Acids, Alkalis and Titrations
5 marks · EasyA student tests a sample of rainwater using universal indicator paper. The paper turns yellow-green.
What does the colour yellow-green indicate about the pH of the rainwater? Estimate the pH value.
Is the rainwater acidic, neutral or alkaline? Explain your answer.
Describe how you would use universal indicator solution, rather than paper, to measure the pH of rainwater.
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.
Show answer
(a) Yellow-green indicates a pH of about 6.
(b) The rainwater is weakly acidic, because pH 6 is below 7.
(c) Add universal indicator solution to the rainwater sample and compare the resulting colour to a pH chart to estimate its pH value.
Common mistakes: putting neutral instead of weakly acidic for yellow-green; omitting to compare to a chart for universal indicator solution
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Yellow-green gives pH below 7 (about 6).
A2 (a) · 1 mark · Correct pH estimate: 6 (accept 5–6).
A3 (b) · 1 mark · Correctly states rainwater is weakly acidic with explanation (pH < 7).
M1 (c) · 1 mark · Describes adding universal indicator solution to rainwater.
M2 (c) · 1 mark · States or describes that the new colour is matched to a pH chart to determine pH value.
Worked solution
a. Universal indicator yellow-green means the pH is around 6.
b. This means rainwater is weakly acidic, as pH 6 is just below neutral (7).
c. To use universal indicator solution, add a few drops to the rainwater sample. Compare the final colour to a universal indicator pH chart to estimate the pH.