The pH scale measures how acidic or basic (alkaline) a solution is, ranging from 0 to 14. A pH of 7 is neutral (like pure water), below 7 is acidic (like lemon juice or vinegar), and above 7 is basic (like soap or baking soda). Acids release hydrogen ions (H⁺) in water; bases release hydroxide ions (OH⁻). Indicators, like litmus paper, change colour to show whether a substance is acidic or basic.
Example
Lemon juice has a pH around 2 (strongly acidic), pure water sits at pH 7 (neutral), and soapy water might sit around pH 9-10 (basic) — you could test each with litmus paper, which turns red in acids and blue in bases.
Key terms
pH scale:
A scale from 0 to 14 measuring how acidic or basic a solution is.
Acid:
A substance that releases hydrogen ions (H⁺) in water; pH below 7.
Base:
A substance that releases hydroxide ions (OH⁻) in water; pH above 7.
Questions
1. The pH scale ranges from:
0 to 14
1 to 10
0 to 100
-10 to 10
2. A pH of 7 is:
Neutral
Strongly acidic
Strongly basic
Impossible
3. A pH below 7 indicates a substance is:
Acidic
Basic
Neutral
Not a real substance
4. A pH above 7 indicates a substance is:
Basic (alkaline)
Acidic
Neutral
Impossible
5. Pure water has a pH of approximately:
7
0
14
100
6. Lemon juice is an example of a substance that is:
Acidic
Basic
Neutral
Not able to be tested
7. An indicator, like litmus paper, is used to:
Show whether a substance is acidic or basic
Measure temperature only
Measure weight only
Replace the need for a pH scale
8. Litmus paper turns which colour in an acid?
Red
Blue
Green
It doesn't change colour
9. Litmus paper turns which colour in a base?
Blue
Red
Yellow
It doesn't change colour
10. Acids release which ion in water?
Hydrogen ions (H⁺)
Hydroxide ions (OH⁻) only
Oxygen ions only
No ions at all
11. Bases release which ion in water?
Hydroxide ions (OH⁻)
Hydrogen ions (H⁺) only
Carbon ions only
No ions at all
12. Soap and baking soda are examples of substances that are:
Basic
Acidic
Neutral only
Unable to be measured
13. A pH of 2 would be considered:
Strongly acidic
Strongly basic
Neutral
Impossible to exist
14. A pH of 12 would be considered:
Strongly basic
Strongly acidic
Neutral
Impossible to exist
15. Each whole number step on the pH scale represents a change in acidity/basicity of approximately:
10 times (the scale is logarithmic)
Exactly 1 time (a simple linear scale)
2 times
100 times for every 2 steps only
16. Why might farmers test and adjust soil pH before planting certain crops?
Different plants grow best within specific pH ranges, affecting nutrient availability
Soil pH has no effect on plant growth
All crops grow equally well at any pH level
Testing soil pH serves no agricultural purpose
17. Why is stomach acid (very low pH) not harmful to the stomach itself under normal conditions?
The stomach lining has protective mechanisms (like mucus) adapted to withstand its own acidity
Stomach acid is not actually acidic at all
The stomach has no biological adaptation to its own acid
Acids never cause any damage to any biological tissue
18. Why might acid rain (abnormally low pH rainfall) damage ecosystems like lakes and forests?
It can disrupt the pH balance many organisms depend on for survival
Acid rain has no real effect on any ecosystem
All organisms thrive equally well regardless of pH changes
Acid rain always improves ecosystem health
19. A neutralisation reaction between an acid and a base typically produces:
Water and a salt
Only a stronger acid
Only a stronger base
No reaction occurs at all
20. Why do swimming pools require regular pH testing and adjustment?
Incorrect pH can irritate skin/eyes and reduce the effectiveness of chlorine disinfection
Pool water pH has no effect on swimmers or water treatment
Pool pH never changes once set, requiring no monitoring
pH testing is unrelated to pool water safety
21. Why might ocean acidification (a drop in seawater pH linked to rising CO2) threaten marine organisms like coral and shellfish?
Many marine organisms rely on stable pH conditions to build and maintain calcium-based shells and skeletons
Ocean pH has no connection to marine organism health
All marine life is completely unaffected by any change in water pH
Ocean acidification always makes shells and skeletons stronger
Answer key (parent copy)
1. 0 to 14
2. Neutral
3. Acidic
4. Basic (alkaline)
5. 7
6. Acidic
7. Show whether a substance is acidic or basic
8. Red
9. Blue
10. Hydrogen ions (H⁺)
11. Hydroxide ions (OH⁻)
12. Basic
13. Strongly acidic
14. Strongly basic
15. 10 times (the scale is logarithmic)
16. Different plants grow best within specific pH ranges, affecting nutrient availability
17. The stomach lining has protective mechanisms (like mucus) adapted to withstand its own acidity
18. It can disrupt the pH balance many organisms depend on for survival
19. Water and a salt
20. Incorrect pH can irritate skin/eyes and reduce the effectiveness of chlorine disinfection
21. Many marine organisms rely on stable pH conditions to build and maintain calcium-based shells and skeletons