What does pH measure?

The concentration of hydrogen ions (H⁺) in a solution.

Give one advantage of a pH probe over universal indicator.

It gives a precise digital reading that does not depend on judging a colour.

Give one advantage of universal indicator over a pH probe.

It is quick, cheap and needs no calibrating.

The pH scaleIB MYP Chemistry: Flashcards

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What range does the pH scale cover?

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What range does the pH scale cover?: 0 to 14.
What pH is neutral?
What does a pH below 7 show?: The solution is acidic.
What does a pH above 7 show?: The solution is alkaline.
What does pH measure?: The concentration of hydrogen ions (H⁺) in a solution.
What happens to H⁺ concentration as pH gets lower?: It gets higher.
How does H⁺ concentration change for each pH unit?: It changes tenfold.
How many times more H⁺ ions does pH 2 have than pH 5?: 1000 times.
What colour is universal indicator at pH 7?: Green.
Give one advantage of a pH probe over universal indicator.: It gives a precise digital reading that does not depend on judging a colour.
Give one advantage of universal indicator over a pH probe.: It is quick, cheap and needs no calibrating.
What happens to the pH of an acid when it is diluted?: It rises towards 7.
By how much does the pH change when an acid is diluted ten times?: It rises by 1.

Exam questions on The pH scale

A swimming pool technician in Dubai measures the water with a digital pH probe and records a pH of 6.4. The recommended range for safe swimming is about 7.2 to 7.8.
Explain why a pH probe is a better choice than universal indicator for the technician.2 marks
A student starts with hydrochloric acid of pH 1.0. She mixes 10 cm³ of this acid with 90 cm³ of distilled water and measures the pH of the new solution with a probe: it is 2.0. She then mixes 10 cm³ of this new solution with another 90 cm³ of distilled water, and the pH is 3.0.
Describe the trend in pH as the acid is diluted and explain it in terms of hydrogen ions.2 marks
A student in Cape Town tests five solutions, P to T, with a pH probe. The readings are: P, pH 1.0; Q, pH 4.0; R, pH 7.0; S, pH 10.0; T, pH 13.0.
State which solution is the most acidic, which is neutral and which is the most alkaline.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).