Chemistry: Acids, Bases, and the pH Scale
Classifying substances and interpreting pH values
Chemistry: Acids, Bases, and the pH Scale
Classifying substances and interpreting pH values
Chemistry - Grade 6-8
- 1
A sample has a pH of 3. Is it acidic, basic, or neutral? Explain how you know.
Compare the pH value to 7.
The sample is acidic because acids have pH values less than 7. - 2
A student tests three liquids. Lemon juice has pH 2, pure water has pH 7, and baking soda solution has pH 9. List the liquids from most acidic to most basic.
The order from most acidic to most basic is lemon juice, pure water, and baking soda solution. - 3
What pH value is considered neutral? Name one common substance that is close to neutral.
Neutral is in the middle of the pH scale.
A pH of 7 is considered neutral. Pure water is a common substance that is close to neutral. - 4
Blue litmus paper turns red when dipped into an unknown liquid. What type of substance is the liquid most likely to be?
The liquid is most likely an acid because acids turn blue litmus paper red. - 5
Red litmus paper stays red when dipped into vinegar. Does this result prove that vinegar is neutral? Explain.
Think about what red litmus paper does in acids and bases.
No, this result does not prove vinegar is neutral. Red litmus paper stays red in acids, and vinegar is acidic. - 6
A cleaning solution has a pH of 12. Describe one safety precaution you should use when handling it and explain why.
You should wear safety goggles and gloves because a solution with pH 12 is a strong base and can irritate or damage skin and eyes. - 7
A student says, "All acids are dangerous." Explain why this statement is too simple.
Think about acids found in foods.
The statement is too simple because some acids are weak and found in foods, such as citric acid in oranges, while strong acids can be dangerous and require careful handling. - 8
Two solutions have pH values of 4 and 6. Which solution is more acidic, and how many times more acidic is it based on the pH scale?
A difference of 2 pH units means 10 times 10.
The solution with pH 4 is more acidic. It is 100 times more acidic than the solution with pH 6 because each pH unit represents a factor of 10. - 9
A pH indicator turns green in a liquid. The indicator chart says green means pH 7. What can you conclude about the liquid?
The liquid is neutral or very close to neutral because the indicator color shows a pH of 7. - 10
An acid and a base are mixed carefully in the correct amounts, and the final solution has a pH of 7. What type of reaction happened?
The pH moved toward 7.
A neutralization reaction happened. The acid and base reacted to form a solution that is neutral or close to neutral. - 11
Classify each substance as acid, base, or neutral: stomach acid pH 1, soap solution pH 10, and salt water pH 7.
Stomach acid is an acid, soap solution is a base, and salt water with pH 7 is neutral. - 12
A fish tank should stay near pH 7. A test shows the water is pH 5. Is the water too acidic or too basic, and what should the owner do before adding any chemical treatment?
Values below 7 are acidic.
The water is too acidic because pH 5 is below 7. The owner should follow safe aquarium testing instructions or ask an expert before adding any chemical treatment. - 13
The table shows four unknown liquids: A has pH 8, B has pH 2, C has pH 7, and D has pH 13. Which liquid is the strongest base?
Liquid D is the strongest base because it has the highest pH value, 13. - 14
Explain the difference between a strong acid and a weak acid in middle school terms.
Strong and weak describe how the acid behaves in water.
A strong acid breaks apart more completely in water and can produce a very low pH. A weak acid does not break apart as much and is usually less reactive at the same concentration. - 15
A student measures orange juice at pH 3 and milk at pH 6. Which is closer to neutral, and how do you know?
Milk is closer to neutral because pH 6 is only 1 unit away from pH 7, while pH 3 is 4 units away from pH 7.