LABORATORY EXERCISE #3
CHEMISTRY OF LIFE
LABORATORY OBJECTIVES
To learn about chemicals and chemical reactions that are a part of both the living and nonliving world around us. Upon completion of this laboratory exercise, the student will be able to:
REFERENCE
Textbook: chapter 3
EARLY PREPARATION
You will prepare a slaked limewater solution just as the laboratory period begins. This solution must sit for an hour or more before it is used later in the lab. You will prepare it without explanation and we will use it and explain it at the appropriate time. Vigorously mix about five ml of calcium hydroxide, Ca(OH)2, with a small beaker 2/3 full of water. Keep it tightly covered with paraffin paper. Some precipitate may collect at the bottom. When you start to use the limewater, carefully pour off the clear portion for use.
INTRODUCTION TO CHEMISTRY
All things on this earth that have weight and take up space are made up of matter that can be in the form of a gas, a liquid or a solid Chemistry is the study of matter. Matter is made up of atoms whose electrons interact with the electrons of other atoms to form molecules or compounds. The compounds then interact with each other to form new compounds. All of these interactions are chemical reactions. In a nutshell, everything around us and all the activities that occur are based on chemical reactions.
Chemistry is an integral part of biology, for every part of every living thing is made up of chemicals. Those chemicals are perpetually cycled among living organisms, the atmosphere, the earth and the water. Those same atoms that make up your body, or the trunk of a tree, previously were components of the air, soil, rivers, and even other living creatures. Indeed, an atom of your brain may once have been an atom of stellar dust or a dinosaur.
Not only are living things made up of chemicals, chemical reactions among different substances are what keep organisms alive and explain all the process of life. Movement, cell division, energy production, food production, food digestion, reproduction, allergic reactions, and even thinking and memory can be explained as chemical reactions that occur within organisms, and between organisms and their environment.
Physical Properties of Water
Life as we know it cannot exist without water. It is a part of every living thing and most of the chemical reactions of life are dependent on water. Organisms either live in water or they go to great lengths to acquire and retain water within their bodies.
Water is so common that it is easy to overlook the fact that it is an exceptional substance with many extraordinary and unique properties. The unique properties of water are important to life.
1. Water is adhesive and cohesive. Water molecules tend to attract and “stick to” one another and to other substances. This causes water drops to form a sphere and it gives water an unusually high surface tension. Because of the high surface tension some insects can scoot about on the surface of a lake. Adhesion and cohesion of water molecules contribute to the transport of water from the roots to the leaves of plants. It also is important in the transport of blood through the tiny vessels in our bodies.
2. Water resists changes in temperature. That is, compared to most other substances it takes a greater input or removal of energy to change the temperature a given amount. The resistance of water to changing temperature means that creatures composed mostly of water, like ourselves, have a certain built-in temperature stability. It also means that organisms that live in water are buffered to some degree by the resistance of water to changes in temperature.
3. Water is less dense as a solid than as a liquid. That is, ice floats. Instead of freezing and solidifying a body of water, the ice floats at the surface, insulating and protecting the life below it.
4. Water is a versatile solvent. It can dissolve many other solutes. This happens in cells, body fluids, tree sap and so on.
Activity
Surface Tension
1. Place a finger bowl of water on a steady surface. Using a pair of forceps (tweezers), pick up a small paper clip. Try to gently lay the clip on the surface of the water and have it not fall to the bottom of the bowl. To accomplish this you must eliminate all sources of vibrations or quick movement. It may take several attempts.
Were you successful? About how may attempts did it take?
Ordinarily objects heavier than the water will sink. Certainly you observed that the paper clip would sink to the bottom. Why then were you able to make it float?
2. Put two drops of water, about an inch apart on a glass slide. Put a second glass slide down on the first, sandwiching the water in between. Match up the four corners of the two slides. Try to pull the two slides apart without sliding them along one another and without prizing them apart with your fingernail. Now slide one glass slide over the other so that you can grasp a corner of each slide. Try to pull the two slides apart.
Are the two slides easy to pull apart? Why or why not?
Acids, Bases and Buffers
Water consists of two atoms of hydrogen and one atom of oxygen – H2O. In pure water a small number of the water molecules break apart resulting in a few hydrogen ions (H+) and a few hydroxyl ions (OH-) in equal numbers. When H+ and OH- are present in equal numbers, the solution is said to be neutral. If H+ ions are present in a higher concentration, the solution is acid. If OH- ions are present in higher concentration, the solution is basic. Scientists use a pH scale to measure and define an acid and a base. The scale ranges from 0 – 14.
Neutral solution – ph 7
Acidic solution – pH less than 7 (0 to 7, but not including 7)
Basic solution – ph greater than 7 (7 to 14, but not including 7)
Extremes of pH in either direction are toxic to living things. The low pH in your stomach (a pH of about 2-3) gives you heartburn when stomach contents regurgitate into your esophagus. Drano (a pH of about 12) does a great job of destroying the hair in your drain but you do not want it on or in your body. A buffer is a compound or mixture of compounds that can absorb H+ or OH- and keep the pH within a desirable range. You might think of it as a chemical sponge; it helps to avoid extremes of pH.
Activity
Determine the pH of Various Substances
To determine the pH of various solutions, you will learn to use pH indicator paper. The pH paper changes shades that correspond with the pH of the solutions with which it comes into contact. Lightly wet the small strip of pH paper with the solution being tested. After 30 seconds compare it with the comparison strip and choose the best match. To avoid contamination that can lead to false pH determinations, be sure to always wash your dropper or container each time you start to use it.
Determine and record the pH of each of the following solutions: distilled water, rainwater, saliva, and dilute vinegar (five parts distilled water to one part vinegar), and diluted bleach (about 20 parts distilled water to one part bleach). Caution: vinegar and bleach can be damaging to skin. Avoid skin contact. Do not allow the vinegar and bleach to come into contact with each other. Test other substances from this list: soapy water, urine, cola, coffee, tomato juice, apple juice; and anything else you want to test.
Record your results on the following table:
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Substance |
pH |
acid or base |
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Tap water |
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Rainwater |
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Saliva |
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Vinegar |
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Demonstrate the Buffering Effect of Soda or Antacid
Pour about twenty ml of distilled water into a small beaker or test tube. Add about seven ml of vinegar. Determine the pH of this solution.
Add about three mil of baking soda (sodium hydrogen carbonate) or an antacid tablet. Stir until the soda goes into solution. Determine the pH of the solution.
What is the pH of the distilled water and vinegar solution?
When you added the baking soda or antacid, did the pH increase, decrease or stay the same?
If you did not observe a distinct change in pH, add more baking soda or antacid, stir, and again measure the pH. (What is the pH now? )
This demonstration illustrates the _________________ effect of the soda or antacid.
How do you think an antacid works to make your stomach ache feel better?
Chemical Reactions
Cells of almost all life forms, including humans, get the energy they need by “burning” glucose molecules. In most cases oxygen taken in from the environment of the organism must be present for this chemical reaction to occur. The oxygen molecules react with the starch molecules producing water and carbon dioxide, and releasing energy. This energy can be used to drive other chemical reactions necessary for life. This important chemical reaction, called cellular respiration, is illustrated by the following equation:
C6H12O2 + O2 à CO2 + H2O + energy
starch + oxygen à carbon dioxide + water + energy
This reaction explains why animals exhale carbon dioxide.
Activity
Detect the Presence of Carbon Dioxide
With another chemical reaction the presence of carbon dioxide can be detected. The limewater that you made at the beginning of class is a solution of calcium hydroxide, Ca(OH)2, or slaked limewater. When Ca(OH)2 comes into contact with CO2 it forms a precipitate – a compound that is insoluble in water – that settles to the bottom. The equation for this chemical reaction is:
Ca(OH)2 + CO2 à H2O + CaCO3
Clear limewater + carbon dioxide à water + calcium carbonate
Pour limewater into a small beaker to a depth of about two inches. Place a stirring straw into the limewater and gently blow for more than one minute. Look for the formation of a precipitate. A precipitate indicates the presence of carbon dioxide from the air you exhaled.
What did you observe when you blew into the limewater?
What is the precipitate?
What will happen to the precipitate in the water if you let it sit for a few hours?
What happened to the CO2 when you exhaled into the limewater?
STUDENT ACTIVITY: CHEMISTRY IN A BAG
Most changes in matter involve energy transfers. When heat must be added before a reaction will occur, it is said to be endergonic. For example, making a cake is an endergonic reaction. After you mix all the ingredients, the cake will not form unless heat is added. Exergonic reactions are reactions where heat is given off. Most occur spontaneously as soon as chemicals are mixed together. Whether in an organism, in the air, in the soil, or in the water, chemical reactions occur all the time. In this activity, four different chemicals will be combined causing an interesting
chemical reaction.
Materials
Plastic ziplock bags
Plastic spoon
10-mL Plastic vial
Phenol red indicator
Water
Calcium chloride
Baking Soda
Small graduated cylinder
Procedure
1. Put a spoonful of baking soda into a ziplock bag. Close the bag, observe, and record observations on a data table.
2. Put 2 spoonfuls of calcium chloride in the same bag and mix them together. Close the bag, observe, and record observations on a data table.
3. In the 10 mL plastic vial, measure 10 mL of phenol red.
4. The next steps require more than one set of hands. Carefully read the directions before beginning.
5. Stand the plastic vial containing phenol red inside the ziplock bag. Keep the vial upright!
6. While keeping the vial upright, slowly squeeze the air out of the bag and seal.
7. Tip over the vial and mix the contents. Record notes in your data table.
8. Now you must investigate further to answer the following questions:
a) Which materials reacted to produce the heat?
b) Which materials reacted to produce the bubbles?
c) Which materials reacted to produce the color change?
Be sure to feel the outside of the bags for temperature changes.
10. When finished, be sure to:
Remove plastic vials; rinse, clean, dry, and return to the lab station.
Empty the bags into the sink and wash the contents down the drain.
Discard the bags in the trash.
Wash your hands.
1. Which materials produced heat? Where did the heat come from?
2. Which materials produced the bubbles? What do you think the bubbles are?
3. Which materials produced the color change? Why do you think the color changed?
4. Did you observe chemical or physical changes? Explain.
5. Which reaction is endergonic; dissolving sodium bicarbonate in water or dissolving
calcium chloride in water?
Explanation
When water is added, a chemical reaction occurs between the calcium chloride and the
baking soda in which a double replacement reaction takes place. The two solids switch
partners and form a new substance--the precipitate calcium carbonate. As the
precipitate is formed, the bicarbonate breaks down first to make hydrogen ions, an acid.
This acid then converts some of the bicarbonate to carbon dioxide gas, which begins to
blow up the plastic bag.
Also, as part of the chemical reaction between these two solids, a change in temperature
can be felt--in one corner of the bag, an exergonic reaction causes the calcium chloride
to become warm, while in the other corner an endergonic reaction makes the baking
soda become cold.
Calcium chloride + baking soda (sodium bicarbonate) à calcium carbonate + sodium
chloride + hydrogen ions
CaCl2 + 2NaHCO3 à CaCO3 + 2NaCl + H+
Hydrogen ions + sodium bicarbonate à carbon dioxide + water + sodium ions
H+ + NaHCO3 à CO2 + H2O + Na+
Answers to Analysis Questions
1. Calcium chloride and water generates heat. Energy came from bonds in calcium chloride.
2. The bubbles came from the reaction of calcium chloride, baking soda and water. They are carbon dioxide.
3. The phenol red changed color from red to yellow when added to sodium bicarbonate. The formation of carbon dioxide produces an acidic solution which causes the color change.
4. Chemical changes. Observations of changes in temperature, gas production and color change were observed.
5. Dissolving sodium bicarbonate in water is an endergonic reaction.
Endothermic and Exothermic Reactions
A physical or chemical change may be accompanied by a change of energy. If the change requires heat from the environment, it is said to be endothermic.
Solute + Solvent + HEAT à Solution (it will feel colder)
If the change releases energy to the environment, it is said to be exothermic.
Solute + Solvent à Solution + HEAT (it will feel hotter)