Strong and Weak Acids
Strong acids ionize completely; weak ones partly.
About this topic
Strong acids such as HCl, HNO3 and H2SO4 ionise completely in water, so [H+] equals the acid concentration. Weak acids, such as acetic acid and carbonic acid, ionise only partially, establishing an equilibrium: HA + H2O ⇌ H3O+ + A−. The acid dissociation constant Ka measures strength; a larger Ka means a stronger acid. Strong and weak describe ionisation, not concentration.
For a weak acid, only a small fraction of molecules donates protons, such as about 1 percent of acetic acid molecules in a 0.1 M solution, so its pH is higher than that of a strong acid at the same concentration: 0.1 M HCl has pH 1, while 0.1 M acetic acid has pH about 2.9. The same ideas apply to bases: sodium hydroxide is strong, while ammonia is weak, with Kb. A strong acid has a very weak conjugate base, and a weak acid has a moderately strong one. Weak acids conduct less electricity than strong acids of the same concentration and react more slowly with metals.
Example: vinegar is about 5 percent acetic acid and is safe to eat, whereas even dilute hydrochloric acid in a battery can cause burns. A solution can be concentrated yet weak, or dilute yet strong.
A common mistake is mixing up strong with concentrated. Memorise the six strong acids: HCl, HBr, HI, HNO3, H2SO4 and HClO4.
Key ideas
- HCl is strong
- Acetic acid is weak
- Ka measures strength
Quick check
Use cases: where Strong and Weak Acids helps
- Homework: review the key idea behind Strong and Weak Acids before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Strong and Weak Acids without notes.
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- Teaching a friend: explain Strong and Weak Acids in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Strong and Weak Acids?
Strong acids ionize completely; weak ones partly.
What will I learn in Strong and Weak Acids?
HCl is strong. Acetic acid is weak. Ka measures strength.
Is Strong and Weak Acids part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Strong and Weak Acids lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Strong and Weak Acids?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
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