Which of the following substances with the corresponding pH value is an alkali? Substance pH

Analyze the question: You're asked to pick the substance whose pH shows it is an alkali (a soluble base).

Concept: The pH scale runs from 0 to 14:

• pH below 7 means acidic
• pH equal to 7 means neutral
• pH above 7 means basic or alkaline

The further the pH is above 7, the stronger the alkalinity.

Solution:

• A. Lime (the fruit), pH 2.4: this is below 7, so it is acidic because it contains citric acid.
• B. Banana, pH 4.6: this is below 7, so it is weakly acidic.
• C. Distilled water, pH 7.0: this is neutral.
• D. Milk of magnesia, pH 10.5: this is above 7, so it is alkaline. It is magnesium hydroxide suspension, used as an antacid to neutralise excess stomach acid.

Answer: D. Milk of magnesia, pH 10.5 is the only substance with a pH above 7.

Tip: Remember "below 7 is acid, 7 is neutral, above 7 is alkali." Antacids such as milk of magnesia are always basic, because they have to neutralise stomach acid. In WASSCE, also watch for "lime" meaning the fruit (acidic) versus "lime" meaning calcium oxide or hydroxide (basic). Here the pH of 2.4 shows it is the fruit.