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What is an acid?

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Quick answer

An acid is a substance that releases hydrogen ions (H⁺) in water, giving it a sour taste and a pH below 7. Acids range from mild ones in food, like the citric acid in lemons, to strong industrial ones like sulfuric acid.

Key things to understand

  1. Acids release hydrogen ions (H⁺) when dissolved in water.
  2. They have a pH below 7 — the lower the pH, the stronger the acid.
  3. Acids taste sour (think lemon or vinegar) and can react with metals and carbonates.
  4. An acid and a base neutralize each other, producing water and a salt.
  5. Strong acids (like hydrochloric acid) ionize completely; weak acids (like the acetic acid in vinegar) only partly.

People also ask

What's the difference between an acid and a base?
Acids release hydrogen ions and have a pH below 7; bases release hydroxide ions (or accept hydrogen ions) and have a pH above 7. Mixed together, they neutralize each other.
What is pH?
pH is a 0–14 scale measuring how acidic or basic a solution is. Below 7 is acidic, 7 is neutral (like pure water), and above 7 is basic.
Are acids dangerous?
Some are — strong acids can burn skin and corrode metal — but many are harmless and everyday, like the citric acid in fruit or the acetic acid in vinegar.