# Acid

A substance that forms hydrogen ions (H+) when it dissolves in water. Acids have a pH below 7 and taste sour.

Chemistry (school) · Part 06: Acids, bases and salts · Card 1 of 9 · Last updated 2026-10-19

## Analogy

Acids are like tiny hydrogen-ion donors, handing out H+ to anything that will take one.

## Example

**Common lab acids**

Hydrochloric acid: HCl
Sulfuric acid: H2SO4
Nitric acid: HNO3

## In practice

Lemon juice contains citric acid. Hydrochloric acid is in your stomach and helps break down food.

## Practice

### Flashcards

- **What ion do all acids produce in water?** Hydrogen ions, H+.

### Interview questions

**Explain the difference between a strong acid and a concentrated acid.**

- Strong means fully ionised in water
- Concentrated means lots of acid per unit of volume
- A weak acid can be concentrated
- A strong acid can be dilute

## Questions people ask

### Are all acids dangerous?

No. Many acids are in food and your body, such as citric acid in oranges and ethanoic acid in vinegar. Strong or concentrated acids like sulfuric acid can burn skin and need goggles. Danger depends on both strength and concentration, so always follow safety rules in the lab.

### What is the difference between an acid and a base?

An acid releases hydrogen ions in water. A base is any substance that can react with and neutralise an acid. Many bases are metal oxides and hydroxides. A base that dissolves in water is called an alkali, so all alkalis are bases but not every base is an alkali.

## Related concepts

- [Strong and weak acids](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-6/chemistry-school-part-6-concept-7)

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