Published on January 9, 2008
What is pH?: What is pH? Acids and Bases: Some of our favorite foods make our tongue curl up because they are SOUR. Acids and Bases 2 Bitter!: Bitter! Some foods have a “bite” of their own because they’re somewhat bitter. WHY? 3 Acidic/Basic: Acidic/Basic There is a scientific reason for this: These foods are either acidic or basic. Other substances besides foods have these characteristics. 4 Acids and Bases: Acids and Bases Chemicals may be classed as acids or bases. Things that are neither acids nor bases are neutral. pH measures how acidic or basic a solution is. 5 Acids: Acids Often taste sour * Strong acids can burn skin & eyes Strong acids can dissolve metals Examples: Lemon juice Vinegar Car battery acid (dangerous!) *Never test an unknown acid by tasting it! 6 Bases: Bases Can taste bitter, sweetish, or salty * Often feel slippery or “soapy” * Strong bases can burn skin & eyes Bases react more easily with protein than with metal; they are often used for cleaning Examples: Milk Baking soda Soap Drain cleaner (dangerous!) *Never test an unknown base by touching or tasting it! 7 No I am Not:: No I am Not: Some substances are not really an acid or a base: For example, pure water 8 Most Substances:: Most Substances: Can be identified as either acidic or basic Like the soil in our backyard. 9 pH: pH A special name is given to the acid or base characteristic that a substance has: It is called: pH 10 How Do We Measure pH?: How Do We Measure pH? We measure pH by using special strips of paper called pH paper 11 How Does It Work?: How Does It Work? The paper is treated with chemicals that change color to show the pH. When the paper touches the substance being tested, it turns a specific color to tell if the substance is an acid or a base. 12 Acid: Acid Any substance which has a pH of value of less than 7 is considered an acid 0--------------7---------------14 Acid Neutral Base 13 Base: Base Any substance which has pH value greater than 7 is a base 0--------------7---------------14 Acid Neutral Base 14 pH 7: pH 7 A pH of 7 is called neutral—neither acid nor base. 0------------7------------14 Acid Neutral Base 15 Acidic or Basic: Acidic or Basic If the number is less than 7 the soil or water is acidic If the number is more than 7 the soil or water is basic 16 The pH Scale: The pH Scale pH scale ranges from 0 -14 pH 7 is neutral; neither acid nor base Pure water is pH 7 Low pH = acid High pH = base The closer to the ends of the scale, the stronger the solution is 17 The pH Scale: The pH Scale 18 The pH Scale: The pH Scale Each pH unit is 10 times as large as the previous one A change of 2 pH units means 100 times more basic or acidic 19 The pH Scale: The pH Scale Careful measurement is important A mistake of one pH unit means 10 times too much or too little! 20 Why is pH important?: Why is pH important? Soil has to be in a certain pH range for plants to grow and stay healthy. Fish can’t live if the pH is too high or too low 21 Slide22: pH and People Water that has too high or low pH may contain harmful dissolved chemicals. Water plant operators keep a careful watch on the pH of our drinking water, to keep it safe. 22 Two papers, two charts: Two papers, two charts Using two different papers gives a more precise reading. 23 Rain/Soil pH: Rain/Soil pH The pH paper comes in a double roll. Orange is for the low range (0-6). Yellow is for high range (6.5-14). Since most rainwater and soils have a low pH, begin with the orange paper. Tear off a 2-inch strip. Do not touch the end you will place in the mixture. Dip the end of the strip into the rainwater or the soil/water mixture. Compare the color to the orange color chart (0-6). If none of the colors match or if there is no color change, repeat the process using the yellow paper and the yellow color chart. Be sure to use the correct color chart! 24 Rainfall pH: Rainfall pH If your school has a 1st grade, work with them to measure rainfall pH when they measure rainfall depth. There must be at least 2 mm of rain in the rain gauge to measure rainfall pH. If your school does not have a 1st grade, you will need to install a rain gauge and define an Atmosphere Study Site to measure rainfall pH. Use the “Precipitation pH Lab Guide” and the “Documenting Your Atmosphere Study Site Field Guide” (from the Teacher’s Guide CD or from www.globe.gov) for instructions on how to define a study site and measure pH. 25 What color is it?: What color is it? Have you tried testing the pH of bleach? What happens? When you try the orange paper, you get a strange (but pretty) purple color that doesn’t match anything on your chart. When you try the yellow paper, you get another shade of purple but it doesn’t match either. Plus, in a couple of seconds the color goes away and the paper turns white! What’s going on? 26 What color is it?: What color is it? Household bleach (Clorox) is a very strong base (around pH 13). It is so strong that even the orange paper will try to match the color. When you use the yellow paper, the pH is still so high the paper can’t get a good color match. Plus, the bleach immediately starts to oxidize, or use up, the chemicals in the paper. So after a couple of seconds, the color goes away completely! Try this: put about 100ml of distilled water in a cup. Measure the pH. Then add about 10 drops of bleach. Swirl or stir and measure the pH. Did you get a reading? What was it? 27
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