A sample of air was bubbled into water. The PH of the water slowly changed from 7 to 6. Which of the following gases in the samples caused the change?

Step 1: Analyze the question Water starts at pH 7 (neutral) and drops to pH 6 (slightly acidic). So one gas in the air must dissolve in water and form an acid.

Step 2: Explain the concept Acidic oxides (usually oxides of non-metals) dissolve in water to form acids, which release H⁺ ions and lower the pH.

Step 3: Check each option

• A. Carbon(IV) oxide (CO₂): It is an acidic oxide. It dissolves in water to form carbonic acid, a weak acid: CO₂ + H₂O ⇌ H₂CO₃ H₂CO₃ ⇌ H⁺ + HCO₃⁻ The H⁺ ions lower the pH slightly, from 7 to about 6. Because carbonic acid is weak, the change is small and gradual, which matches "slowly changed from 7 to 6". This is also why rainwater is naturally slightly acidic (about pH 5.6).
• B. Carbon(II) oxide (CO): It is a neutral oxide and does not react with water to form an acid, so the pH would not change.
• C. Nitrogen (N₂): It is unreactive and almost insoluble in water, so it has no effect on pH.
• D. Oxygen (O₂): It is only slightly soluble and does not form an acid in water, so it has no effect on pH.

Step 4: Answer The gas responsible is carbon(IV) oxide, which is option A.

Exam tip: Remember that CO₂, SO₂ and NO₂ are acidic gases, while CO and N₂O are neutral. In air, CO₂ is the common gas that turns water mildly acidic. Whenever you see "pH drops slightly when air is bubbled into water", think CO₂ and carbonic acid.