H⁺ needs water

The idea

An acid releases H+ ions only in the presence of water; hydrogen ions cannot exist alone but combine with water molecules, so they are shown as H+(aq) or hydronium ion (H3O+).

Dry gas, dry paper

Dry hydrogen chloride gas (HCl) is brought near dry blue litmus paper. There is no water anywhere.

What happens to the litmus?

• It stays blue
• It turns red at once
• It burns

Add the water

Watch one HCl molecule. Press Add water and follow the hydrogen.

H⁺ needs water to appear

An acid releases H⁺ ions only in the presence of water. Hydrogen ions cannot exist alone: they combine with water molecules, so they are shown as H⁺(aq) or hydronium ion (H₃O⁺).

H⁺(aq) and H₃O⁺ mean the same thing: a hydrogen ion in water.

• Step | Equation
• H⁺ meets water | H⁺ + H₂O → H₃O⁺
• HCl in water | HCl + H₂O → H₃O⁺ + Cl⁻

Notes

An acid releases H⁺ ions only in the presence of water; hydrogen ions cannot exist alone but combine with water molecules, so they are shown as H⁺(aq) or hydronium ion (H₃O⁺).

Check yourself

When does HCl release H⁺ ions?

H⁺ cannot exist alone. What does it join to form H₃O⁺?

What do H⁺(aq) and H₃O⁺ mean?

Dry HCl gas meets dry blue litmus paper. What do you see?

• Only when water is present — correct. Yes!
• Always, even when dry. Dry HCl gas does not turn dry blue litmus red.
• Only when it is heated. Water is what makes it happen, not heat.
• A chloride ion. The chloride ion stays as Cl⁻. The H⁺ joins a water molecule.
• A water molecule — correct. Yes! H⁺ + H₂O → H₃O⁺.
• Another H⁺. Two H⁺ ions repel each other. The H⁺ joins a water molecule.
• Two different ions. They are two ways to write the same ion.
• H₃O⁺ is a gas. H₃O⁺ is an ion in water, not a gas.
• The same thing: a hydrogen ion in water — correct. Yes!
• No change: the paper stays blue — correct. Yes! No water, no H⁺.
• The paper turns red. It needs water for H⁺ to be released.
• The paper turns purple. Purple is litmus solution that is neither acid nor base.