Not All Acids Are Dangerous
Why IGCSE Chemistry students assume all acids are corrosive, how weak and dilute acids are safe and common in food, and how to distinguish hazard from chemical classification on 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
What students typically write
“All acids are corrosive and dangerous, so you must always wear goggles when using them.” Or: “Acids burn your skin.”
Students associate the word “acid” with danger, imagining fuming liquids that dissolve metal. This leads to blanket safety statements and prevents them from understanding the range of acid behaviour that the syllabus covers.
Why it is wrong
Acids vary enormously in their hazard level. The key factors are strength and concentration:
- Strong acids (HCl, H2SO4, HNO3) fully ionise in water, producing a high concentration of H+ ions. Concentrated strong acids are corrosive.
- Weak acids (citric acid, ethanoic acid, carbonic acid) only partially ionise. Even at high concentrations, they produce far fewer H+ ions than strong acids.
- Dilute solutions of any acid are much less hazardous. A 0.1 mol/dm3 solution of hydrochloric acid is an irritant, not corrosive.
You consume weak acids every day. Citric acid (in citrus fruits), ethanoic acid (in vinegar), lactic acid (in yoghurt), and carbonic acid (in sparkling water) are all acids with pH values below 7 but are perfectly safe to eat and drink.
The syllabus expects you to know that acids produce H+ ions in solution and that pH measures the concentration of those ions. It does not want you to treat all acids as equally hazardous.
What the mark scheme actually wants
A typical question: “Give an example of an acid found in food and state its pH range.” [2]
Marking points:
- Named acid: citric acid / ethanoic acid / lactic acid (1 mark)
- pH below 7 / weakly acidic (1 mark)
No mark is available for saying “all acids are dangerous”. The question tests whether you know acids exist in safe, everyday contexts.
Worked example: wrong vs right
Question: Ethanoic acid has a concentration of 0.5 mol/dm3 and a pH of about 3. Hydrochloric acid at the same concentration has a pH of about 0.3. Explain this difference. [2]
Wrong answer: “Hydrochloric acid is more dangerous so it has a lower pH.” This scores 0/2. Danger is not a chemical explanation.
Right answer: “Hydrochloric acid is a strong acid and fully ionises in water, producing a high concentration of H+ ions and a very low pH (1). Ethanoic acid is a weak acid and only partially ionises, producing fewer H+ ions at the same concentration, so its pH is higher (1).” This scores 2/2.
How to avoid this mistake
When you see the word “acid”, do not default to “dangerous”. Instead, ask: is it strong or weak? Is it concentrated or dilute? Only concentrated strong acids merit the corrosive hazard symbol.
Remember: the pH scale is a measure of H+ ion concentration. Low pH means many H+ ions; high pH (but still below 7) means fewer H+ ions. The scale tells you about chemistry, not about danger.
For the full treatment of acids, bases, and pH, see acids, bases and indicators and the pH scale.
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