NAME_____________________DATE_______________PER______ Measuring pH of Various Substances Academic Objective: SWBAT define acids and bases and relate their importance to biological systems Roles: (10 points of grade) Name Roles Description Reader/Task Master Reads the group’s material aloud. Makes sure the group stays on task Recorder/Gatekeeper Writes the group’s consensus of the answers. Makes sure everyone has a chance to speak or participate. Materials Handler/ Obtains and returns materials and Timekeeper equipment needed by the group. Ensures the lab station is cleaned up. Keeps track of time for the group to ensure the group completes the assigned task. Checker/Prober Verifies that everyone agrees with the answer. May try to get students to elaborate and asks for other possible answers. Introduction There’s a lot more going on in water than meets the eye! Most of the molecules are present at H20, but at any instant some of them break into two parts: H+ and OH-. In pure water, the amount of H+ and OH- must be exactly the same, since every time a molecule splits, one of each type of ion is produced. But in some fluids containing other dissolved materials, this balance is lost: the fluid can have more H+ or more OH-. The amount of H+ or OH- in a fluid gives it some important properties. In particular, the amount of H+ in a solution is a measure of its acidity and is called pH. The greater the number of free hydrogen ion floating around, the more acidic the solution is. Pure water is in the middle of the pH scale, with a pH of 7.0. Any fluid with a pH below 7.0 has more H+ ions (and fewer OH- ions) and is considered an acid. Any fluid with a pH about 7.0 has fewer H+ ions (and more OH- ions) and is considered a base. The pH scale, like the Richter scale for earthquakes, is logarithmic, although in the case of pH, the lower the number the greater the acidity; a decrease of 1 on the scale represents a 10 fold increase in the hydrogen ion concentration. A decrease of 2 represents a 100 fold increase in hydrogen ion concentration. The pH of blood is usually 7.4. Given that most cellular reactions produce or consume H+ ions, there ought to be great swings in the pH of our blood. Unfortunately, our bodies cannot tolerate such swings. Most of the chemicals that aid in the chemical reactions in our blood or cells stop functioning properly if the pH swings up or down by less than half a point. Fortunately, our bodies contain some chemicals that act like bank accounts for H+ ions. Called buffers, these chemicals can quickly absorb excess H+ ions to keep a solution from becoming too acidic, and they can quickly release H+ ions to counteract any increases in OH- concentration. Therefore, buffers are chemicals that act to resist changes in pH. Visit each station to test each of the following samples to determine the pH. Record your results in the table provided. Data Table: Substance pH Acid or Base Lemon Juice Tea Coffee Soda Bottled Water Milk of Magnesia Mr. Clean Tap Water Vinegar Laundry Detergent Bleach Orange Juice Ocean Spray Juice Milk ACT Mouthwash Analysis: 1) Indicate where each of the tested items would appear on the below pH scale: 2) Which solutions are acids? 3) Which solutions are bases? 4) What ions in the solution caused the pH paper to change? 5) What solution has the highest concentration of hydroxide ions (OH-)? How do you know? 6) What solution has the highest concentration of hydrogen (H+) ions? How do you know? 7) What does it mean in regards to the H+ and OH- concentrations if a substance is neutral? 8) Pure water has a pH of 7. If the tap water or bottled water you tested does not have a pH of 7, how might you explain this? 9) Where does stomach acid fall on the pH scale? Why do you think it has this pH? 10) What is an antacid and how does it function? 11) What is a buffer and how does it work? What is its importance in the human body? 12) By far the most important buffer for maintaining acid-base balance in the blood is the carbonic-acid-bicarbonate buffer. Research this buffering system and explain how it works. 13) What is acidosis and alkalosis? What causes these conditions? Can these conditions be corrected? If so, how?