Name: ______________________________________
Date: ________________________
Student Exploration: pH Analysis Vocabulary: acid, acidic, alkaline, base, indicator, neutral, pH
Prior Knowledge Questions (Do these BEFORE using the Gizmo.) 1. Acids are substances that produce hydrogen ions (H+) when dissolved in water. Lemon juice is an example of an acid. A. What does lemon juice taste like? ________________________________________ B. What does it feel like if lemon juice gets in your eye? _________________________ 2. Bases are substances that produce hydroxide ions (OH-) when dissolved in water. Hand soap is an example of a base. A. What does soap feel like? ______________________________________________ B. What does soap taste like? _____________________________________________ C. What does it feel like if soap gets in your eye? ______________________________ Gizmo Warm-up The strength of an acid or base is measured on the pH scale. The term “pH” is short for “potential of hydrogen.” It is a measure of how many excess H+ ions there are in a solution. The pH scale runs from 0 to 14, with 0 representing the highest concentration of hydrogen ions. Acidic substances have a pH below 7, while alkaline substances (bases) have a pH above 7. Pure water has a pH of 7 and is considered neutral. The pH Analysis Gizmo™ allows you to find the pH of a variety of liquids. In the Gizmo, check that the Substance in the tube is Ammonia, and click Test. Wait until the animation is finished. 1. Indicators change color in acids or bases. What is the color of the pH paper? ___________
2. Compare the paper to the pH color chart. What is the pH of ammonia? _____________
3. Is ammonia acidic or alkaline? _____________
Get the Gizmo ready:
Activity A:
Click Reset. Check that the 0-14 paper is selected.
Measuring pH
Goal: Find the pH of 18 common substances. 1. Test: Use the Gizmo to find the pH of each of the available substances. Classify each substance as acidic (pH < 7), alkaline (ph > 7), or neutral (pH = 7).
0-14 pH indicator paper Material in the tube
pH value
Acidic, alkaline, or neutral?
Baking soda Bleach Coffee Cola Drain cleaner Hand soap Juice (lemon) Juice (orange) Juice (tomato) Milk Milk of magnesia Oven cleaner Saliva (human) Shampoo Stomach acid Vinegar Water (distilled) Water (ocean)
2. Summarize: Compare all the acidic substances and all the alkaline substances. A. What substances in the table are acidic? _______________________ ___________________________________________________________________ B. What substances are alkaline? ______________________________ ___________________________________________________________________
Get the Gizmo ready:
Activity B: More accurate pH
Click Reset. Select the 4.5-7.5 paper.
Goal: Find the pH of substances in a more accurate way. 1. Test: Before you begin testing with the 4.5-7.5 paper, list the pH values of the substances below that you found using the 0-14 pH indicator paper. Then find the pH of each substance with the 4.5-7.5 paper.
4.5-7.5 pH indicator paper Material in the tube
pH value (0 to 14 paper)
pH value (4.5 to 7.5 paper)
Coffee Milk Oven cleaner Saliva (human) Shampoo Stomach acid Water (distilled)
2. Analyze: Compare the pH values in each column. A. How do these values compare? _________________________________________ ___________________________________________________________________ B. What is an advantage of using the 4.5-7.5 paper? ___________________________ ___________________________________________________________________ C. What is a disadvantage of using the 4.5-7.5 paper? __________________________ ___________________________________________________________________ D. Given the results from two kinds of indicator paper, which substances appear to be neutral (pH = 7)? _____________________________________________________
DO NOT FORGET TO DO THE QUIZ AT THE END OF THIS GIZMO ACTIVITY!!!!!
ADDITIONAL RESEARCH You will need to use your textbook and notes to answer the following questions… The pH scale is used to describe how acidic a solution is. It is a measure of the concentration of hydrogen ions (H+) in solutions. The basis for the pH scale is the tendency of water to dissociate into hydrogen ions (H+) and hydroxide ions (OH-). Pure water has equal concentrations of H+ and OH- ions and a neutral pH of 7 on a scale of 0 to 14. One pH unit represents a difference or relative acidity of 10 times the H+ concentration. In other words, an acid with a pH of 3 is 10 times more acidic and has 10 times more hydrogen ions than an acid with a pH of 4. 1.
Explain the concentration of hydrogen ions to hydroxide ions of an acid.
2.
Explain the concentration of hydrogen ions to hydroxide ions of a base.
3.
What is an ion?
4.
How are hydrogen ions and hydroxide ions formed?
5.
If Base ‘A’ has a pH of 11 and Base ‘B’ has a pH of 14; which base is stronger? How many times stronger is it based upon relative acidity?
Vocabulary: pH Analysis Vocabulary
Acid – a water-soluble compound that donates protons (H+ ions) to a base. o
Acids are often sour in taste, can burn the skin and eyes, and react with a base to produce a salt and water.
o
Concentrated acids are corrosive to metals.
o
The chemical formulae of acids usually begin with H. Examples are HCl (hydrochloric acid, found in your stomach), H2SO4 (sulfuric acid, found in car batteries) and H2CO3 (carbonic acid, found in soft drinks).
Acidic – being or containing an acid. o
Alkaline – being or containing a base. o
Alkaline substances have a pH value greater than 7.
Base – a water-soluble chemical compound that accepts protons from an acid. o
Bases are often bitter in taste, have a slippery texture, and react with acids to product a salt and water.
o
Concentrated bases are corrosive to organic matter.
o
The chemical formulae of bases usually end with OH. Examples are NaOH (sodium hydroxide, found in drain cleaners), KOH (potassium hydroxide, used to make soap) and Ca(OH)2 (calcium hydroxide, found in plaster).
Indicator – a substance that changes color when in contact with an acid or base. o
Acidic substances have a pH value less than 7.
Examples of indicators include litmus, bromthymol blue, methyl yellow, phenol red, red cabbage juice, curry powder, and many others.
Neutral – neither acidic nor basic. o
Neutral substances do not yield excess H+ or OH- ions when dissolved in water.
o
Pure water is neutral and has a pH value of 7.0.
pH – a measure of how many hydrogen ions there are in a solution. The greater the number of hydrogen ions, the more acidic the solution. o
The symbol “pH” stands for “potential of hydrogen” or “power of hydrogen.”
o
The pH scale is a negative logarithmic scale:
The lower the pH, the greater the concentration of hydrogen ions, and the more acidic the solution.
Because the scale is logarithmic, a substance with a pH of 4 is 10 times more acidic than a substance with a pH of 5.
