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

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
• 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

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.

You must create your own procedures for this lab. Write down what you do in a few words in your notebook. You must accomplish the following tasks:

1. Determine whether mass is gained, lost, or stays the same when an ice cube melts.
2. Determine whether mass is gained, lost, or stays the same when water is boiled for five minutes.
3. Determine whether mass is gained, lost, or stays the same when you burn a candle for five minutes.
4. Determine whether mass is gained, lost, or stays the same when your teacher burns magnesium.
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).

Citric Acid Reaction Procedure

1. Take a moment to describe on paper the two chemicals you will be reacting.
2. Place the ¼ tsp baking soda and ¼ 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. Take the temperature of the materials before the reaction by putting the bulb of the thermometer in the colored water for about 1 minute. Record the temperature in degrees Celsius.
7. 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.
8. 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.
9. 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.
10. 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.
11. Take the temperature of the substances in the bag after the reaction has finished.

The Report

Write a formal lab report. In your Analysis section answer the following questions.

Make a table showing the mass of each substance before the change, the mass after the change, the change in mass, and the percent change in mass.

• Change in mass is Δm = m_f - m_i. This means that change in mass (Δm) equals the the final mass (m_f, 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 in 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 percent difference using the formula below. This is the percent change in mass.
  

     Δm
   —————— × 100%
     mi
  

  This means the change in mass divided by the mass you started with. You multiply by 100% to turn the decimal answer into a percentage. Put the numbers you calculate into your data table. Use these numbers to help you answer the questions below. The percent change in mass tells you whether the change in mass was large or small compared to the mass you started with. If the change is smaller than 1% then it is probably just an experimental error. If it is larger than that then it means that the mass definitively changed.

Questions

Write your answers in paragraph form in the Analysis section of the lab report.

1. For each experiment describe what happened and refer to the change in mass. In each case, decide whether or not mass is conserved and try to explain what you observed. If the mass increased, explain where the extra mass came from. If the mass decreased then explain where the mass went. If you wish to claim that matter was really created from nothing or that it was completely destroyed then you must find a way to justify your claims. In any case, provide evidence for your explanations.
   
   As you know, matter is made of tiny particles called atoms and molecules. Molecules are just groups of atoms bound together. Try to explain your results by referring to atoms and molecules. What happened to the atoms in cases where the mass either increased or decreased?
   
   Very important point: Be sure to use your quantitative (numerical) data to justify your answers to this question. Use the change in mass (Δm) and percent change in mass in your answers.
2. Background: Physical and chemical changes often absorb or give off heat. In addition to absorbing or giving off heat a chemical reaction produces new substances. When a process gives off heat (that is, it makes its surroundings warmer than before) it is called exothermic. The heat in an exothermic process is not provided by an outside source like a heater. When a process absorbs heat (that is, it makes its surroundings colder than before) it is called endothermic. An endothermic process takes in heat from the surroundings and can’t work without it. Sometimes this happens because you heat it directly with a heater. Sometimes this happens all by itself and the process feels cold because it absorbs heat from your body.
   Question: For each process you observed determine whether the process was exothermic or endothermic. Justify your answer using your observations.
3. 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).
4. 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).
5. Which of these experiments were a good test of the hypothesis that matter is conserved in physical and chemical changes? Why?
6. Which of these experiments were a poor test of the hypothesis that matter is conserved in physical and chemical changes? Why?
7. For each of the poor tests invent a way to make them better and describe it. Your new experiments should be practical and performable.
8. 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 a 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.]