What is pH?
• What is it like to eat a lemon?
• Scientists realized that the sour taste of a
lemon is related to certain type of chemical
when it is present (hint: hydrogen ions H+).
• And, vice versa, they also realized that the
bitter taste of something is related to certain
type of chemical when it is present (hint:
hydroxide ions OH-).
“potential for hydrogen (H)”
pH-scale that measures hydrogen
ions (H+=acidic) and hydroxide ions
(OH-=alkaline/basic)
12/9/13
• You can find the pH of any solution using
universal indicator.
• The more OH- ions (hydroxide ions), the more
alkaline an alkali will be.
• The more H+ ions (hydrogen ions), the more
acidic and acid will be.
pH Scale
• Each one-unit change in the pH scale
corresponds to a ten-fold change in hydrogen
ion concentration.
Properties of acids
1. They are liquids.
2. If concentrated they can be corrosive.
3. Acids taste sour (for example, vinegar, lemon juice).
4. Turn blue litmus paper red - this is an easy test for an
acid!
5. Usually react with metals to form salts. (experiment)
6. Acids contain hydrogen ions (H+).
7. Turn Universal Indicator from green to red, and have a
pH less than 7.
Properties of alkalis
1. Like acids, they can burn the skin.
2. They turn red litmus blue - this is how you test
for an alkali!
3. Alkalis contain hydroxide ions (OH-).
4. They taste bitter. (for example: Baking soda,
tums, bleach)
5. Turns Universal Indicator from green to blue or
purple, and have a pH higher than 7.
Properties of neutral substances
1. Litmus paper is not affected by neutral paper.
2. Tend to be harmless.
3. Universal Indicator stays green, and pH of 7.
Buffers
• A molecule that tends to either bind or
release hydrogen ions (H+) to maintain pH.
• Important to help maintain
homeostasis/balance. Human pH=7.4
• If our blood is above or below 7.35-7.45,
negative affects will occur.
• Humans can tolerate pH between 7.2-7.6
• Can be either acidic or basic buffer.
Questions – 1PP/question
1. What is the pH of a neutral compound? An
acidic compound? A basic compound?
2. What are some common substances that are
acidic?
3. What are some common substances that are
basic?
4. What natural compounds can neutralize acids
or bases?
pH mini-lab
Each group of 4 will receive a chemical tray with 4
liquids (as designated on the table).
• Make a guess as to whether you think it might be
acidic or basic.
• Test one of the chemicals for pH using the strips.
• Compare your pH strip with the pH chart.
• Tell what pH you found it to be.
• Go back and write the pH in your data table and
decide if it is acidic, basic or neutral.
• Test the other three, one at a time.
Predict whether each chemical is basic, neutral, or acid.
Chemical
Prediction
(basic, neutral,
acid)
Color on
indicator/pH #
H2O
Your guess before
testing
Color
Coffee
Orange juice
Milk
Category
(basic, neutral,
acid)
pH #