What acids have in common
Lime juice and lab acid look nothing alike. Both turn a blue strip red. Both free the same kind of ion, a charged particle, in water.
- A spoonful of acid drops into water.
- It frees H⁺ ions into the water.
- A spoonful of base drops into water instead.
- It frees OH⁻ ions into the water.
Pick a substance below. Switch the water on and off.
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The rule
Every acid frees H⁺ ions in water. A base that dissolves in water frees OH⁻ ions.
A base that dissolves in water is also called an alkali.
An ion is a particle that carries a charge. In water, H⁺ joins a water molecule and makes H₃O⁺. Books write H⁺ for short.
H⁺ is a hydrogen ion. OH⁻ is a hydroxide ion.
Dry, with no water, an acid shows none of its tests. Dry hydrogen chloride gas does not turn dry litmus red. Water lets the ions form.
The ions also carry electric current. That is why acid solutions conduct electricity.
| Substance | Family | Ion it frees in water |
|---|---|---|
| Lime juice | acid | H⁺ |
| Tamarind water | acid | H⁺ |
| Dilute hydrochloric acid | acid | H⁺ |
| Sodium hydroxide solution | base | OH⁻ |
| Lime water | base | OH⁻ |
| Glucose | neither | none |
Watch out!
Hydrogen in the formula does not make an acid. Glucose has hydrogen, but it frees no H⁺ in water. Alcohol is the same.