The pH scaleIB MYP Chemistry: Revision notes
Section 1
The pH scale
The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14.
- Below 7: acidic. The lower the number, the stronger the acid. Strong acids have a pH of 0 to 2.
- 7: neutral, such as pure water.
- Above 7: alkaline. The higher the number, the stronger the alkali. Strong alkalis have a pH of 12 to 14.
Examples: stomach acid has a pH of about 1 to 2, lemon juice about 2, pure water 7, soapy water about 9 to 10, and drain cleaner about 13 to 14.
Section 2
Measuring pH: indicator and probe
There are two main ways to measure pH.
Universal indicator changes colour: red (pH 1 to 2), orange (3 to 4), yellow (5 to 6), green (7), blue (8 to 11), purple (12 to 14). It is quick and cheap, but it only gives an approximate value and the colour match is a judgement. A coloured solution can hide the colour change.
A pH probe (pH meter) gives a digital reading, often to one or two decimal places. It is more precise and objective, and can be connected to a data logger, but it must be calibrated and costs more.
If a question needs a precise number, or a small difference such as pH 7.2 and 7.8, use a pH probe.
Section 3
pH and hydrogen ion concentration
pH is a measure of the concentration of hydrogen ions (H⁺) in a solution.
- The lower the pH, the higher the concentration of H⁺ ions.
- Each change of one pH unit is a tenfold change in H⁺ concentration.
Worked example: how does a solution with pH 2 compare with pH 5? The pH differs by 3, so the concentration of H⁺ ions differs by 10 × 10 × 10 = 1000 times. The pH 2 solution has 1000 times as many hydrogen ions.
Alkaline solutions have fewer H⁺ ions and more OH⁻ ions than neutral water.
A change of 3 pH units is not 3 times stronger. It is 10 × 10 × 10 = 1000 times.
Section 4
Diluting acids and alkalis
Dilution means adding water. It lowers the concentration of the acid, so there are fewer hydrogen ions per cm³ and the pH rises.
If you dilute an acid ten times (for example 10 cm³ of acid plus 90 cm³ of water), the H⁺ concentration falls to a tenth and the pH goes up by 1 unit. Diluting 100 times raises the pH by 2.
An acid moves towards pH 7 when diluted, but it never becomes alkaline. In the same way, diluting an alkali lowers its pH towards 7.
That's the notes covered.
Carry on to the next subtopic.
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
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).