BACKGROUND ph AND WATER ph 15 The ph of a solution is a measure of its hydrogen ion (H + ) concentration. A solution with the same amount of H + as pure water has a ph value of 7 and is said to be neutral. Solutions with increasingly more H + than water are assigned progressively lower ph values and are defined as acidic. Conversely, solutions with less H + than water are given higher ph values and are described as basic or alkaline. Concentrations of the hydroxyl ion (OH - ) equal hydrogen ion concentration in pure water, but decrease in acids and increase in bases. Thus a strong acid has a high concentration of H + and a low concentration of OH -, whereas a strong base has these ions in reverse proportions. Note that ph values represent ten-fold changes in ion concentration. This means a solution with a ph of 2 has ten times more H + than a ph 3 solution and 100 times more H + than a solution with a ph of 4. The ph Scale ph Value Quality Comparable substance H + Concentration (moles/liter) OH - Concentration (moles/liter) 0 most acidic 1 (= 10-0 ) 0.00000000000001 (= 10-14 ) 1 battery acid 0.1 (= 10-1 ) 0.0000000000001 (= 10-13 ) 2 stomach acid 0.01 (= 10-2 ) 0.000000000001 (= 10-12 ) 3 vinegar 0.001 (= 10-3 ) 0.00000000001 (= 10-11 ) 4 0.0001 (= 10-4 ) 0.0000000001 (= 10-10 ) 5 0.00001 (= 10-5 ) 0.000000001 (= 10-9 6 least acidic saliva 0.000001 (= 10-6 ) 0.00000001 (= 10-8 ) 7 neutral pure water 0.0000001 (= 10-7 ) 0.0000001 (= 10-7 ) 8 least basic sea water 10-8 10-6 9 intestinal fluids 10-9 10-5 10 10-10 10-4 11 10-11 10 3 12 10-12 10 2 13 10-13 10 1 14 most basic lye (Drano) 10-14 10 0
ph and Water 16
ph and Water 17 WATER Since 1929, when a man named Henderson wrote a book entitled The Fitness of the Environment, biologists have been aware that life depends heavily upon certain physical properties of water. Keep in mind that not only are all living things around 60-90% water, but also that most places where organisms live are at least partially wet. The purpose of this exercise is to learn about some of these special water properties and how they are important to life on earth. This should help you to understand why scientists regard water as a necessary prerequisite for life on other planets. From lecture (and reading) you have learned that water molecules are polar, meaning that they have slight electrochemical charges at each of their four poles, two negative charges on the oxygen atom and a positive charge on each of the two hydrogen atoms. A positive charge on one water molecule forms a weak hydrogen bond with a negative charge on another water molecule. In this fashion, each water molecule forms four hydrogen bonds with other water molecules. PROCEDURES ph OF COMMON HOUSEHOLD SOLUTIONS One way to get a better understanding of ph is to relate the properties of familiar substances to their ph values as we have done in the diagram above. This exercise will attempt to carry this process a few steps further. Aqueous (water) solutions of a variety of household substances will be provided on the lab counter tops. Strips of ph paper that react to different ph values by changing color will also be provided. The ph paper should be handled with forceps (tweezers), as the salts present on your fingers will react with them, producing incorrect results. Dip a strip of paper into one of the sample solutions then rapidly compare its color to one of the color keys on the counter. Delay of more than 5 or 6 seconds before making the color comparison can lead to error. Record your results in the worksheet at the end of this lab. Place used paper strips in the containers labeled Used ph Paper.
ph and Water 18 It is not necessary (or reasonable) that all students measure each of the substances. Rather, each student should measure a select group of solutions and then trade results with other students to get the data for other substances. ACIDIFICATION OF LIQUIDS BY CARBONATION The carbonation of soft drinks is done by "injecting" carbon dioxide gas into the beverage. In a similar fashion, the carbon dioxide produced by yeast during fermentation gives beer and sparkling wines their "natural" carbonation. When carbon dioxide and water are mixed, a weak acid (carbonic acid) is formed. This happens in all carbonated beverages. The reaction is as follows: CO 2 + H 2 O -----------> H 2 CO 3 Because Earth s atmosphere is about one percent carbon dioxide, rainwater falling through it becomes both naturally carbonated and acidified. The process only lowers the ph of rainwater to about 6. True "acid rain" is created when sulfur and nitrogen oxides (air pollutants) react with atmospheric water vapor to create sulfuric and nitric acids. BUFFERS Buffers have the ability to resist ph change by chemically incorporating excess H + or OH -. The addition of acid or base to a ph buffered solution will have relatively little effect on the original ph. Many fluids, including blood and milk, contain natural buffer systems. The following short experiment demonstrates this feature in milk. Pour 10 ml of milk into a 30-ml beaker and 10 ml of tap water into another. Measure the ph of each (using ph paper) and record the results on the lab worksheet. Now add 5 drops of dilute hydrochloric acid (HCl) to each beaker, stir and measure their ph's again. Finally, add 10 drops of weak base (sodium hydroxide, NaOH) to each beaker, stir, and once again measure the resulting ph's. There are questions about your results on the lab worksheet. Specific Heat THE PROPERTIES OF WATER The specific heat of a substance is the amount of heat necessary to raise the temperature of one gram of that substance one degree Centigrade. Substances with high specific heats require lots of heat to raise their temperatures. They are hard to heat up (and hard to cool off). Incredible though it may seem, ordinary water has the highest specific heat of any known substance except ammonia. Try a simple experiment, comparing the specific heats of water and another common solvent, ethyl alcohol. This may lead us to consider the biological disadvantages of hypothetical bodies and environments containing alcohol! 1. Add about 2 cm of crushed ice to one of the large glass bowls you have been provided. Add enough cold tap water to just cover the ice.
ph and Water 19 2. With a wax pencil, label the two small beakers you have been provided; one as water and the other as alcohol. Add three cm of tap water to the water beaker and 3 cm of ethyl alcohol to the alcohol beaker. Alcohol is available in a large flask on the side counter. 3. Place the beakers of alcohol and water into the bowl of ice water, making sure the fluid levels in the beakers are higher than the fluid level in the bowl (otherwise the beakers tend to float and tip over). Place one thermometer securely into each of the two beakers. Allow about 10 minutes for the temperatures of the two beakers to equalize. This procedure will equilibrate the contents of both beakers to the same low temperature. It is critical that the beginning temperatures in the beakers be equal and cold! If a thermometer breaks, notify your instructor at once - do not attempt to clean up broken glass yourself. 4. After 10 minutes of chilling record the temperatures of the two beakers as the 0:00 temperature on the data sheet provided for specific heat. 5. Now add two cm of hot tap water to the second large glass bowl, the hotter the better. Shift the alcohol and water beakers into the bowl of hot water and begin recording the temperatures of the two beakers every 30 seconds. The Specific Heat Data Sheet provides a space to record each of these measures. 6. After five minutes of exposure to hot water, shift the beakers back to the bowl of ice water. Continue to record the beaker temperatures at 30-second intervals for another five minutes. The first temperature measurement recorded after returning to cold water should be entered in the 5:30" time slot on the data sheet. You will be asked to graph the results of this experiment on the lab worksheet. 7. Set the beakers of alcohol and water aside, along with the thermometers, for use in other experiments. Heat of Vaporization The heat of vaporization is the amount of heat necessary to cause a liquid to evaporate (change in state from a liquid to a gas). Since water has a very high heat of vaporization it is very slow to evaporate. Since the evaporation of water requires a great deal of heat, water is a very efficient evaporative cooler. It is easy to be confused on this point, so let's try a simple experiment. 1. Have your lab partner rub a small amount of ethyl alcohol on the back of one of your hands and - at the same time - an equal amount of tap water on the back of your other hand. Your partner should do this in such a fashion that you are not aware which is which. (This is called a blind experiment; the subject being unaware of the nature of the treatment cannot bias the results. To make the experiment double blind your lab partner - the researcher, in this case, must also not know which is which. A third party labels the solutions "A" and "B," and only reveals which is which after the experiment is finished.)
ph and Water 20 2. Record which liquid (water or alcohol) evaporates most rapidly and which produces the greatest sensation of cooling. A table is provided for these data in the lab worksheet. Your results may at first seem contradictory. Although the cooling produced by the alcohol seemed great, it was only a brief effect. In the long run, the slower-evaporating water takes away more heat. Surface Tension and Capillary Action Capillary action, the seemingly magical way in which water is drawn up by tubes with small diameters, involves forces known as cohesion and adhesion. Adhesion causes water molecules to cling to the sides of small pores, cylinders, and other tiny spaces. Cohesion enables water molecules to pull other water molecules up behind them. COHESION AND ADHESION The ability of water molecules to bond to each other is known as cohesion. These strong forces cause water's high specific heat and heat of vaporization by making individual water molecules resistant to movement and difficult to release into the air (heat content is proportional to molecular movement, and evaporation occurs when water molecules break free from the liquid state into the air above). Cohesion also allows water molecules to form an almost membrane-like outer layer known as surface tension. This is why mosquitoes are able to stand on water. Water molecules at the surface are unable to form hydrogen bonds above themselves, so the unused bonding capacity is filled on a plane parallel to the water's surface. This extraadhesive layer makes the surface tension. The charged poles of water molecules can also bond to other substances, assuming those substances are also charged or polar. This phenomenon is known as adhesion. Adhesion enables water to "wet" other substances, a process defined as adsorption. Adhesion also enables water molecules to hold other molecules and ions in solution.
ph and Water 21 The following experiment will demonstrate the interaction of charged surfaces and polar molecules in the process of capillary action. 1. Bring the following tubes to your work station: Glass, large diameter...1 Glass, small diameter ( capillary tube )...2 Glass, medium diameter...1 Plastic, medium diameter (drinking straw)...2 2. Now make pair-wise comparisons of water and alcohol in the various tubes. Following the chart below, place pairs of tubes into the beakers of ethyl alcohol and water that you set aside after the specific heat experiment. Record your findings in the corresponding data table provided in the lab worksheet. Comparison Tube diameter Tube type Liquid type versus Tube diameter Tube type Liquid type 1 Large Glass Water Small Glass Water 2 Small Glass Alcohol Small Glass Water 3 Medium Plastic Water Medium Plastic Alcohol Note: water is strongly polar, alcohol is weakly polar, plastic is non-polar, and glass is an electrically charged compound.
ph and Water 22