain a change in mass.
   If there was no change in mass then explain why not.

8. Which of these experiments were a good test of the hypothesis that matter is conserved in physical and chemical changes? Why?
9. Which of these experiments were a poor test of the hypothesis that matter is conserved in physical and chemical changes? Why?
10. For each of the poor tests invent a way to make them better and describe it. Your new experiments should be practical.
11. There is a buoyant force on objects submerged in a fluid that is proportional to the volume of the object and the density of the fluid. Air is a fluid with a density at 25°C of about 1.2 g/L. The buoyant force on an object with a volume of 1 L would lead to underestimating its mass by 1.2 g. The buoyant force on an object with a volume of 0.5 L would make it appear to weigh about 0.6 g less than its true weight. Does this information help to explain the loss of mass found in one of the experiments in this lab? Which one? And is it enough to account for the change in mass?
    
    [This question not included on the printout because students had too much trouble figuring it out. Nevertheless, if students see a mass loss of between 0.6 g and 1.2 g the buoyancy of the bag in air is the culprit.]