The H⁺ in every acid

The idea

All acids contain H+ as the cation and produce hydrogen ions, H+(aq), in solution; these ions are responsible for their acidic properties.

What do acids share?

Hydrochloric acid, nitric acid and sulphuric acid all taste sour, turn blue litmus red and make hydrogen with metals. Their formulae are HCl, HNO₃ and H₂SO₄.

Which part do all three have in common?

• Hydrogen (H)
• Chlorine (Cl)
• Oxygen (O)

Acids in water

Add water to each one and watch it break up. Glucose has hydrogen too: does it break up the same way?

Every acid makes H⁺

All acids contain H⁺ as the cation and produce hydrogen ions, H⁺(aq), in solution. These ions are responsible for their acidic properties.

Having hydrogen is not enough. An acid must release it as H⁺ ions.

• Acid | In water
• HCl | H⁺ + Cl⁻
• HNO₃ | H⁺ + NO₃⁻
• H₂SO₄ | 2H⁺ + SO₄²⁻
• Glucose | no H⁺: not an acid

Notes

All acids contain H⁺ as the cation and produce hydrogen ions, H⁺(aq), in solution; these ions are responsible for their acidic properties.

Check yourself

Which ion do all acids give in water?

H₂SO₄ is added to water. What does it make?

Glucose has hydrogen in it. Is it an acid?

Which ions make a solution show acidic properties?

• OH⁻. OH⁻ comes from bases.
• H⁺ — correct. Yes! Hydrogen ions.
• Cl⁻. Only hydrochloric acid has chloride ions.
• H₂ gas and SO₄. It breaks into ions. It does not give hydrogen gas.
• One H⁺ and one OH⁻. OH⁻ comes from bases. An acid gives H⁺ and its own negative ion.
• Two H⁺ and one SO₄²⁻ — correct. Yes!
• No, it does not give H⁺ ions in water — correct. Yes! Only substances that release H⁺ are acids.
• Yes, because it has hydrogen. Having hydrogen is not enough. It has to be released as H⁺.
• Yes, because it is sweet. Taste is not the test. The test is whether it gives H⁺ in water.
• Chloride ions. Chloride is only one acid’s negative ion. Hydrogen ions are in every acid.
• Hydrogen ions — correct. Yes!
• Sodium ions. Sodium ions come from bases and salts, not from acids.