Lesson 2.3: A Scale for Acids and Bases

You have learned about the properties of acids and bases.  Can you tell if an acid or a base is dangerous?  Vinegar and battery acid are both acids. Vinegar is used in food and is quite safe; battery acid is used in car batteries and is very dangerous.  Baking soda and oven cleaner are bases.  Baking soda is used in cooking and is quite safe; oven cleaner is used to clean ovens and is very dangerous.  Scientists have developed a scale to indicate how acidic or basic a substance is.  While it cannot be used exclusively to tell if a substance is safe or dangerous, it is useful in determining how acidic or basis a substance is. 

Read page 65 in your textbook to learn about the pH scale.  

Question 1. What does the pH of a substance tell you?

Question 2. What is the number range of the pH scale?

Question 3. How does the pH of a substance tell you if it is an acid or a base?

Question 4. If the pH of a substance is 7, what does that mean?

Question 5. Use the pH scale in figure 4.6 to complete the table giving the pH of the following substances.

milk

Substance

pH

Substance

pH

ammonia

 
apples   milk of magnesia  
baking soda   normal Great Lakes water  
battery acid   oven cleaner  
human blood   tomatoes  
lemon juice   vinegar  
normal rain      

 

Question 6. A substance called Spider's Milk has a pH of 2.7.  Another substance called Fly's Milk has a pH of 3.8.  Which of the two substances is more acidic? 

Check your answers with those that follow.

 

 

 

 

 

 

 

 

 

 

 

Answers to Questions:

Question 1. What does the pH of a substance tell you?

The pH of a substance tells you how acidic or basic the substance is.

Question 2. What is the number range of the pH scale?

The number range is from 0 to 14.

Question 3. How does the pH of a substance tell you if it is an acid or a base?

If the pH is less than 7, it is an acid.  If the pH is more than 7 it is a base.

Question 4. If the pH of a substance is 7, what does that mean?

If the pH is 7, it is neutral.  The text indicates substances close to pH = 7 are neutrals.  Because there is no indication of how close to pH = 7 a substance has to be to be called a neutral, this course uses a different definition to keep things simple.  At pH = 7 the substance is neutral.  When the pH is less than 7 it is an acid.  When the pH is more than 7 it is a base.

Question 5. Use the pH scale in figure 4.6 to complete the table giving the pH of the following substances.

Substance

pH

Substance

pH

ammonia

11.1

 milk
6.6
apples

3.0

 milk of magnesia

10.5

baking soda

8.2

 normal Great Lakes water

8.5

battery acid

0

 oven cleaner

14.0
human blood

7.4

 tomatoes

4.2
lemon juice

2.0

 vinegar
2.2
normal rain

5.6

    

          

Question 6. A substance called Spider's Milk has a pH of 2.7.  Another substance called Fly's Milk has a pH of 3.8.  Which of the two substances is more acidic? 

Spider's Milk is more acidic because it has a lower pH.

 


Click on the page forward to continue Lesson 2.