Pure water contaminated with quicklime will have a pH of

Analyze the question Quicklime is calcium oxide (CaO). The question asks what pH pure water will have once it is contaminated with it.

Explain the concept

• Quicklime is a basic oxide. When it dissolves in water, it forms calcium hydroxide (slaked lime), which is an alkali:

CaO + H₂O → Ca(OH)₂

• Ca(OH)₂ ionizes to give hydroxide ions:

Ca(OH)₂ → Ca²⁺ + 2OH⁻

• Extra OH⁻ ions make the solution basic, so the pH is greater than 7.

Step-by-step solution

1. Pure water has a pH of 7 (neutral).
2. Adding CaO produces Ca(OH)₂, which releases OH⁻ ions.
3. The solution becomes alkaline, so the pH rises above 7.
4. Checking the options: • A (1): strongly acidic, so wrong. • B (6): weakly acidic, so wrong. • C (7): neutral, so wrong. • D (8): slightly alkaline, which fits.

Note on the value: In reality, a saturated calcium hydroxide solution has a pH of about 12 to 12.5. But among the options given, only D is above 7, so D is the only one that fits an alkaline solution.

Answer: D (8)

Exam tip: Remember that metal oxides are basic and non-metal oxides are acidic. For example, CaO, MgO and Na₂O give alkaline solutions, while CO₂, SO₂ and P₂O₅ give acidic ones. When a question asks about the pH after adding a metal oxide, pick the option above 7.