Question detail
Explain why the pH of a solution increases when an acid is neutralised by an alkali.
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
Question
Type
exam_style
Style
Topic
Reactions of acids
Question
Explain why the pH of a solution increases when an acid is neutralised by an alkali.
Answer
The pH of a solution increases when an acid is neutralised by an alkali because the hydrogen ions (H+) from the acid react with the hydroxide ions (OH-) from the alkali to form water. This reduces the concentration of free hydrogen ions in the solution, leading to a higher pH.
Explanation
This question requires students to understand the relationship between hydrogen ions and pH, as well as the concept of neutralisation. It tests their ability to explain the chemical changes that occur during the reaction and how these changes affect pH.
Common mistake
Confusing ionic and molecular equations
Students write the neutralisation reaction as H⁺ + OH⁻ → H₂O instead of the full ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) and omit the state symbols, or they write a molecular equation such as HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) and then claim it is the ionic equation.
Remind students that the ionic equation shows only the ions that actually participate in the reaction. The correct ionic equation for neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l). The molecular equation is fine for describing the overall reaction, but the ionic form is required when describing the mechanism of neutralisation.
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