MYP Chemistry: Acids, Bases & the pH Scale FAQ

Most students don't struggle with definitions — they struggle mixing three skills at once: reading indicator colours, doing log-based pH maths, and explaining particle-level theory in the same answer. I've marked papers where a student nails the calculation but can't explain why pH 3 is ten times more acidic than pH 4.

An acid releases hydrogen ions (H+) when dissolved in water and turns litmus red. A base accepts H+ ions, or releases hydroxide ions (OH−), and turns litmus blue. At MYP 4-5 you'll use this simplified model — bases that dissolve in water are specifically called alkalis.

The pH scale runs from 0 to 14 and measures hydrogen ion concentration logarithmically — each whole step changes acidity tenfold. Below 7 is acidic, 7 is neutral (pure water), above 7 is alkaline. A solution at pH 2 is ten times more acidic than pH 3, not just 'one unit worse'.

Strong acids like hydrochloric acid fully ionise in water, releasing all their H+ ions at once, giving a lower pH for the same concentration. Weak acids like ethanoic acid only partially ionise, so more molecules stay intact and the pH sits higher even at an identical concentration.

Litmus only tells you acid or base — red or blue, nothing more precise. Universal indicator gives a full colour range matched to a 0-14 pH chart, so you can estimate an actual pH value rather than just the category. Examiners expect the colour AND the approximate pH in extended responses.

A neutralisation reaction is an acid reacting with a base to form a salt and water, moving the pH toward 7. They matter because farmers use lime to neutralise acidic soil, and your stomach lining relies on similar chemistry to manage excess acid. MYP examiners often build Criterion A tasks around exactly this context.

pH equals the negative logarithm of the hydrogen ion concentration in mol/dm³: pH = −log[H+]. A solution with [H+] = 1×10⁻³ mol/dm³ has a pH of 3. Getting comfortable with this formula now pays off directly in DP Chemistry, where pH, Ka and Kb calculations appear across both SL and HL.

Start with definitions and the pH scale, then move to indicator colours, then practise writing neutralisation word and symbol equations, and finish with data-based questions interpreting a pH graph or titration result. I tell every student: don't just memorise 'red equals acid' — practise explaining why, in full sentences.

Acids and bases tasks are usually assessed against MYP Sciences Criterion A (Knowing and understanding) and Criterion C (Processing and evaluating) when there's a practical or data component. Tasks linking to environmental impacts like acid rain or ocean acidification often bring in Criterion D as well.

Expect 'state' for a quick fact (state the pH of a neutral solution), 'describe' for a labelled process, 'explain' for a reasoned cause-and-effect answer, and 'compare' when contrasting strong and weak acids. Explain answers need a reason word — 'because' — not just a restated fact.

Yes — MYP treats acids and bases conceptually: definitions, the pH scale, indicators and basic neutralisation. DP Chemistry adds quantitative depth: pH calculations, Ka/Kb, buffers and titration curves. According to the IB, the current DP Chemistry guide has first exams in 2025, and it builds directly on the MYP foundations covered here.

Acid rain, ocean acidification, indigestion remedies and soil pH for farming are the classic MYP contexts examiners build into Criterion A and D tasks. If you can explain one real example with the actual chemistry behind it — not just the headline fact — you'll stand out in an extended response.

Look for topic-specific revision notes that separate the theory (pH scale, indicators) from the maths (pH calculations), plus a worksheet mixing command terms so your child practises explaining, not just labelling. On RevisionPrep, MYP Chemistry Revision Notes and Topical Worksheets are built around exactly that split.

No lab needed for most MYP-level practice — a basic calculator with a log function covers pH = −log[H+] questions at this stage. Practical skills like using pH probes or running a titration are assessed separately under Criterion C, usually from real experiment data your teacher supplies in class.