Science: pH Scale and Acids and Bases
Using the pH scale to classify acids, bases, and neutral substances
Science: pH Scale and Acids and Bases
Using the pH scale to classify acids, bases, and neutral substances
Science - Grade 9-12
- 1
A solution has a pH of 3. Classify it as acidic, basic, or neutral, and explain how you know.
Use 7 as the dividing point on the pH scale.
A solution with a pH of 3 is acidic because any pH value below 7 is considered acidic on the pH scale. - 2
A substance has a pH of 7. What does this tell you about the substance?
A substance with a pH of 7 is neutral, which means it is neither acidic nor basic. - 3
A cleaning solution has a pH of 11. Is it an acid or a base? Explain your answer.
Numbers greater than 7 indicate one side of the pH scale.
A cleaning solution with a pH of 11 is a base because substances with pH values above 7 are basic. - 4
Compare a solution with pH 4 to a solution with pH 6. Which one is more acidic, and by how much in terms of hydrogen ion concentration?
The solution with pH 4 is more acidic. Because each pH unit represents a tenfold change in hydrogen ion concentration, a pH 4 solution has 100 times greater hydrogen ion concentration than a pH 6 solution. - 5
If a sample changes from pH 8 to pH 5, did it become more acidic or more basic? Explain the change.
Lower pH means greater acidity.
The sample became more acidic because its pH decreased from above 7 to below 7. This change also means the hydrogen ion concentration increased. - 6
Name one common property of acids and one common property of bases.
One common property of acids is that they donate hydrogen ions in solution. One common property of bases is that they accept hydrogen ions or produce hydroxide ions in solution. - 7
Litmus paper turns red in one solution and blue in another. What can you conclude about each solution?
Red and blue litmus colors are used to identify acids and bases.
If litmus paper turns red, the solution is acidic. If litmus paper turns blue, the solution is basic. - 8
A student says that a solution with pH 2 is only slightly more acidic than a solution with pH 3. Is the student correct? Explain.
The student is not correct. A solution with pH 2 is ten times more acidic in terms of hydrogen ion concentration than a solution with pH 3. - 9
Why is the pH scale considered logarithmic rather than linear?
Think about how concentration changes from one pH number to the next.
The pH scale is logarithmic because each change of 1 pH unit represents a tenfold change in hydrogen ion concentration rather than an equal additive change. - 10
A sample of rainwater has a pH of 5.6. Is this sample acidic, basic, or neutral? Explain.
A rainwater sample with a pH of 5.6 is slightly acidic because its pH is below 7. - 11
If one solution has a pH of 9 and another has a pH of 12, which solution is more basic, and how much lower is its hydrogen ion concentration?
A difference of 3 pH units means multiplying by 10 three times.
The solution with pH 12 is more basic. Compared with the pH 9 solution, it has 1000 times lower hydrogen ion concentration because the difference is 3 pH units. - 12
Describe what happens in a neutralization reaction between an acid and a base.
In a neutralization reaction, an acid and a base react to form water and a salt. The reaction reduces the acidic and basic properties of the original substances. - 13
A student mixes equal amounts of a strong acid and pure water. How will the pH likely change, and why?
Dilution changes concentration without changing the identity of the substance.
The pH will likely increase because adding pure water dilutes the acid, which lowers the hydrogen ion concentration. - 14
Household ammonia is a base. Predict whether its pH is more likely to be 2, 7, or 11, and explain your choice.
Its pH is more likely to be 11 because ammonia is a base, and bases have pH values greater than 7. - 15
Explain why a substance with pH 1 can be dangerous to skin and materials.
Very low pH values indicate strong acidity.
A substance with pH 1 is highly acidic and has a very high hydrogen ion concentration. This makes it corrosive, so it can damage skin and react with some materials.