New Chemistry Exercises
By D. Michael Byler, Assistant Professor of Chemistry
In the 2007-2008 academic year, the Chemistry Department at CCP used the handout below as one of the ten laboratory exercises for Chemistry 110. In 2008 this document was published in our Custom Laboratory Manual for Chemistry 110:
“Laboratory Manual To Accompany Chemistry 110,” Community College of Philadelphia, 2008: Cengage Learning, Mason, OH 45050.
Chem 110 Lab Handout Experiment 3
Physical and Chemical Changes; Law of Conservation of Matter (Mass)
Everything in the universe is composed of matter and energy. Matter occupies space (volume) and has mass (weight). Matter can undergo two basic types of changes: physical and chemical.
Physical Changes:
Physical changes may change the appearance of a substance, but do NOT involve any change in its chemical composition or identity. Some examples are given below
1. A phase change (or change in state): Converting a solid to a liquid and then to a gas involves only a change in the amount of energy in the sample. There is no effect on the chemical identity of the substance. For example, ice, liquid water, and water vapor or steam are all the same substance: H2O.
2. Crushing a solid into a powder. Or consider the opposite, melting small pellets of plastic and allowing the liquid resolidify into one larger object, such as a cup or a bottle.
3. Some metals (including iron, Fe, and nickel, Ni) can be magnetized. Magnetized iron behaves chemically just like iron that is not magnetic. For instance, both rust easily.
4. Dissolving some crystals of salt in a beaker of water, or the reverse, evaporating the water from a salt water solution leaving behind crystals of salt.
5. Heating some platinum or nichrome wire in a flame until it glows bright red-orange.
Chemical Changes:
A chemical change has occurred whenever a chemical reaction has taken place. After a chemical reaction, new substances will have formed from the original substances that existed before the reaction occurred. Often the new substances do not resemble the old ones in any way. In all cases, the total number of atoms of each kind of element remains the same, but how they are arranged or connected together to form new substances or compounds is distinctly different from their arrangement in the starting materials before the chemical change happened. Signs or observations that often indicate that a chemical change has taken place include:
1. The formation of a gas when liquids or solids are mixed without adding any heat.
2. The formation of a solid (precipitate) when two solutions are mixed.
3. A color change.
4. A change in the acidity or basicity (pH) of a solution.
5. A change in temperature (the evolution or absorption of heat), particularly when flames appear. All chemical changes are accompanied by a change in energy.
For example, paper is made up of giant molecules of cellulose, a compound composed of carbon, hydrogen, and oxygen. If the paper is burned in air (oxygen), a chemical change occurs in which carbon dioxide gas and water vapor are formed. The latter can be condensed to liquid water. Concurrently, much energy is released in the form of light and heat (a flame). A second example of a chemical change is much slower and less dramatic: the silvery metal iron turns to dark orange-brown rust when it is exposed to moist air for a long time.
Law of Conservation of Matter (Mass):
One of the best known and most widely accepted scientific laws is the Law of Conservation of Matter (Mass). It states that during a chemical reaction (change), matter is neither created nor destroyed. An alternative expression of this law declares that the combined total measured mass of all products formed during a chemical change or reaction is equal to the total mass of all the starting materials used up during the reaction.
PROCEDURE
Record all observations and answers to questions in your laboratory notebook. Be sure to measure all masses to three decimal places (that is, to the nearest 0.001 g or nearest 1 mg.)
Part I: Physical & Chemical Changes
1A. Record the mass of a clean, dry aluminum, Al, weighing boat. Cut a small piece of wax from a candle and place it into the Al boat. Measure the mass of the boat and the wax. Heat the boat with the wax on a warm hot plate until the wax melts completely. Describe the liquid that forms. Then allow the sample to cool. Again measure the mass of the boat and the wax. Describe the appearance of the wax. What kind of change do each of these transformations represent, physical or chemical? Explain your answers. How much did the total mass change?
1B. Place the Al boat with the wax back onto the warm hot plate until the wax again melts. Carefully take the boat and liquid wax off the hot plate and place it on the lab bench. Immediately stand the candle in the melted wax in the boat. Hold it in a vertical position until the wax cools and resolidifies. Now weigh the candle and the boat. Use a match to light the candle, and let it burn for at least one minute. Record everything you observe. Be sure to sketch the flame and describe its color. After about one minute, extinguish the candle. Record any changes you observe now. Finally, reweigh the candle and the weighing boat. How much did the total mass change? Does this involve physical or chemical changes? Explain.
2A. Tear a small piece of paper (about 5 cm × 5 cm square) into smaller pieces. What kind of change is involved here?
2B. Put these pieces of paper onto a watch glass and then place the glass on a wire gauze. Ignite (light on fire) the paper with a match and allow it to burn. Record your observations. What kind of change has occurred here?
3A. Add a small spatula full of salt (sodium chloride, NaCl) about the size of a green pea to a test tube containing about 5 mL of distilled water. Carefully swirl or stir the contents until all the salt has dissolved. Record your observations. Does this involve physical or chemical changes?
3B. Now use a disposable plastic pipette (dropper) to add approximately ½ mL (about 10 drops) of aqueous silver nitrate (0.1 M AgNO3) to the test tube with the salt water. What happens? Record your observations. Now swirl or stir the contents of the tube. What do you observe? Gently stand the test tube and its contents in a small beaker or test tube rack. Let it stand undisturbed for several minutes. Have any further changes taken place? Were these chemical or physical changes? [CAUTION: Avoid direct contact with the silver nitrate solution. Silver nitrate can stain your skin or clothing. It is also mildly toxic.]
4. Obtain a ribbon of magnesium metal [Mg] about 2 cm long. Describe its physical properties. Cut this sample into 2 or 3 small pieces. Place these bits of metal into a test tube and use a plastic disposable pipette to add about 2 mL of dilute hydrochloric acid [1.0M HCl (aq)]. Record all your observations and state which changes are physical and which are chemical.
CAUTION: Even though such a dilute solution of acid is relatively safe, it could cause chemical burns to your eyes or to sensitive skin, especially about your face. It can also do permanent damage to cotton clothing, ultimately causing holes to form where the acid comes in contact with the fabric.
5A. Place 5-10 crystals of CuSO4·5H2O (copper (II) sulfate pentahydrate) into a mortar. Describe their appearance. Now use a pestle to grind these crystals of into a uniform, fine powder. Note any changes you observe. Are these physical or chemical?
5B. Place a small spatula full of CuSO4·5H2O crystals into a clean, dry test tube. (The sample should just cover the bottom of the test tube). Use a test tube clamp or holder to gently heat the sample in a small Bunsen burner flame. Be sure to hold the test tube at about a 45º angle and move it constantly in and out of the flame to avoid overheating the sample. Also only heat the bottom part of the test tube. Continue heating gently until the original color of the sample has completely changed. Record your observations. Note also what has occurred near the top of the test tube where it has not been heated by the flame. What kind of changes did you observe?
CAUTION: While heating a sample in a test tube with a Bunsen burner flame, avoid pointing the open end of the test tube at yourself or any other person. Remember hot glass looks no different than cold glass!
5C. Allow the sample to cool in a test tube rack or small beaker until the test tube is safe to touch. With a pipette place one or two drops of water directly on the sample. What happens? Feel the bottom of the test tube with your fingertip. Record your observations. Is this a chemical or physical change?
Part II: Law of Conservation of Matter (Mass)
1A. Use a 10. mL graduated cylinder to measure 4.0 mL of 1.0M NaOH (aq) [sodium hydroxide] solution into a medium-sized (18 ´ 150 mm) test tube. Then measure 4.0 mL of 0.5 M CuSO4 (aq) [copper sulfate]. Record the color and appearance of both solutions.
CAUTION: Although a dilute solution of base like NaOH is relatively safe, it could cause chemical burns to your eyes or to sensitive skin, especially about your face. It may also damage clothing, ultimately causing holes to form where the base comes in contact with the fabric.
1B. Stand the two test tubes with the solutions from 1A in a 100 mL beaker. Now place the beaker with the test tubes and solutions on a balance and measure their combined total mass.
1C. Next pour the NaOH solution into the test tube with the CuSO4 solution. Describe what happens.
1D. Now thoroughly stir the mixture by flicking the test tube with the tip of your finger. [Do NOT use a stirring rod.] Let the mixture stand quietly for a minute or so. Describe any additional changes that you observe.
1E. Weigh the 100 mL beaker with the two test tubes (the empty one and the one containing the mixture) again to three decimal places. Did the weight change? Why or why not?
2. Repeat steps 1A-1E again, but this time use 4.0 mL of 2.0M NH4OH (aq) [ammonium hydroxide] and 4.0 mL of 0.5 M CuSO4 (aq). Measure the total mass as in step 1B both before and after mixing the two solutions. Again carefully record everything that you observe before and after mixing the two solutions in your notebook.
CAUTION: Although a dilute aqueous solution of NH4OH is relatively safe, ammonia vapor can irritate the eyes and lungs; the liquid solution may cause chemical burns to your eyes or to sensitive skin, especially around your face. It may also damage clothing and other fabric.
3. Repeat steps 1A-1E for a third time, but now use 4.0 mL of 1.0M Na2CO3 (aq) [sodium carbonate] and 4.0 mL of 0.5 M CuSO4 (aq). Measure the total mass as in step 1B both before and after mixing the two solutions. Again carefully record everything that you observe before and after mixing the two solutions in your notebook.
4. Repeat steps 1A-1E for a fourth time. For this last experiment we need two clean beakers. Use the 10 mL graduated cylinder and measure 10.0 mL of 1.0M Na2CO3 (aq) and pour it into a 50 mL beaker; next measure 20.0 mL of 1.0M HCl (aq) into a 100 mL beaker. Now measure the total mass of the two beakers and their contents. Next slowly mix the two solutions, carefully pouring the Na2CO3 (aq) into the beaker with the HCL (aq). After the reaction is complete, once again measure the total mass of the two beakers and their contents. Make a detailed record of whatever you observe in your notebook before and after mixing the two solutions.
Pre-Lab Questions:
1. A clear, colorless solution of chemical A and a colorless, but cloudy suspension of chemical B had a combined total mass of 1.623 g. After mixing, the new mixture was cloudy and green and had a mass of 1.615 g. Was the Law of Conservation of Mass obeyed during this experiment? What might explain the small change in mass?
2. A clear, colorless solution of chemical D and a clear, red solution of chemical E had a total mass of 1.840 g. After mixing the two solutions, a new orange solution was formed with evolution of many small bubbles of gas. The new solution had a mass of only 1.671 g. What might account for the loss of mass that was observed in this experiment? Was the Law of Conservation of Mass obeyed? Do we have enough information to give a definitive answer the previous question? Please explain.
D. Michael Byler, Chem. Dept., CCP, 20 Sep 2007; revd. 31 Jan 2008
