pH and indicators
5 marks · HardAn unknown solution is tested with universal indicator. The indicator turns blue-green and the estimated pH is 8.
Use the pH scale to classify the solution as strongly acidic, weakly acidic, neutral, weakly alkaline, or strongly alkaline. Give a reason for your answer.
State what the pH value tells you about the concentration of hydrogen ions compared to a solution of pH 7.
Describe how you would use universal indicator to estimate the pH of an aqueous solution. Include what you would observe and what you would compare it to.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Weakly alkaline, because pH 8 lies in the range 8–10 on the pH scale
(b) The concentration of hydrogen ions is lower than in a neutral solution
(c) Add universal indicator to the solution, observe the colour change (e.g., blue-green), then compare the colour to a pH colour chart to estimate the pH value.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Classifies pH 8 as weakly alkaline (accept weak alkali)
B2 (a) · 1 mark · States reason: pH 8 is in the weakly alkaline range (8–10)
B3 (b) · 1 mark · States that the hydrogen ion concentration is lower than pH 7 or than in a neutral solution (accept 'fewer H+ ions').
B4 (c) · 1 mark · Describes adding universal indicator to the solution and observing colour change.
B5 (c) · 1 mark · Mentions comparing the colour to a pH colour chart to estimate the pH value.
Worked solution
a) pH 8 falls in the range 8–10, which is classified as weakly alkaline.
Reason: Weakly alkaline solutions have pH values between 8 and 10.
b) The solution has a lower hydrogen ion concentration than a neutral solution (pH 7).
c) Add universal indicator to the solution. Observe the colour change (here, blue-green). Compare this colour to a pH colour chart to estimate the pH value.