Lab #2 Biology 10 BCC Topic: Chemistry in Practice

Lab #2 Biology 10 BCC Topic: Chemistry in Practice Chemistry is a vast field of study in and of itself. In Biology, we use chemistry as a tool to help us understand how things relate to each other on a smaller level. In this lab, we are going to take the aspects of chemistry that relate to our studies so we can better understand the basics. Part I: Covalent vs. Ionic Bonding, Water and it s properties. 1. Go to www.bio10bcc.weebly.com, find and watch the following podcasts Covalent vs. Ionic bonding, Water as a polar molecule and properties of water and surface tension. Use class notes to help you through the following lessons. Part II: Unique Properties of Water Water has some very unique properties. Much of this uniqueness stems from the fact that water, although covalent, has polarity. What does it mean to have polarity? Write out the definition of a polar covalent bond: On the letters below, draw lines to show which elements are bonded together on these water molecules. What kind of bond did you draw? Do you remember how many electrons both elements have in their valence shells? O H O H H H H O Add the polarity to both water molecules. Which side is positive and which side is negative? What does this mean for how the molecules relate to each other? Label that too. The fact that water molecules are attracted to each other is due to the cohesive properties of water. Cohesion is the molecular force between particles within a body or substance that acts to unite them. Water molecules are equally attracted to each other in all directions, if they are surrounded by other water molecules. What about the surface of the water? Page 1 of 10

~~~~~~~~~~~~~~~~~~~ Draw water molecules in this beaker. Show attraction between the water molecules by using dotted lines to demonstrate the attraction. What kind of bonds did you draw? How is this attraction different at the top of the water column? What do you call that type of tension? Because water is polar, it also has adhesive properties. Adhesion is the molecular force of attraction in the area of contact between unlike bodies that acts to hold them together. Let s see what this means. Go to the materials table. Grab two slides and a dropper of water and bring them back to your table. Put the two slides together. Do they stick? Is it easy to separate the slides? Why? Now, drop water on the slides and put them together. Is it still easy to separate them? Why? Since water is attracted to itself and to other things, it can do cool stuff like climb walls. Go to the materials table and get a small beaker and a capillary tube. Fill the beaker with a small amount of water. Now, place the tube into the beaker, holding it so it just touches the top of the water. Draw what happened to the water How can you explain what happened? What force do you think leads to this phenomena? The combination of cohesive and adhesive properties leads to something called capillarity. Can you find a definition for capillarity in the reference materials? If so, what did you find? What is the definition of capillarity? Page 2 of 10

Think about what this might mean for organisms like plants and animals. When you water a plant, where do you put the water? You probably know that water has to move up to the leaves, so what action do you think helps the water move up to the leaves? What properties of water help move water against gravity? Part III: Density of Water Now let s think about the fact that water is attracted to itself. Think about food items in your freezer and in your refrigerator. If you had a small, frozen fish in your freezer, you could probably break it in half with some effort. However, if the same fish were just refrigerated (and not frozen), would it be as easy to snap? Why? What do you think it means about the bonds if a frozen fish is easier to break? Do you think the water molecules in the frozen fish are as attracted to each other the same as in a refrigerated fish? Why or why not? If water molecules are like magnets, why do you think frozen water molecules don t pull toward each other? Do you think it has anything to do with how close together the molecules are? What is the word we use to describe how many of something is in one place (or mass per unit volume)? Page 3 of 10

Use the above concepts to explain why ice floats (feel free to draw if that helps). What do you think will happen to global water levels when glaciers melt? Part IV: Water and Energy Storage We ve heard that there is energy in the bonds between atoms. That energy holds them together, so when those bonds break, that energy is released. We just considered that there is a difference in the attraction of water molecules when water is frozen. Let s now consider what happens when water goes from a liquid to a gaseous state instead. At some point, you ve probably heard that the steam coming off a pot of boiling water is hotter than the water in the pot.you may have even experienced this (steam burns really hurt!). When water boils, liquid is converted into a gas, which is the steam. In order for water to turn into steam, what must happen to the hydrogen bonds of the water molecule? What do you think happens to the energy that holds those bonds together? Think about a rainy, calm day around here in the winter. Now, compare this temperature to that of a day when it isn t raining. You might have experienced that rainy days are actually much warmer. What do you think happens to water droplets as they fall and eventually evaporate from the ground? Rainy days are warmer because which bonds are broken? and they release when broken. This means that water has a high specific heat, or it can hold a lot of energy before changing. It also means that water can help regulate temperature in everything, including you. Water has what s called thermostatic properties, meaning it can keep climates moderate. Why do you think the temperature in Barstow is more drastic than the temperature in Los Angeles? Page 4 of 10

Mountains also affect climate because they influence air flow over land. When moist, warm air moves from the water toward a mountain, the air rises, releasing the moisture as it moves. On the other side, the cool, dry air falls on the other creating what s called a rain shadow. Why do you think there is more fog west of the San Bernardino Mountains? Why does Barstow get less rain than say San Francisco? Part V: Importance of ph 1. Go to bio10bcc.weebly.com watch the podcast on ph: Acids and Bases. What is ph? ph is a measurement of how acidic or how basic a solution is. Specifically, it is the measurement of how many H+ ions there are in solution. The ph scale starts at 0 and goes up to 14. Halfway between 0 and 14 is 7, which is neutral. Compounds are acidic if they have a ph lower than 7. Compounds with a ph higher than 7 are said to be basic or alkaline. Exactly what makes a compound an acid or a base? To understand this you must understand water. Water is a molecule made up of three atoms covalently bonded together. Think of water as HOH. Some compounds can cause water molecules to break apart into H+ and OH- ions. The H+ ion is called a hydrogen ion. It is actually proton with no electrons. The OH- ion is called a hydroxide ion. If you mixed hydroxide and hydrogen ions together, they would immediately pair up and make water molecules. H + + OH - ------------> HOH ----> H20 This is called a neutralization reaction. Hydroxide ions neutralize hydrogen ions. If, after the neutralization reaction is complete, there are H + ions left over, then the solution is acidic. If, after the neutralization reaction is complete, their are OH - ions left over, then the solution is basic. The scale we use is a logarithmic scale meaning a ph of 1 is really 10-1 or 0.1. Therefore a ph of 4 is really 10-4 or 0.0001. That means something with a ph of 1 has 1000x more H+ to donate than something with a ph of 4. 1 ACIDIC 7 BASIC 14 Page 5 of 10

Why is ph important to biology? Most cells can only survive within a certain range of ph. For example, human blood usually has a ph of about 7.3, which is slightly basic. A healthy person s blood often has a ph range of 7.35 and 7.45. Any ph higher or lower and the blood cells would be injured or killed. Acids denature, or change the shape of proteins in much the same way heat does. As a matter of fact, strong acids like vinegar and lemon juice can be used to actually cook meats like fish and eggs. Seviche is a dish made by mixing raw fish and lime juice and letting it sit for a few hours breaking down the proteins in the fish meat. Acids are used by your digestive system work in much the same way; to break down food molecules into simpler units called monomers. Bases cause oils and fats to fall apart. Your digestive system uses bile, a basic compound to help in the digestion of fats and grease. Oven cleaners and drain cleaners contain lye, a strong base that dissolves baked on grease and burned fats. Buffers are an important component in any biological system. Since the reactions in cells only work in a narrow range of ph, buffers exist to help keep ph within that range. A buffer is a mixture of molecules that release or bind H + in order to maintain a relatively stable ph. Note that the function of a buffer is NOT to keep a solution neutral (at ph 7); its function is to minimize the change in ph when base or acid is added to the solution. In humans, for example, buffers act to maintain blood ph between 7.35 and 7.45 even though acids and bases are continually being added to and removed from the blood as it travels through the body. ph Questions 1. A student mixes strawberry koolaid and water. A ph meter is used to measure ph of 5.4. What kind of solution is strawberry Koolaid? 2. In the koolaid mixture, what must there be more of, hydrogen ions or hydroxide ions? 3. A student adds an alka-seltzer to the koolaid and stirs. The ph meter now reads 8.3. What was released by the alka-seltzer tablet to cause this change? 4. Baking soda is a weak base. Hydrochloric acid is a strong acid. What would happen if these two were mixed? Page 6 of 10

5. A student takes large glass of tap water and measures and records the ph in the chart below after conducting various experiments with it. Note: Bromthymol blue is a chemical indicator that is blue in basic and neutral solutions, and turns greenish and then yellow as the solution becomes more and more acidic. Fill in what color you think bromthymol blue would be in each of the situations in the chart. Situation ph Bromthymol blue indicator Water directly out of tap ph =7.2 Water after exhaled air is blown ph=5.1 through a straw into it for 5 minutes. Water after a snail has lived in it for three days Water with 2mL of bleach added Water with instant coffee added ph=5.8 ph=9.4 ph=5.0 Water after an aquatic plant is grown ph=7.7 in it for three days in bright sunlight Use the table above to help you answer these questions: 6. What effect does carbon dioxide have on tap water? 7. Is bleach an acid or a base? 8. Is coffee an acid or a base? 9. Do you think your body would have uses for buffers? If so, what might those uses be? 10. Why do you think understanding ph is important? 11. Why do you think buffers are important? Page 7 of 10

Part VI: ph of Common Substances Intro: You will use chemical indicators to show whether a solution is acidic or basic. Red litmus paper turns blue when wet with basic solutions Blue litmus paper turns red when wet with an acidic solution ph paper will be compared against a chart to get ph value Universal indicator dropped into a liquid, and changes color of the solution based on its ph; which can then be compared against a ph chart. What do chemical indicators do? Procedure: 1. Copy the data chart ph of Common Substances onto a paper towel 2. You will test the ph of various common substances on the paper towel and then record your results on the chart below. 3. Place 1 piece of blue litmus, red litmus and ph indicator in the appropriate boxes for each substance on your paper towel. 4. Using the stirring rod in each solution, transfer a drop of solution first to a strip of blue litmus, then to a strip of red litmus paper, and finally to the ph paper. 5. Copy your results onto your chart below. 6. Obtain a ph dispenser to determine the ph number of each solution. Determine the ph by matching the color of the solution to the appropriate color on the ph dispenser. Record the ph value on your chart. 7. Once all the values are recorded, throw your paper towel and all the indicators in the trash. Clean up your lab area and wash your hands Solution Egg Milk Orange Juice Baking Soda Ammonia Water Coffee Vinegar Bleach Cola Tap Water Blue Litmus Color? Acid or base? ph of Common Substances Red Litmus ph paper (color Color? Acid and ph number) or base? Universal Indicator (color and ph number) Page 8 of 10

On the ph scale below, record the ph of each solution from the list above. 1 ACIDIC 7 BASIC 14 Post-Lab Questions 1. Which of the liquids had the lowest ph? 2. Which of the liquids had the highest ph? 3. Which of the liquids were closest to being neutral? 4. If the ph of a sample was 3, how MANY MORE TIMES acidic is it than a solution with a ph of 6? 5. If the ph of a sample were 6, how MANY MORE TIMES acidic is it than a solution with a ph of 13? 6. How might one correct the ph of a lake with a reading of 3? (hint: what could you add?) Page 9 of 10

Part VII: Final Questions 1. Explain how hydrogen bonds influence at least 2 unique qualities of water 2. How do these qualities affect climate? 3. What does ph stand for? 4. Why is ph important to us? 5. How do you think your body uses buffers? 6. Were there any surprises in the lab? Page 10 of 10